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Lock free shared data

This presentation discusses lock-free data exchange algorithms for real-time applications. It begins with an introduction that outlines what will be covered, including an overview of lock-free algorithms and their performance advantages. Next, it discusses traditional locking approaches and their limitations. It then introduces the concept of lock-free algorithms as an alternative that can provide better real-time determinism and scalability. In the last section, it provides examples of how lock-free algorithms can be implemented to allow shared data access without the need for locks.

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Introduction Lock-Free Explained Lock-Free Performance Summary Lock-Free Data Exchange for Real-Time Applications Peter Soetens Flander’s Mechatronics Technology Centre Leuven 25 Feb 2006 Free and Open Source Developers Meeting Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Lock-Free Performance Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What You Will Learn. What lock-free data exchange offers. When to use it. Why it is better. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Will Offer. Give an introduction to lock-free algorithms for non-experts. Show an experimental comparison between traditional locks and lock-free algorithms. Provide good leads for lock-free application and kernel development. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Assume About You. You. . . Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Assume About You. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Assume About You. You. . . know what multi-threaded applications are. care about real-time or embedded application design. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Definitions An algorithm is lock-free if. . . it does not ’block’ threads every step taken achieves global progress. it allows individual threads to starve (loop forever) but denies livelock. Redefinition (2003): “Obstruction Free” Real-Time A term to denote execution time determinism of an action or sequence of actions in response to an event. This means that the action always completes (and/or starts) within a bounded time interval. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary When to Use Lock-Free Algorithms Lock-Free is especially useful for multi-threaded or -process applications blob data- or pointer-exchange OS kernels and applications Real-Time (time determinism) Embedded (simple scheduler) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary When to Use Lock-Free Algorithms Lock-Free is especially useful for multi-threaded or -process applications blob data- or pointer-exchange OS kernels and applications Real-Time (time determinism) Embedded (simple scheduler) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Requires Lock-Free requires Processor support A Compare-And-Swap (CAS, CMPXCH) or Load-Linked/Store-Conditional (LL/SC) processor instruction or Atomic increment/decrement and test. Architecture support A piece of physically Shared Memory. An algorithm Fortunately, they all look the same. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Requires Lock-Free requires Processor support A Compare-And-Swap (CAS, CMPXCH) or Load-Linked/Store-Conditional (LL/SC) processor instruction or Atomic increment/decrement and test. Architecture support A piece of physically Shared Memory. An algorithm Fortunately, they all look the same. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Requires Lock-Free requires Processor support A Compare-And-Swap (CAS, CMPXCH) or Load-Linked/Store-Conditional (LL/SC) processor instruction or Atomic increment/decrement and test. Architecture support A piece of physically Shared Memory. An algorithm Fortunately, they all look the same. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Does Not Require Lock-Free does not require OS support Scheduler support Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Example Uses Common uses are: From Orocos.org: FIFO/LIFO data buffers (Single) linked lists Pointer queues Shared data From Boost.org (0.33.0) Smart pointers . . . in many readers/many writers environments Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary The Example Application D Shared Data Writer Reader Thread Thread ’Thread’ : May be an interrupt, thread, process,... ’Data’ : Modifyable in a single (composite) transaction. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Unprotected Access D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Unprotected Access Unrestricted Access D Corrupted Data Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks The Lock D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Is this scalable ? Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks Lowest D Priority ! Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks Pre−empting threads always have higher Lowest priority ! D Priority ! Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks Lowest Priority ! D Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Lowest Priority ! Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary Effect on priorities ? A higher priority means a worse delay Best case access time ? Grab the lock, modify, release lock (Taccess ) Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay Best case access time ? Grab the lock, modify, release lock (Taccess ) Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) - Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) - Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) + Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) - Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) + Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Let’s get rid of locks! Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D 2 Reader Threads Shared 1 Writer Data Thread Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Mark most recent D D D Read most recent Pre-allocate data blocks to avoid run-time allocation (not real-time). No heap required. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Write... modify D D D Yep, that’s RCU (Read-Copy-Update) for you experts. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Done! D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Mark Writer modify D D D D Reader Reader Worst case number of required data blocks = 2 + R For a single writer - many readers setup. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary Effect on priorities ? None Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead + Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead + Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead - Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead + Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead - Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers 2 Writer D 2 Reader Threads Threads Shared Data Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Which one is most recent ? D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 modify D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 Writer 2 Higher modify priority D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Compare Writer 1 Writer 2 −and− Swap: modify D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 Done! modify D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 CAS Fails ! modify D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 modify again! D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Done! D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers D D D Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary Effect on priorities ? Highest priority wins! Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! + Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! + Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! + Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. - Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Recap: Definition Real-Time A term to denote execution time determinism of an action or sequence of actions in response to an event. This means that the action always completes (and/or starts) within a bounded time interval. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation Experiment Real-time Machine Controller Pentium III 750MHz, 128MB RAM (this is vastly oversized for our purpose, but allowed on-target data capturing) Software Linux 2.4.18 with RTAI/LXRT 3.0 Patch Orocos configured for LXRT Many readers / many writers test applications Both FIFO buffers and shared data exchange Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Flow Sma ll NRT RT RT 500Hz 1KHz RT RT 500Hz 500Hz Concu rr ent NRT RT NRT RT 2KHz 2KHz RT RT 1Hz 1Hz Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: No Communication 1e+06 1ms/0.5ms 100000 5ms/1ms 10000 Occurences 1000 100 Execution 10 1 latencies 0.1 1e-05 1e-04 0.001 Latency time ( s ). Bucket size: 5 us 0.01 For a small (2 RT 1e+06 0.5ms/0.1ms threads) and 100000 1ms/0.2ms 10000 2ms/0.3ms concurrent (3 RT Occurences 1000 100 threads) 10 application. 1 0.1 1e-05 1e-04 0.001 0.01 Latency time ( s ). Bucket size: 5 us Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 1e+06 1ms/0.5ms 100000 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 1e+06 Latency time ( s ). Bucket size: 5 us latencies 1ms/0.5ms 100000 Lock based (top) 10000 Occurences 1000 and lock free 100 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Small application, high priority thread. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 5ms/0.5ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 100000 Latency time ( s ). Bucket size: 5 us latencies 5ms/0.5ms 10000 Lock based (top) Occurences 1000 100 and lock-free 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Small application, low priority thread. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 0.5ms/0.1ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 100000 Latency time ( s ). Bucket size: 5 us latencies 0.5ms/0.1ms 10000 Lock based (top) Occurences 1000 100 and lock free 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Concurrent application, high priority thread. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 1ms/0.2ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 1e+06 Latency time ( s ). Bucket size: 5 us latencies 1ms/0.2ms 100000 Lock based (top) 10000 Occurences 1000 and lock free 100 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Concurrent application, medium priority thread. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 2ms/0.3ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 100000 Latency time ( s ). Bucket size: 5 us latencies 2ms/0.3ms 10000 Lock based (top) Occurences 1000 100 and lock free 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Concurrent application, medium priority thread. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Conclusions Small Applications Lock-free performs on average better Lock-free performs worst case better Concurrent Applications Lock-free performs on average better Lock-free performs worst case better Lock-free prevents dead-line failures Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Memory Overhead Memory consumption increases linearly OK for most real-time and embedded applications, number of threads is well known. Worse, if not catastrophic, for OS Reference counted kernels, number of threads is unknown. D D D Reference counted memory Both readers and writers need to reference count data blocks. Requires ’atomic’ processor instructions. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Overhead for Readers Increase reference count Since a data block may not be freed before all readers are done reading it, a reference counting Reference counted implementation is required. Analogous to RCU D D D Detect moved ’Most Recent’ pointer. If a reader ’locks’ the data block but detects that the refcount is one, it must retry, since the block may be in re-use already. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Overhead for Writers Find an empty data block Possibly race against other writers Increase reference count Copy and update the data Reference counted Large data blocks will reduce D D D performance Retry if necessary (W > 1) In case source data block changed, startover with the copy-update from the new data block. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues + Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues + Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues + Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. + Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Lock-Free Performance Summary Summary Lock-Free algorithms are a drastic improvement for real-time applications Lock-Free algorithms don’t require any scheduler intervention. But be aware of memory requirements. Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Lock-Free Performance Summary Thank you for your attention ! Peter Soetens Lock-Free Data Exchange
Introduction Lock-Free Explained Lock-Free Performance Summary References http://www.orocos.org Anderson, J., S. Ramamurthy, and K. Jeffay (1995). Real-time com- puting with lock-free shared objects. Proceedings of the 16th IEEE Real-Time Systems Symposium. Herlihy, M. (1991). Wait-free synchronization. ACM Trans. Program. Lang. Syst. 13 (1), 124-149. Herlihy, M., V. Luchangco, and M. Moir (2003). Obstruction-free synchronization: Double-ended queues as an example. In 03: Proceedings of the 23rd International Conference on Dis- tributed Computing Systems, Washington, DC, USA, pp. 522. IEEE Computer Society. Peter Soetens Lock-Free Data Exchange

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Lock free shared data

  • 1.
    Introduction Lock-Free Explained Lock-Free Performance Summary Lock-Free Data Exchange for Real-Time Applications Peter Soetens Flander’s Mechatronics Technology Centre Leuven 25 Feb 2006 Free and Open Source Developers Meeting Peter Soetens Lock-Free Data Exchange
  • 2.
    Introduction Lock-Free Explained Lock-Free Performance Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
  • 3.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
  • 4.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What You Will Learn. What lock-free data exchange offers. When to use it. Why it is better. Peter Soetens Lock-Free Data Exchange
  • 5.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Will Offer. Give an introduction to lock-free algorithms for non-experts. Show an experimental comparison between traditional locks and lock-free algorithms. Provide good leads for lock-free application and kernel development. Peter Soetens Lock-Free Data Exchange
  • 6.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Assume About You. You. . . Peter Soetens Lock-Free Data Exchange
  • 7.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Assume About You. Peter Soetens Lock-Free Data Exchange
  • 8.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What I Assume About You. You. . . know what multi-threaded applications are. care about real-time or embedded application design. Peter Soetens Lock-Free Data Exchange
  • 9.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
  • 10.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Definitions An algorithm is lock-free if. . . it does not ’block’ threads every step taken achieves global progress. it allows individual threads to starve (loop forever) but denies livelock. Redefinition (2003): “Obstruction Free” Real-Time A term to denote execution time determinism of an action or sequence of actions in response to an event. This means that the action always completes (and/or starts) within a bounded time interval. Peter Soetens Lock-Free Data Exchange
  • 11.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary When to Use Lock-Free Algorithms Lock-Free is especially useful for multi-threaded or -process applications blob data- or pointer-exchange OS kernels and applications Real-Time (time determinism) Embedded (simple scheduler) Peter Soetens Lock-Free Data Exchange
  • 12.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary When to Use Lock-Free Algorithms Lock-Free is especially useful for multi-threaded or -process applications blob data- or pointer-exchange OS kernels and applications Real-Time (time determinism) Embedded (simple scheduler) Peter Soetens Lock-Free Data Exchange
  • 13.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Requires Lock-Free requires Processor support A Compare-And-Swap (CAS, CMPXCH) or Load-Linked/Store-Conditional (LL/SC) processor instruction or Atomic increment/decrement and test. Architecture support A piece of physically Shared Memory. An algorithm Fortunately, they all look the same. Peter Soetens Lock-Free Data Exchange
  • 14.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Requires Lock-Free requires Processor support A Compare-And-Swap (CAS, CMPXCH) or Load-Linked/Store-Conditional (LL/SC) processor instruction or Atomic increment/decrement and test. Architecture support A piece of physically Shared Memory. An algorithm Fortunately, they all look the same. Peter Soetens Lock-Free Data Exchange
  • 15.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Requires Lock-Free requires Processor support A Compare-And-Swap (CAS, CMPXCH) or Load-Linked/Store-Conditional (LL/SC) processor instruction or Atomic increment/decrement and test. Architecture support A piece of physically Shared Memory. An algorithm Fortunately, they all look the same. Peter Soetens Lock-Free Data Exchange
  • 16.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary What Lock-Free Does Not Require Lock-Free does not require OS support Scheduler support Peter Soetens Lock-Free Data Exchange
  • 17.
    Introduction Lock-Free Explained About You and Me Lock-Free Performance Application Domain Summary Example Uses Common uses are: From Orocos.org: FIFO/LIFO data buffers (Single) linked lists Pointer queues Shared data From Boost.org (0.33.0) Smart pointers . . . in many readers/many writers environments Peter Soetens Lock-Free Data Exchange
  • 18.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
  • 19.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary The Example Application D Shared Data Writer Reader Thread Thread ’Thread’ : May be an interrupt, thread, process,... ’Data’ : Modifyable in a single (composite) transaction. Peter Soetens Lock-Free Data Exchange
  • 20.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Unprotected Access D Peter Soetens Lock-Free Data Exchange
  • 21.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Unprotected Access Unrestricted Access D Corrupted Data Peter Soetens Lock-Free Data Exchange
  • 22.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks The Lock D Peter Soetens Lock-Free Data Exchange
  • 23.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
  • 24.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
  • 25.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
  • 26.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Peter Soetens Lock-Free Data Exchange
  • 27.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
  • 28.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
  • 29.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
  • 30.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks ZZzzz... D A worst case scenario. . . Peter Soetens Lock-Free Data Exchange
  • 31.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Is this scalable ? Peter Soetens Lock-Free Data Exchange
  • 32.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
  • 33.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks Lowest D Priority ! Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
  • 34.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks Pre−empting threads always have higher Lowest priority ! D Priority ! Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
  • 35.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks Lowest Priority ! D Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
  • 36.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Using Locks D Lowest Priority ! Which thread has the lowest priority ? Peter Soetens Lock-Free Data Exchange
  • 37.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary Effect on priorities ? A higher priority means a worse delay Best case access time ? Grab the lock, modify, release lock (Taccess ) Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
  • 38.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay Best case access time ? Grab the lock, modify, release lock (Taccess ) Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
  • 39.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
  • 40.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) - Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
  • 41.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) - Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) + Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
  • 42.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Based Reader-Writer Summary - Effect on priorities ? A higher priority means a worse delay - Best case access time ? Grab the lock, modify, release lock (Taccess ) - Worst case access time ? = 2 ∗ Taccess using priority inheritance or ∞ without priority inheritance (priority inversions) + Memory requirements ? = sizeof (D) + sizeof (Lock ) + sizeof (SchedAlgorithms) Peter Soetens Lock-Free Data Exchange
  • 43.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Let’s get rid of locks! Peter Soetens Lock-Free Data Exchange
  • 44.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
  • 45.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D 2 Reader Threads Shared 1 Writer Data Thread Peter Soetens Lock-Free Data Exchange
  • 46.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Mark most recent D D D Read most recent Pre-allocate data blocks to avoid run-time allocation (not real-time). No heap required. Peter Soetens Lock-Free Data Exchange
  • 47.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Write... modify D D D Yep, that’s RCU (Read-Copy-Update) for you experts. Peter Soetens Lock-Free Data Exchange
  • 48.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Done! D D D Peter Soetens Lock-Free Data Exchange
  • 49.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
  • 50.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
  • 51.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
  • 52.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer D D D Peter Soetens Lock-Free Data Exchange
  • 53.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 1 Writer Mark Writer modify D D D D Reader Reader Worst case number of required data blocks = 2 + R For a single writer - many readers setup. Peter Soetens Lock-Free Data Exchange
  • 54.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary Effect on priorities ? None Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
  • 55.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
  • 56.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
  • 57.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead + Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
  • 58.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead + Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead - Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
  • 59.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Single Writer Summary + Effect on priorities ? None + Best case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead + Worst case access time ? Writer: TFindpointer + TModify + TPointercopy Reader: TRead - Memory requirements ? = sizeof (D) ∗ (2 + R) Did I use CAS ? No Peter Soetens Lock-Free Data Exchange
  • 60.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers 2 Writer D 2 Reader Threads Threads Shared Data Peter Soetens Lock-Free Data Exchange
  • 61.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Which one is most recent ? D D D Peter Soetens Lock-Free Data Exchange
  • 62.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 modify D D D Peter Soetens Lock-Free Data Exchange
  • 63.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 Writer 2 Higher modify priority D D D Peter Soetens Lock-Free Data Exchange
  • 64.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Compare Writer 1 Writer 2 −and− Swap: modify D D D Peter Soetens Lock-Free Data Exchange
  • 65.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 Done! modify D D D Peter Soetens Lock-Free Data Exchange
  • 66.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 CAS Fails ! modify D D D Peter Soetens Lock-Free Data Exchange
  • 67.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Writer 1 modify again! D D D Peter Soetens Lock-Free Data Exchange
  • 68.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers Done! D D D Peter Soetens Lock-Free Data Exchange
  • 69.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Away with Locks: 2 Writers D D D Peter Soetens Lock-Free Data Exchange
  • 70.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary Effect on priorities ? Highest priority wins! Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
  • 71.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
  • 72.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! + Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
  • 73.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! + Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
  • 74.
    Introduction Lock-Free Explained The Good ’Ol Days Lock-Free Performance A New Kind of Computer Science Summary Lock-Free Multiple Writer Summary + Effect on priorities ? Highest priority wins! + Best case access time ? = TFindpointer + TModify + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. - Memory requirements ? = sizeof (D) ∗ (2 ∗ W + R) Peter Soetens Lock-Free Data Exchange
  • 75.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
  • 76.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Recap: Definition Real-Time A term to denote execution time determinism of an action or sequence of actions in response to an event. This means that the action always completes (and/or starts) within a bounded time interval. Peter Soetens Lock-Free Data Exchange
  • 77.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
  • 78.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
  • 79.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
  • 80.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Going Real-Time Given one of these Real-Time schedulers: Rate Monotonic Scheduler (RMS) Deadline Monotonic Scheduler (DMS) Earliest Deadline First Scheduler (EDFS) The following properties are always true for any lock-free algorithm: The highest priority writer thread has best case access time. The other writer threads have bounded access time. Any reader has always best case access time. Peter Soetens Lock-Free Data Exchange
  • 81.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation Experiment Real-time Machine Controller Pentium III 750MHz, 128MB RAM (this is vastly oversized for our purpose, but allowed on-target data capturing) Software Linux 2.4.18 with RTAI/LXRT 3.0 Patch Orocos configured for LXRT Many readers / many writers test applications Both FIFO buffers and shared data exchange Peter Soetens Lock-Free Data Exchange
  • 82.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Flow Sma ll NRT RT RT 500Hz 1KHz RT RT 500Hz 500Hz Concu rr ent NRT RT NRT RT 2KHz 2KHz RT RT 1Hz 1Hz Peter Soetens Lock-Free Data Exchange
  • 83.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: No Communication 1e+06 1ms/0.5ms 100000 5ms/1ms 10000 Occurences 1000 100 Execution 10 1 latencies 0.1 1e-05 1e-04 0.001 Latency time ( s ). Bucket size: 5 us 0.01 For a small (2 RT 1e+06 0.5ms/0.1ms threads) and 100000 1ms/0.2ms 10000 2ms/0.3ms concurrent (3 RT Occurences 1000 100 threads) 10 application. 1 0.1 1e-05 1e-04 0.001 0.01 Latency time ( s ). Bucket size: 5 us Peter Soetens Lock-Free Data Exchange
  • 84.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 1e+06 1ms/0.5ms 100000 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 1e+06 Latency time ( s ). Bucket size: 5 us latencies 1ms/0.5ms 100000 Lock based (top) 10000 Occurences 1000 and lock free 100 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Small application, high priority thread. Peter Soetens Lock-Free Data Exchange
  • 85.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 5ms/0.5ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 100000 Latency time ( s ). Bucket size: 5 us latencies 5ms/0.5ms 10000 Lock based (top) Occurences 1000 100 and lock-free 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Small application, low priority thread. Peter Soetens Lock-Free Data Exchange
  • 86.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 0.5ms/0.1ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 100000 Latency time ( s ). Bucket size: 5 us latencies 0.5ms/0.1ms 10000 Lock based (top) Occurences 1000 100 and lock free 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Concurrent application, high priority thread. Peter Soetens Lock-Free Data Exchange
  • 87.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 1ms/0.2ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 1e+06 Latency time ( s ). Bucket size: 5 us latencies 1ms/0.2ms 100000 Lock based (top) 10000 Occurences 1000 and lock free 100 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Concurrent application, medium priority thread. Peter Soetens Lock-Free Data Exchange
  • 88.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Data Exchange 100000 2ms/0.3ms 10000 Occurences 1000 100 10 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Communication 100000 Latency time ( s ). Bucket size: 5 us latencies 2ms/0.3ms 10000 Lock based (top) Occurences 1000 100 and lock free 10 (bottom). 1 0.1 1e-06 1e-05 1e-04 0.001 0.01 0.1 Latency time ( s ). Bucket size: 5 us Concurrent application, medium priority thread. Peter Soetens Lock-Free Data Exchange
  • 89.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Real-Time Validation: Conclusions Small Applications Lock-free performs on average better Lock-free performs worst case better Concurrent Applications Lock-free performs on average better Lock-free performs worst case better Lock-free prevents dead-line failures Peter Soetens Lock-Free Data Exchange
  • 90.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Outline 1 Introduction About You and Me Application Domain 2 Lock-Free Explained The Good ’Ol Days A New Kind of Computer Science 3 Lock-Free Performance Time determinism Algorithm Overhead Peter Soetens Lock-Free Data Exchange
  • 91.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Memory Overhead Memory consumption increases linearly OK for most real-time and embedded applications, number of threads is well known. Worse, if not catastrophic, for OS Reference counted kernels, number of threads is unknown. D D D Reference counted memory Both readers and writers need to reference count data blocks. Requires ’atomic’ processor instructions. Peter Soetens Lock-Free Data Exchange
  • 92.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Overhead for Readers Increase reference count Since a data block may not be freed before all readers are done reading it, a reference counting Reference counted implementation is required. Analogous to RCU D D D Detect moved ’Most Recent’ pointer. If a reader ’locks’ the data block but detects that the refcount is one, it must retry, since the block may be in re-use already. Peter Soetens Lock-Free Data Exchange
  • 93.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Overhead for Writers Find an empty data block Possibly race against other writers Increase reference count Copy and update the data Reference counted Large data blocks will reduce D D D performance Retry if necessary (W > 1) In case source data block changed, startover with the copy-update from the new data block. Peter Soetens Lock-Free Data Exchange
  • 94.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
  • 95.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues + Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
  • 96.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues + Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
  • 97.
    Introduction Lock-Free Explained Time determinism Lock-Free Performance Algorithm Overhead Summary Exception: Lock-free Pointer Queues + Best case access time ? = TFindpointer + TPointercopy Always the case for the highest priority thread. + Worst case access time ? = WHigherPriority ∗ TBestcase Depends on the number of higher priority writers. + Memory requirements ? = sizeof (D) ∗ Nqueue ⇒ Best of both worlds ! Independent of number of threads. Peter Soetens Lock-Free Data Exchange
  • 98.
    Introduction Lock-Free Explained Lock-Free Performance Summary Summary Lock-Free algorithms are a drastic improvement for real-time applications Lock-Free algorithms don’t require any scheduler intervention. But be aware of memory requirements. Peter Soetens Lock-Free Data Exchange
  • 99.
    Introduction Lock-FreeExplained Lock-Free Performance Summary Thank you for your attention ! Peter Soetens Lock-Free Data Exchange
  • 100.
    Introduction Lock-Free Explained Lock-Free Performance Summary References http://www.orocos.org Anderson, J., S. Ramamurthy, and K. Jeffay (1995). Real-time com- puting with lock-free shared objects. Proceedings of the 16th IEEE Real-Time Systems Symposium. Herlihy, M. (1991). Wait-free synchronization. ACM Trans. Program. Lang. Syst. 13 (1), 124-149. Herlihy, M., V. Luchangco, and M. Moir (2003). Obstruction-free synchronization: Double-ended queues as an example. In 03: Proceedings of the 23rd International Conference on Dis- tributed Computing Systems, Washington, DC, USA, pp. 522. IEEE Computer Society. Peter Soetens Lock-Free Data Exchange