Role of locking- cds

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Role of locking- cds

  1. 1. Overview  Introduction  Synchronization  Non-blocking Synchronization  Is Non-blocking Synchronization performancebeneficial for Parallel Applications?  NOBLE: A Non-blocking Synchronization Interface. How can we make non-blocking synchronization accessible to the parallel programmer?  Lock-free Skip lists  Conclusions, Future Work
  2. 2. Systems: SMP  Cache-coherent distributed shared memory multiprocessor systems:  UMA  NUMA
  3. 3. Synchronization Barriers  Locks, semaphores,… (mutual exclusion)  “A significant part of the work performed by today’s parallel applications is spent on synchronization.” ...
  4. 4. Lock-Based Synchronization: Sequential
  5. 5. Non-blocking Synchronization  Lock-Free Synchronization  Optimistic approach • Assumes it’s alone and prepares operation which later takes place (unless interfered) in one atomic step, using hardware atomic primitives • Interference is detected via shared memory • Retries until not interfered by other operations • Can cause starvation
  6. 6. Slide provided by Jim Anderson Example: Shared Queue The usual approach is to implement operations using retry loops. Here’s an example: type Qtype = record v: valtype; next: pointer to Qtype end type Qtype = record v: valtype; next: pointer to Qtype end shared var Tail: pointer to Qtype; shared var Tail: pointer to Qtype; local var old, new: pointer to Qtype local var old, new: pointer to Qtype procedure Enqueue (input: valtype) procedure Enqueue (input: valtype) new := (input, NIL); new := (input, NIL); repeat old := Tail repeat old := Tail until CAS2(&Tail, &(old->next), old, NIL, new, new) until CAS2(&Tail, &(old->next), old, NIL, new, new) old Tail new old Tail new
  7. 7. Non-blocking Synchronization  Lock-Free Synchronization  Avoids problems that locks have  Fast  Starvation?  (not in the Context of HPC) Wait-Free Synchronization  Always finishes in a finite number of its own steps. • Complex algorithms • Memory consuming • Less efficient on average than lock-free
  8. 8. Overview  Introduction  Synchronization  Non-blocking Synchronization  Is Non-blocking Synchronization performancebeneficial for Parallel Scientific Applications?  NOBLE: A Non-blocking Synchronization Interface. How can we make non-blocking synchronization accessible to the parallel programmer?  Conclusions, Future Work
  9. 9. Non-blocking Synchronisation Synchronisation:  An alternative approach for synchronisation introduced 25 years ago  Many theoretical results Evaluation:  Micro-benchmarks shows better performance than mutual exclusion in real or simulated multiprocessor systems.
  10. 10. Practice   Non-blocking synchronization is still not used in practical applications Non-blocking solutions are often  complex  having non-standard or un-clear interfaces  non-practical ? ?
  11. 11. Practice Question? ”How the performance of parallel scientific applications is affected by the use of non-blocking synchronisation rather than lock-based one?” ? ? ?
  12. 12. Answers How the performance of parallel scientific applications is affected by the use of nonblocking synchronisation rather than lockbased one?     The identification of the basic locking operations that parallel programmers use in their applications. The efficient non-blocking implementation of these synchronisation operations. The architectural implications on the design of non-blocking synchronisation. Comparison of the lock-based and lock-free versions of the respective applications
  13. 13. Applications Ocean simulates eddy currents in an ocean basin. Radiosity computes the equilibrium distribution of light in a scene using the radiosity method. Volrend renders 3D volume data into an image using a raycasting method. Water Evaluates forces and potentials that occur over time between water molecules. Spark98 a collection of sparse matrix kernels. Each kernel performs a sequence of sparse matrix vector product operations using matrices that are derived from a family of three-dimensional finite element earthquake applications.
  14. 14. Removing Locks in Applications  Many locks are “Simple Locks”.  Many critical sections contain shared floatingpoint variables.  Large critical sections.    CAS, FAA and LL/SC can be used to implement non-blocking version. Floating-point synchronization primitives are needed. A DoubleFetch-and-Add primitive was designed. Efficient Non-blocking implementations of big ADT are used.
  15. 15. Experimental Results: Speedup 58P 58P 32P 24P 24P 58P 58P
  16. 16. SPARK98 Before: spark_setlock(lockid); w[col][0] += A[Anext][0][0]*v[i][0] + A[Anext][1][0]*v[i][1] + A[Anext][2][0]*v[i][2]; w[col][1] += A[Anext][0][1]*v[i][0] + A[Anext][1][1]*v[i][1] + A[Anext][2][1]*v[i][2]; w[col][2] += A[Anext][0][2]*v[i][0] + A[Anext][1][2]*v[i][1] + A[Anext][2][2]*v[i][2]; spark_unsetlock(lockid); After: dfad(&w[col][0], A[Anext][0][0]*v[i][0] + A[Anext][1][0]*v[i][1] + A[Anext][2][0]*v[i][2]); dfad(&w[col][1], A[Anext][0][1]*v[i][0] + A[Anext][1][1]*v[i][1] + A[Anext][2][1]*v[i][2]); dfad(&w[col][2], A[Anext][0][2]*v[i][0] + A[Anext][1][2]*v[i][1] + A[Anext][2][2]*v[i][2]);
  17. 17. Overview  Introduction  Synchronization  Non-blocking Synchronization  Is Non-blocking Synchronization beneficial for Parallel Scientific Applications?  NOBLE: A Non-blocking Synchronization Interface. How can we make non-blocking synchronization accessible to the parallel programmer?  Conclusions, Future Work
  18. 18. Practice   Non-blocking synchronization is still not used in practical applications Non-blocking solutions are often  complex  having non-standard or un-clear interfaces  non-practical ? ?
  19. 19. NOBLE: Brings Non-blocking closer to Practice  Create a non-blocking inter-process communication interface with the properties:  Attractive functionality  Programmer friendly  Easy to adapt existing solutions  Efficient  Portable  Adaptable for different programming languages
  20. 20. NOBLE Design: Portable Noble.h #define NBL... #define NBL... #define NBL... Exported definitions Identical for all platforms Platform in-dependent QueueLF.c StackLF.c #include “Platform/Primitives.h” … #include “Platform/Primitives.h” … ... Platform dependent SunHardware.asm IntelHardware.asm CAS, TAS, Spin-Locks … CAS, TAS, Spin-Locks ... ...
  21. 21. Using NOBLE • First create a global variable handling the shared data object, for example a stack: • Create the stack with the appropriate implementation: Globals #include <noble.h> ... NBLStack* stack; Main stack=NBLStackCreateLF(10000); ... Threads • When some thread wants to do some operation: NBLStackPush(stack, item); or item=NBLStackPop(stack);
  22. 22. Using NOBLE Globals #include <noble.h> ... NBLStack* stack; Main  When the data structure is not in use anymore: stack=NBLStackCreateLF(10000); ... NBLStackFree(stack);
  23. 23. Using NOBLE Globals #include <noble.h> ... NBLStack* stack; • To change the synchronization mechanism, only one line of code has to be changed! Main stack=NBLStackCreateLB(); ... NBLStackFree(stack); Threads NBLStackPush(stack, item); or item=NBLStackPop(stack);
  24. 24. Design: Attractive functionality  Data structures for multi-threaded usage  FIFO Queues  Priority Queues  Dictionaries  Stacks  Singly linked lists  Snapshots  MWCAS  ...  Clear specifications
  25. 25. Status  Multiprocessor support  Sun Solaris (Sparc)  Win32 (Intel x86)  SGI (Mips)  Linux (Intel x86) Availiable for academic use: http://www.noble-library.org/
  26. 26. Did our Work have any Impact? 1) 2) 3) Industry has initialized contacts and uses a test version of NOBLE. Free-ware developers has showed interest. Interest from research organisations. NOBLE is freely availiable for research and educational purposes.
  27. 27. A Lock-Free Skip list  Presented as part of the: H. Sundell, Ph. Tsigas Fast and Lock-Free Concurrent Priority Queues for Multi-Thread Systems. 17th IEEE/ACM International Parallel and Distributed Processing Symposium (IPDPS ´03), May 2003 (TR 2002). Best Paper Award A very similar lock-free skip list algorithm will be presented this August at the ACM Symposium on Principles of Distributed Computing (PODC 2004): ”Lock-Free Linked Lists and Skip Lists” Mikhail Fomitchev, Eric Ruppert
  28. 28. Randomized Algorithm: Skip Lists  William Pugh: ”Skip Lists: A Probabilistic Alternative to Balanced Trees”, 1990  Layers of ordered lists with different densities, achieves a tree-like behavior Head Tail 1 2  Time 3 4 5 6 7 complexity: O(log2N) – probabilistic! … 25% 50%
  29. 29. Our Lock-Free Concurrent Skip List  Define node state to depend on the insertion status at lowest level as well as a deletion flag 1 3 2 1 p D 2 D  Insert  Set 3 D 4 D 5 D 6 D 7 D from lowest level going upwards deletion flag. Delete from highest level going downwards 3 2 1 p D
  30. 30. Concurrent Insert vs. Delete operations  b) 1 Problem: 2 Delete 3 Insert - both nodes are deleted!  4 a) Solution (Harris et al): Use bit 0 of pointer to mark deletion status 1 b) 2 * c) a) 3 4
  31. 31. Dynamic Memory Management Problem: System memory allocation functionality is blocking!  Solution (lock-free), IBM freelists:   Pre-allocate a number of nodes, link them into a dynamic stack structure, and allocate/reclaim using CAS Allocate Head Mem 1 Mem 2 Reclaim Used 1 © Ph. Tsigas 2003-2004 … Mem n
  32. 32. The ABA problem  Problem: Because of concurrency (pre-emption in particular), same pointer value does not always mean same node (i.e. CAS succeeds)!!! Step 1: 1 6 7 3 7 4 Step 2: 2 4 © Ph. Tsigas 2003-2004
  33. 33. The ABA problem  Solution: (Valois et al) Add reference counting to each node, in order to prevent nodes that are of interest to some thread to be reclaimed until all threads have left the node New Step 2: 1 * 6 * 1 1 CAS Failes! 2 3 ? 7 ? 4 1 © Ph. Tsigas 2003-2004 ?
  34. 34. Helping Scheme  Threads need to traverse safely 2 * 1 4 or 1  4 ? ?  2 * Need to remove marked-to-be-deleted nodes while traversing – Help! Finds previous node, finish deletion and continues traversing from previous node 1 2 * 4 © Ph. Tsigas 2003-2004
  35. 35. Overlapping operations on Insert 2 shared data 2  Example: Insert operation 1 4 - which of 2 or 3 gets inserted?  Solution: Compare-And-Swap atomic primitive: CAS(p:pointer to word, old:word, new:word):boolean atomic do if *p = old then *p := new; return true; else return false; © Ph. Tsigas 2003-2004 3 Insert 3
  36. 36. Experiments 1-30 threads on platforms with different levels of real concurrency  10000 Insert vs. DeleteMin operations by each thread. 100 vs. 1000 initial inserts  Compare with other implementations:   Lotan and Shavit, 2000  Hunt et al “An Efficient Algorithm for Concurrent Priority Queue Heaps”, 1996 © Ph. Tsigas 2003-2004
  37. 37. Full Concurrency © Ph. Tsigas 2003-2004
  38. 38. Medium Pre-emption © Ph. Tsigas 2003-2004
  39. 39. High Pre-emption © Ph. Tsigas 2003-2004
  40. 40. Lessons Learned     The Non-Blocking Synchronization Paradigm can be suitable and beneficial to large scale parallel applications. Experimental Reproducable Work. Many results claimed by simulation are not consistent with what we observed. Applications gave us nice problems to look at and do theoretical work on. (IPDPS 2003 Algorithmic Best Paper Award) NOBLE helped programmers to trust our implementations. © Ph. Tsigas 2003-2004
  41. 41. Future Work Extend NOBLE for loosely coupled systems.  Extend the set of data structures supported by NOBLE based on the needs of the applications.  Reactive-Synchronisation  © Ph. Tsigas 2003-2004
  42. 42. Questions?  Contact Information:  Address: Philippas Tsigas Computing Science Chalmers University of Technology  Email:  Web: tsigas @ cs.chalmers.se http://www.cs.chalmers.se/~tsigas http://www.cs.chalmers.se/~dcs http://www.noble-library.org © Ph. Tsigas 2003-2004
  43. 43. Pointers:        NOBLE: A Non-Blocking Inter-Process Communication Library. ACM Workshop on Languages, Compilers, and Run-time Systems for Scalable Computers (LCR ´02). Evaluating The Performance of Non-Blocking Synchronization on Shared Memory Multiprocessors. ACM SIGMETRICS 2001/Performance2001 Joint International Conference on Measurement and Modeling of Computer Systems (SIGMETRICS 2001). Integrating Non-blocking Synchronization in Parallel Applications: Performance Advantages and Methodologies. ACM Workshop on Software and Performance (WOSP ´01). A Simple, Fast and Scalable Non-Blocking Concurrent FIFO queue for Shared Memory Multiprocessor Systems, ACM Symposium on Parallel Algorithms and Architectures (SPAA ´01). Fast and Lock-Free Concurrent Priority Queues for Multi-Thread Systems. 17th IEEE/ACM International Parallel and Distributed Processing Symposium (IPDPS ´03). Fast, Reactive and Lock-free Multi-word Compare-and-swap Algorithms. 12th EEE/ACM International Conference on Parallel Architectures and Compilation Techniques (PACT ´03) Scalable and Lock-free Cuncurrent Dictionaries. Proceedings of the 19th ACM Symposium on Applied Computing (SAC ’04). © Ph. Tsigas 2003-2004

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