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Operating System 4
 

Operating System 4

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    Operating System 4 Operating System 4 Presentation Transcript

    • Chapter 4: Threads
    • What’s in a process?
      • A process consists of (at least):
        • an address space
        • the code for the running program
        • the data for the running program
        • an execution stack and stack pointer (SP)
          • traces state of procedure calls made
        • the program counter (PC), indicating the next instruction
        • a set of general-purpose processor registers and their values
        • a set of OS resources
          • open files, network connections, sound channels, …
    • Concurrency
      • Imagine a web server, which might like to handle multiple requests concurrently
        • While waiting for the credit card server to approve a purchase for one client, it could be retrieving the data requested by another client from disk, and assembling the response for a third client from cached information
      • Imagine a web client (browser), which might like to initiate multiple requests concurrently
        • The IT home page has 10 “src= …” html commands, each of which is going to involve a lot of sitting around! Wouldn’t it be nice to be able to launch these requests concurrently?
      • Imagine a parallel program running on a multiprocessor, which might like to employ “physical concurrency”
        • For example, multiplying a large matrix – split the output matrix into k regions and compute the entries in each region concurrently using k processors
    • What’s needed?
      • In each of these examples of concurrency (web server, web client, parallel program):
        • Everybody wants to run the same code
        • Everybody wants to access the same data
        • Everybody has the same privileges (most of the time)
        • Everybody uses the same resources (open files, network connections, etc.)
      • But you’d like to have multiple hardware execution states:
        • an execution stack and stack pointer (SP)
          • traces state of procedure calls made
        • the program counter (PC), indicating the next instruction
        • a set of general-purpose processor registers and their values
    • How could we achieve this?
      • Given the process abstraction as we know it:
        • fork several processes
        • cause each to map to the same physical memory to share data
      • This is really inefficient!!
        • space: PCB, page tables, etc.
        • time: creating OS structures, fork and copy address space, etc.
      • So any support that the OS can give for doing multi-threaded programming is a win
    • Can we do better?
      • Key idea:
        • separate the concept of a process (address space, etc.)
        • … from that of a minimal “ thread of control ” (execution state: PC, etc.)
      • This execution state is usually called a thread , or sometimes, a lightweight process
    • Single-Threaded Example
      • Imagine the following C program:
      • main() {
      • ComputePI(“pi.txt”);
      • PrintClassList(“clist.text”);
      • }
      • What is the behavior here?
        • Program would never print out class list
        • Why? ComputePI would never finish
    • Use of Threads
      • Version of program with Threads:
      • main() {
      • CreateThread(ComputePI(“pi.txt”));
      • CreateThread(PrintClassList(“clist.text”));
      • }
      • What does “CreateThread” do?
        • Start independent thread running for a given procedure
      • What is the behavior here?
        • Now, you would actually see the class list
        • This should behave as if there are two separate CPUs
      CPU1 CPU2 CPU1 CPU2 Time CPU1 CPU2
    • Threads and processes
      • Most modern OS’s (NT, modern UNIX, etc) therefore support two entities:
        • the process , which defines the address space and general process attributes (such as open files, etc.)
        • the thread , which defines a sequential execution stream within a process
      • A thread is bound to a single process / address space
        • address spaces, however, can have multiple threads executing within them
        • sharing data between threads is cheap: all see the same address space
        • creating threads is cheap too!
    • Single and Multithreaded Processes
    • Benefits
      • Responsiveness - Interactive applications can be performing two tasks at the same time (rendering, spell checking)
      • Resource Sharing - Sharing resources between threads is easy
      • Economy - Resource allocation between threads is fast (no protection issues)
      • Utilization of MP Architectures - seamlessly assign multiple threads to multiple processors (if available). Future appears to be multi-core anyway.
    • Thread Design Space address space thread one thread/process many processes many threads/process many processes one thread/process one process many threads/process one process MS/DOS Java older UNIXes Mach, NT, Chorus, Linux, …
    • (old) Process address space 0x00000000 0x7FFFFFFF address space code (text segment) static data (data segment) heap (dynamic allocated mem) stack (dynamic allocated mem) PC SP
    • (new) Address space with threads 0x00000000 0x7FFFFFFF address space code (text segment) static data (data segment) heap (dynamic allocated mem) thread 1 stack PC (T2) SP (T2) thread 2 stack thread 3 stack SP (T1) SP (T3) PC (T1) PC (T3) SP PC
    • Types of Threads
    • Thread types
      • User threads : thread management done by user-level threads library. Kernel does not know about these threads
      • Kernel threads : Supported by the Kernel and so more overhead than user threads
        • Examples: Windows XP/2000, Solaris, Linux, Mac OS X
      • User threads map into kernel threads
    • Multithreading Models
      • Many-to-One: Many user-level threads mapped to single kernel thread
        • If a thread blocks inside kernel, all the other threads cannot run
        • Examples: Solaris Green Threads, GNU Pthreads
    • Multithreading Models
      • One-to-One: Each user-level thread maps to kernel thread
    • Multithreading Models
      • Many-to-Many: Allows many user level threads to be mapped to many kernel threads
        • Allows the operating system to create a sufficient number of kernel threads
    • Two-level Model
      • Similar to M:M, except that it allows a user thread to be bound to kernel thread
      • Examples
        • IRIX
        • HP-UX
        • Tru64 UNIX
        • Solaris 8 and earlier
    • Threads Implementation
      • Two ways:
        • Provide library entirely in user space with no kernel support.
          • Invoking function in the API ->local function call
        • Kernel-level library supported by OS
          • Invoking function in the API -> system call
      • Three primary thread libraries:
        • POSIX Pthreads (maybe KL or UL)
        • Win32 threads (KL)
        • Java threads (UL)
    • Threading Issues
    • Threading Issues
      • Semantics of fork() and exec() system calls
      • Thread cancellation
      • Signal handling
      • Thread pools
      • Thread specific data
      • Scheduler activations
    • Semantics of fork() and exec()
      • Does fork() duplicate only the calling thread or all threads?
    • Thread Cancellation
      • Terminating a thread before it has finished
      • Two general approaches:
        • Asynchronous cancellation terminates the target thread immediately
        • Deferred cancellation allows the target thread to periodically check if it should be cancelled
    • Signal Handling
      • Signals are used in UNIX systems to notify a process that a particular event has occurred
      • A signal handler is used to process signals
        • Signal is generated by particular event
        • Signal is delivered to a process
        • Signal is handled
      • Every signal maybe handled by either:
        • A default signal handler
        • A user-defined signal handler
    • Signal Handling
      • In Multi-threaded programs, we have the following options:
        • Deliver the signal to the thread to which the signal applies (e.g. synchronous signals)
        • Deliver the signal to every thread in the process (e.g. terminate a process)
        • Deliver the signal to certain threads in the process
        • Assign a specific thread to receive all signals for the process
      • In *nix: Kill –signal pid (for process), pthread_kill tid (for threads)
    • Thread Pools
      • Do you remember the multithreading scenario in a web server? It has two problems:
        • Time required to create the thread
        • No bound on the number of threads
    • Thread Pools
      • Create a number of threads in a pool where they await work
      • Advantages:
        • Usually slightly faster to service a request with an existing thread than create a new thread
        • Allows the number of threads in the application(s) to be bound to the size of the pool
    • Thread Specific Data
      • Allows each thread to have its own copy of data
      • Useful when you do not have control over the thread creation process (i.e., when using a thread pool)
    • Scheduler Activations
      • Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application
      • Use intermediate data structure called LWP (lightweight process)
        • CPU Bound -> one LWP
        • I/O Bound -> Multiple LWP
    • Scheduler Activations
      • Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library
      • This communication allows an application to maintain the correct of number kernel threads
    • OS Examples
    • Windows XP Threads
      • Implements the one-to-one mapping
      • Each thread contains
        • A thread id
        • Register set
        • Separate user and kernel stacks
        • Private data storage area
    • Linux Threads
      • Linux refers to them as tasks rather than threads
      • Thread creation is done through clone() system call
      • clone() allows a child task to share the address space of the parent task (process)
    • Conclusion
      • Kernel threads:
        • More robust than user-level threads
        • Allow impersonation
        • Easier to tune the OS CPU scheduler to handle multiple threads in a process
        • A thread doing a wait on a kernel resource (like I/O) does not stop the process from running
      • User-level threads
        • A lot faster if programmed correctly
        • Can be better tuned for the exact application
      • Note that user-level threads can be done on any OS
    • Conclusion
      • Each thread shares everything with all the other threads in the process
        • They can read/write the exact same variables, so they need to synchronize themselves
        • They can access each other’s runtime stack, so be very careful if you communicate using runtime stack variables
        • Each thread should be able to retrieve a unique thread id that it can use to access thread local storage
      • Multi-threading is great, but use it wisely