Chapter 4 Threads Operating System Concepts 9 th

Chapter 4: Threads n Overview n Multicore Programming n Multithreading Models n Thread Libraries

Objectives n To introduce the notion of a thread—a fundamental unit of CPU utilization

Motivation n Most modern applications are multithreaded n Threads run within application n Multiple

Multithreaded Server Architecture Operating System Concepts – 9 th Edition 4. 5 Silberschatz, Galvin

Benefits n Responsiveness – may allow continued execution if part of process is blocked,

Multicore Programming n Multicore or multiprocessor systems putting pressure on programmers, challenges include: l

Concurrency vs. Parallelism n Concurrent execution on single-core system: n Parallelism on a multi-core

Single and Multithreaded Processes Operating System Concepts – 9 th Edition 4. 9 Silberschatz,

Amdahl’s Law n Identifies performance gains from adding additional cores to an application that

User Threads and Kernel Threads n User threads - management done by user-level threads

Multithreading Models n Many-to-One n One-to-One n Many-to-Many Operating System Concepts – 9 th

Many-to-One n Many user-level threads mapped to single kernel thread n One thread blocking

One-to-One n Each user-level thread maps to kernel thread n Creating a user-level thread

Many-to-Many Model n Allows many user level threads to be mapped to many kernel

Two-level Model n Similar to M: M, except that it allows a user thread

Thread Libraries n Thread library provides programmer with API for creating and managing threads

Pthreads n May be provided either as user-level or kernel-level n A POSIX standard

Pthreads Example Operating System Concepts – 9 th Edition 4. 19 Silberschatz, Galvin and

Pthreads Example (Cont. ) Operating System Concepts – 9 th Edition 4. 20 Silberschatz,

Pthreads Code for Joining 10 Threads Operating System Concepts – 9 th Edition 4.

Win 32 API Multithreaded C Program Operating System Concepts – 9 th Edition 4.

Win 32 API Multithreaded C Program (Cont. ) Operating System Concepts – 9 th

Java Threads n Java threads are managed by the JVM n Typically implemented using

Java Multithreaded Program Operating System Concepts – 9 th Edition 4. 25 Silberschatz, Galvin

Java Multithreaded Program (Cont. ) Operating System Concepts – 9 th Edition 4. 26

Implicit Threading n Growing in popularity as numbers of threads increase, program correctness more

Thread Pools n Create a number of threads in a pool where they await

Open. MP n Set of compiler directives and an API for C, C++, FORTRAN

Grand Central Dispatch n Apple technology for Mac OS X and i. OS operating

Threading Issues n Semantics of fork() and exec() system calls n Signal handling l

Semantics of fork() and exec() n Does fork()duplicate only the calling thread or all

Signal Handling n Signals are used in UNIX systems to notify a process that

Thread Cancellation n Terminating a thread before it has finished n Thread to be

Thread Cancellation (Cont. ) n Invoking thread cancellation requests cancellation, but actual cancellation depends

Thread-Local Storage n Thread-local storage (TLS) allows each thread to have its own copy

Scheduler Activations n Both M: M and Two-level models require communication to maintain the

Operating System Examples n Windows XP Threads n Linux Thread Operating System Concepts –

Windows Threads n Windows implements the Windows API – primary API for Win 98,

Windows XP Threads Data Structures Operating System Concepts – 9 th Edition 4. 40

Linux Threads n Linux refers to them as tasks rather than threads n Thread

End of Chapter 4 Operating System Concepts – 9 th Edition Silberschatz, Galvin and

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Chapter 4: Threads n Overview n Multicore Programming n Multithreading Models n Thread Libraries n Implicit Threading n Threading Issues n Operating System Examples Operating System Concepts – 9 th Edition 4. 2 Silberschatz, Galvin and Gagne © 2013

Objectives n To introduce the notion of a thread—a fundamental unit of CPU utilization that forms the basis of multithreaded computer systems n To discuss the APIs for the Pthreads, Windows, and Java thread libraries n To explore several strategies that provide implicit threading n To examine issues related to multithreaded programming n To cover operating system support for threads in Windows and Linux Operating System Concepts – 9 th Edition 4. 3 Silberschatz, Galvin and Gagne © 2013

Motivation n Most modern applications are multithreaded n Threads run within application n Multiple tasks with the application can be implemented by separate threads l Update display l Fetch data l Spell checking l Answer a network request n Process creation is heavy-weight while thread creation is light-weight n Can simplify code, increase efficiency n Kernels are generally multithreaded Operating System Concepts – 9 th Edition 4. 4 Silberschatz, Galvin and Gagne © 2013

Benefits n Responsiveness – may allow continued execution if part of process is blocked, especially important for user interfaces n Resource Sharing – threads share resources of process, easier than shared memory or message passing n Economy – cheaper than process creation, thread switching lower overhead than context switching n Scalability – process can take advantage of multiprocessor architectures Operating System Concepts – 9 th Edition 4. 6 Silberschatz, Galvin and Gagne © 2013

Multicore Programming n Multicore or multiprocessor systems putting pressure on programmers, challenges include: l Dividing activities l Balance l Data splitting l Data dependency l Testing and debugging n Parallelism implies a system can perform more than one task simultaneously n Concurrency supports more than one task making progress l n n Single processor / core, scheduler providing concurrency Types of parallelism l Data parallelism – distributes subsets of the same data across multiple cores, same operation on each l Task parallelism – distributing threads across cores, each thread performing unique operation As # of threads grows, so does architectural support for threading l CPUs have cores as well as hardware threads l Consider Oracle SPARC T 4 with 8 cores, and 8 hardware threads per core Operating System Concepts – 9 th Edition 4. 7 Silberschatz, Galvin and Gagne © 2013

Amdahl’s Law n Identifies performance gains from adding additional cores to an application that has both serial and parallel components n S is serial portion n N processing cores n I. e. if application is 75% parallel / 25% serial, moving from 1 to 2 cores results in speedup of 1. 6 times n As N approaches infinity, speedup approaches 1 / S Serial portion of an application has disproportionate effect on performance gained by adding additional cores n But does the law take into account contemporary multicore systems? Operating System Concepts – 9 th Edition 4. 10 Silberschatz, Galvin and Gagne © 2013

User Threads and Kernel Threads n User threads - management done by user-level threads library n Three primary thread libraries: l POSIX Pthreads l Win 32 threads l Java threads n Kernel threads - Supported by the Kernel n Examples – virtually all general purpose operating systems, including: l Windows l Solaris l Linux l Tru 64 UNIX l Mac OS X Operating System Concepts – 9 th Edition 4. 11 Silberschatz, Galvin and Gagne © 2013

Many-to-One n Many user-level threads mapped to single kernel thread n One thread blocking causes all to block n Multiple threads may not run in parallel on muticore system because only one may be in kernel at a time n Few systems currently use this model n Examples: l Solaris Green Threads l GNU Portable Threads Operating System Concepts – 9 th Edition 4. 13 Silberschatz, Galvin and Gagne © 2013

One-to-One n Each user-level thread maps to kernel thread n Creating a user-level thread creates a kernel thread n More concurrency than many-to-one n Number of threads per process sometimes restricted due to overhead n Examples l Windows NT/XP/2000 l Linux l Solaris 9 and later Operating System Concepts – 9 th Edition 4. 14 Silberschatz, Galvin and Gagne © 2013

Many-to-Many Model n Allows many user level threads to be mapped to many kernel threads n Allows the operating system to create a sufficient number of kernel threads n Solaris prior to version 9 n Windows NT/2000 with the Thread. Fiber package Operating System Concepts – 9 th Edition 4. 15 Silberschatz, Galvin and Gagne © 2013

Two-level Model n Similar to M: M, except that it allows a user thread to be bound to kernel thread n Examples l IRIX l HP-UX l Tru 64 UNIX l Solaris 8 and earlier Operating System Concepts – 9 th Edition 4. 16 Silberschatz, Galvin and Gagne © 2013

Thread Libraries n Thread library provides programmer with API for creating and managing threads n Two primary ways of implementing l Library entirely in user space l Kernel-level library supported by the OS Operating System Concepts – 9 th Edition 4. 17 Silberschatz, Galvin and Gagne © 2013

Pthreads n May be provided either as user-level or kernel-level n A POSIX standard (IEEE 1003. 1 c) API for thread creation and synchronization n Specification, not implementation n API specifies behavior of the thread library, implementation is up to development of the library n Common in UNIX operating systems (Solaris, Linux, Mac OS X) Operating System Concepts – 9 th Edition 4. 18 Silberschatz, Galvin and Gagne © 2013

Java Threads n Java threads are managed by the JVM n Typically implemented using the threads model provided by underlying OS n Java threads may be created by: l Extending Thread class l Implementing the Runnable interface Operating System Concepts – 9 th Edition 4. 24 Silberschatz, Galvin and Gagne © 2013

Implicit Threading n Growing in popularity as numbers of threads increase, program correctness more difficult with explicit threads n Creation and management of threads done by compilers and run-time libraries rather than programmers n Three methods explored n l Thread Pools l Open. MP l Grand Central Dispatch Other methods include Microsoft Threading Building Blocks (TBB), java. util. concurrent package Operating System Concepts – 9 th Edition 4. 27 Silberschatz, Galvin and Gagne © 2013

Thread Pools n Create a number of threads in a pool where they await work n Advantages: l Usually slightly faster to service a request with an existing thread than create a new thread l Allows the number of threads in the application(s) to be bound to the size of the pool l Separating task to be performed from mechanics of creating task allows different strategies for running task 4 n i. e. Tasks could be scheduled to run periodically Windows API supports thread pools: Operating System Concepts – 9 th Edition 4. 28 Silberschatz, Galvin and Gagne © 2013

Open. MP n Set of compiler directives and an API for C, C++, FORTRAN n Provides support for parallel programming in shared-memory environments n Identifies parallel regions – blocks of code that can run in parallel #pragma omp parallel Create as many threads as there are cores #pragma omp parallel for(i=0; i<N; i++) { c[i] = a[i] + b[i]; } Run for loop in parallel Operating System Concepts – 9 th Edition 4. 29 Silberschatz, Galvin and Gagne © 2013

Grand Central Dispatch n Apple technology for Mac OS X and i. OS operating systems n Extensions to C, C++ languages, API, and run-time library n Allows identification of parallel sections n Manages most of the details of threading n Block is in “^{ }” - ˆ{ printf("I am a block"); } n Blocks placed in dispatch queue l n Assigned to available thread in thread pool when removed from queue Two types of dispatch queues: l serial – blocks removed in FIFO order, queue is per process, called main queue 4 l Programmers can create additional serial queues within program concurrent – removed in FIFO order but several may be removed at a time 4 Three system wide queues with priorities low, default, high Operating System Concepts – 9 th Edition 4. 30 Silberschatz, Galvin and Gagne © 2013

Threading Issues n Semantics of fork() and exec() system calls n Signal handling l n Synchronous and asynchronous Thread cancellation of target thread l Asynchronous or deferred n Thread-local storage n Scheduler Activations Operating System Concepts – 9 th Edition 4. 31 Silberschatz, Galvin and Gagne © 2013

Semantics of fork() and exec() n Does fork()duplicate only the calling thread or all threads? l n Some UNIXes have two versions of fork Exec() usually works as normal – replace the running process including all threads Operating System Concepts – 9 th Edition 4. 32 Silberschatz, Galvin and Gagne © 2013

Signal Handling n Signals are used in UNIX systems to notify a process that a particular event has occurred. n A signal handler is used to process signals n n 1. Signal is generated by particular event 2. Signal is delivered to a process 3. Signal is handled by one of two signal handlers: 1. default 2. user-defined Every signal has default handler that kernel runs when handling signal l User-defined signal handler can override default l For single-threaded, signal delivered to process Where should a signal be delivered for multi-threaded? l Deliver the signal to the thread to which the signal applies l Deliver the signal to every thread in the process l Deliver the signal to certain threads in the process l Assign a specific thread to receive all signals for the process Operating System Concepts – 9 th Edition 4. 33 Silberschatz, Galvin and Gagne © 2013

Thread Cancellation n Terminating a thread before it has finished n Thread to be canceled is target thread n Two general approaches: n l Asynchronous cancellation terminates the target thread immediately l Deferred cancellation allows the target thread to periodically check if it should be cancelled Pthread code to create and cancel a thread: Operating System Concepts – 9 th Edition 4. 34 Silberschatz, Galvin and Gagne © 2013

Thread Cancellation (Cont. ) n Invoking thread cancellation requests cancellation, but actual cancellation depends on thread state n If thread has cancellation disabled, cancellation remains pending until thread enables it n Default type is deferred l n Cancellation only occurs when thread reaches cancellation point 4 I. e. pthread_testcancel() 4 Then cleanup handler is invoked On Linux systems, thread cancellation is handled through signals Operating System Concepts – 9 th Edition 4. 35 Silberschatz, Galvin and Gagne © 2013

Thread-Local Storage n Thread-local storage (TLS) allows each thread to have its own copy of data n Useful when you do not have control over the thread creation process (i. e. , when using a thread pool) n Different from local variables n l Local variables visible only during single function invocation l TLS visible across function invocations Similar to static data l TLS is unique to each thread Operating System Concepts – 9 th Edition 4. 36 Silberschatz, Galvin and Gagne © 2013

Scheduler Activations n Both M: M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application n Typically use an intermediate data structure between user and kernel threads – lightweight process (LWP) l Appears to be a virtual processor on which process can schedule user thread to run l Each LWP attached to kernel thread l How many LWPs to create? n Scheduler activations provide upcalls - a communication mechanism from the kernel to the upcall handler in the thread library n This communication allows an application to maintain the correct number kernel threads Operating System Concepts – 9 th Edition 4. 37 Silberschatz, Galvin and Gagne © 2013

Windows Threads n Windows implements the Windows API – primary API for Win 98, Win NT, Win 2000, Win XP, and Win 7 n Implements the one-to-one mapping, kernel-level n Each thread contains l A thread id l Register set representing state of processor l Separate user and kernel stacks for when thread runs in user mode or kernel mode l Private data storage area used by run-time libraries and dynamic link libraries (DLLs) n The register set, stacks, and private storage area are known as the context of the thread n The primary data structures of a thread include: l ETHREAD (executive thread block) – includes pointer to process to which thread belongs and to KTHREAD, in kernel space l KTHREAD (kernel thread block) – scheduling and synchronization info, kernel-mode stack, pointer to TEB, in kernel space l TEB (thread environment block) – thread id, user-mode stack, thread-local storage, in user space Operating System Concepts – 9 th Edition 4. 39 Silberschatz, Galvin and Gagne © 2013

Linux Threads n Linux refers to them as tasks rather than threads n Thread creation is done through clone() system call n clone() allows a child task to share the address space of the parent task (process) l n Flags control behavior struct task_struct points to process data structures (shared or unique) Operating System Concepts – 9 th Edition 4. 41 Silberschatz, Galvin and Gagne © 2013