Table of Contents 1. Abstract 2. Pthreads Overview 1. What is a Thread? 2. What are Pthreads? 3. Why Pthreads? 4. Designing Threaded Programs 3. The Pthreads API 4. Compiling Threaded Programs 5. Thread Management 1. Creating and Terminating Threads 2. Passing Arguments to Threads 3. Joining and Detaching Threads 4. Stack Management 5. Miscellaneous Routines 6. Mutex Variables 1. Mutex Variables Overview 2. Creating and Destroying Mutexes 3. Locking and Unlocking Mutexes 7. Condition Variables 1. Condition Variables Overview 2. Creating and Destroying Condition Variables 3. Waiting and Signaling on Condition Variables 8. LLNL Specific Information and Recommendations 9. Topics Not Covered 10. Pthread Library Routines Reference 11. References and More Information 12. Exercise In shared memory multiprocessor architectures, such as SMPs, threads can be used to implement parallelism. Historically, hardware vendors have implemented their own proprietary versions of threads, making portability a concern for software developers. For UNIX systems, a standardized C language threads programming interface has been specified by the IEEE POSIX 1003.1c standard. Implementations that adhere to this standard are referred to as POSIX threads, or Pthreads. The tutorial begins with an introduction to concepts, motivations, and design considerations for using Pthreads. Each of the three major classes of routines in the Pthreads API are then covered: Thread Management, Mutex Variables, and Condition Variables. Example codes are used throughout to demonstrate how to use most of the Pthreads routines needed by a new Pthreads programmer. The tutorial concludes with a discussion of LLNL specifics and how to mix MPI with pthreads. A lab exercise, with numerous example codes (C Language) is also included. Level/Prerequisites: Ideal for those who are new to parallel programming with threads. A basic understanding of parallel programming in C is assumed. For those who are unfamiliar with Parallel Programming in general, the material covered in EC3500: Introduction To Parallel Computing would be helpful. What is a Thread? Technically, a thread is defined as an independent stream of instructions that can be scheduled to run as such by the operating system. But what does this mean? To the software developer, the concept of a "procedure" that runs independently from its main program may best describe a thread. To go one step further, imagine a main program (a.out) that contains a number of procedures. Then imagine all of these procedures being able to be scheduled to run simultaneously and/or independently by the operating system. That would describe a "multi-threaded" program. How is this accomplished? Before understanding a thread, one first needs to understand a UNIX process. A process is created by the operating system, and requires a fair amount of "overhead". Processes contain information about program resources and program execution state, including: Process ID, process group ID, user ID, and group ID Environment Working directory. Program instructions POSIX Threads Programming Tutorials | Exercises | Abstracts | LC Workshops | Comments | Search | Privacy & Legal Notice Blaise Barney, Lawrence Livermore National Laboratory UCRL-MI-133316 Abstract Pthreads Overview Page 1 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ Registers Stack Heap File descriptors Signal actions Shared libraries Inter-process communication tools (such as message queues, pipes, semaphores, or shared memory). Threads use and exist within these process resources, yet are able to be scheduled by the operating system and run as independent entities largely because they duplicate only the bare essential resources that enable them to exist as executable code. This independent flow of control is accomplished because a thread maintains its own: Stack pointer Registers Scheduling properties (such as policy or priority) Set of pending and blocked signals Thread specific data. So, in summary, in the UNIX environment a thread: Exists within a process and uses the process resources Has its own independent flow of control as long as its parent process exists and the OS supports it Duplicates only the essential resources it needs to be independently schedulable May share the process resources with other threads that act equally independently (and dependently) Dies if the parent process dies - or something similar Is "lightweight" because most of the overhead has already been accomplished through the creation of its process. Because threads within the same process share resources: Changes made by one thread to shared system resources (such as closing a file) will be seen by all other threads. Two pointers having the same value point to the same data. Reading and writing to the same memory locations is possible, and therefore requires explicit synchronization by the programmer. What are Pthreads? Historically, hardware vendors have implemented their own proprietary versions of threads. These implementations differed substantially from each other making it difficult for programmers to develop portable threaded applications. In order to take full advantage of the capabilities provided by threads, a standardized programming interface was required. For UNIX systems, this interface has been specified by the IEEE POSIX 1003.1c standard (1995). Implementations adhering to this standard are referred to as POSIX threads, or Pthreads. Most hardware vendors now offer Pthreads in addition to their proprietary API's. The POSIX standard has continued to evolve and undergo revisions, including the Pthreads specification. The latest version is known as IEEE Std 1003.1, 2004 Edition. Some useful links: POSIX FAQs: www.opengroup.org/austin/papers/posix_faq.html Download the Standard: www.unix.org/version3/ieee_std.html Pthreads are defined as a set of C language programming types and procedure calls, implemented with a pthread.h header/include file and a thread library - though this library may be part of another library, such as libc, in some implementations. UNIX PROCESS THREADS WITHIN A UNIX PROCESS Pthreads Overview Page 2 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ Why Pthreads? The primary motivation for using Pthreads is to realize potential program performance gains. When compared to the cost of creating and managing a process, a thread can be created with much less operating system overhead. Managing threads requires fewer system resources than managing processes. For example, the following table compares timing results for the fork() subroutine and the pthreads_create() subroutine. Timings reflect 50,000 process/thread creations, were performed with the time utility, and units are in seconds, no optimization flags. Note: don't expect the sytem and user times to add up to real time, because these are SMP systems with multiple CPUs working on the problem at the same time. At best, these are approximations run on local machines, past and present. fork_vs_thread.txt All threads within a process share the same address space. Inter-thread communication is more efficient and in many cases, easier to use than interprocess communication. Threaded applications offer potential performance gains and practical advantages over non-threaded applications in several other ways: Overlapping CPU work with I/O: For example, a program may have sections where it is performing a long I/O operation. While one thread is waiting for an I/O system call to complete, CPU intensive work can be performed by other threads. Priority/real-time scheduling: tasks which are more important can be scheduled to supersede or interrupt lower priority tasks. Asynchronous event handling: tasks which service events of indeterminate frequency and duration can be interleaved. For example, a web server can both transfer data from previous requests and manage the arrival of new requests. The primary motivation for considering the use of Pthreads on an SMP architecture is to achieve optimum performance. In particular, if an application is using MPI for on-node communications, there is a potential that performance could be greatly improved by using Pthreads for onnode data transfer instead. For example: MPI libraries usually implement on-node task communication via shared memory, which involves at least one memory copy operation (process to process). For Pthreads there is no intermediate memory copy required because threads share the same address space within a single process. There is no data transfer, per se. It becomes more of a cache-to-CPU or memory-to-CPU bandwidth (worst case) situation. These speeds are much higher. Some local comparisons are shown below: Designing Threaded Programs Parallel Programming: Pthreads Overview Platform fork() pthread_create() real user sys real user sys AMD 2.3 GHz Opteron (16cpus/node) 12.5 1.0 12.5 1.2 0.2 1.3 AMD 2.4 GHz Opteron (8cpus/node) 17.6 2.2 15.7 1.4 0.3 1.3 IBM 4.0 GHz POWER6 (8cpus/node) 9.5 0.6 8.8 1.6 0.1 0.4 IBM 1.9 GHz POWER5 p5-575 (8cpus/node) 64.2 30.7 27.6 1.7 0.6 1.1 IBM 1.5 GHz POWER4 (8cpus/node) 104.5 48.6 47.2 2.1 1.0 1.5 INTEL 2.4 GHz Xeon (2 cpus/node) 54.9 1.5 20.8 1.6 0.7 0.9 INTEL 1.4 GHz Itanium2 (4 cpus/node) 54.5 1.1 22.2 2.0 1.2 0.6 Platform MPI Shared Memory Bandwidth (GB/sec) Pthreads Worst Case Memory-to-CPU Bandwidth (GB/sec) AMD 2.3 GHz Opteron 1.8 5.3 AMD 2.4 GHz Opteron 1.2 5.3 IBM 1.9 GHz POWER5 p5-575 4.1 16 IBM 1.5 GHz POWER4 2.1 4 Intel 2.4 GHz Xeon 0.3 4.3 Intel 1.4 GHz Itanium 2 1.8 6.4 Pthreads Overview Page 3 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ On modern, multi-cpu machines, pthreads are ideally suited for parallel programming, and whatever applies to parallel programming in general, applies to parallel pthreads programs. There are many considerations for designing parallel programs, such as: What type of parallel programming model to use? Problem partitioning Load balancing Communications Data dependencies Synchronization and race conditions Memory issues I/O issues Program complexity Programmer effort/costs/time ... Covering these topics is beyond the scope of this tutorial, however interested readers can obtain a quick overview in the Introduction to Parallel Computing tutorial. In general though, in order for a program to take advantage of Pthreads, it must be able to be organized into discrete, independent tasks which can execute concurrently. For example, if routine1 and routine2 can be interchanged, interleaved and/or overlapped in real time, they are candidates for threading. Programs having the following characteristics may be well suited for pthreads: Work that can be executed, or data that can be operated on, by multiple tasks simultaneously Block for potentially long I/O waits Use many CPU cycles in some places but not others Must respond to asynchronous events Some work is more important than other work (priority interrupts) Pthreads can also be used for serial applications, to emulate parallel execution. A perfect example is the typical web browser, which for most people, runs on a single cpu desktop/laptop machine. Many things can "appear" to be happening at the same time. Several common models for threaded programs exist: Manager/worker: a single thread, the manager assigns work to other threads, the workers. Typically, the manager handles all input and parcels out work to the other tasks. At least two forms of the manager/worker model are common: static worker pool and dynamic worker pool. Pipeline: a task is broken into a series of suboperations, each of which is handled in series, but concurrently, by a different thread. An automobile assembly line best describes this model. Peer: similar to the manager/worker model, but after the main thread creates other threads, it participates in the work. Shared Memory Model: All threads have access to the same global, shared memory Threads also have their own private data Programmers are responsible for synchronizing access (protecting) globally shared data. Page 4 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ Thread-safeness: Thread-safeness: in a nutshell, refers an application's ability to execute multiple threads simultaneously without "clobbering" shared data or creating "race" conditions. For example, suppose that your application creates several threads, each of which makes a call to the same library routine: This library routine accesses/modifies a global structure or location in memory. As each thread calls this routine it is possible that they may try to modify this global structure/memory location at the same time. If the routine does not employ some sort of synchronization constructs to prevent data corruption, then it is not thread-safe. The implication to users of external library routines is that if you aren't 100% certain the routine is thread-safe, then you take your chances with problems that could arise. Recommendation: Be careful if your application uses libraries or other objects that don't explicitly guarantee thread-safeness. When in doubt, assume that they are not thread-safe until proven otherwise. This can be done by "serializing" the calls to the uncertain routine, etc. The original Pthreads API was defined in the ANSI/IEEE POSIX 1003.1 - 1995 standard. The POSIX standard has continued to evolve and undergo revisions, including the Pthreads specification. The latest version is known as IEEE Std 1003.1, 2004 Edition. Copies of the standard can be purchased from IEEE or downloaded for free from www.unix.org/version3/ieee_std.html. The subroutines which comprise the Pthreads API can be informally grouped into four major groups: 1. Thread management: Routines that work directly on threads - creating, detaching, joining, etc. They also include functions to set/query thread attributes (joinable, scheduling etc.) 2. Mutexes: Routines that deal with synchronization, called a "mutex", which is an abbreviation for "mutual exclusion". Mutex functions provide for creating, destroying, locking and unlocking mutexes. These are supplemented by mutex attribute functions that set or modify The Pthreads API Page 5 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ attributes associated with mutexes. 3. Condition variables: Routines that address communications between threads that share a mutex. Based upon programmer specified conditions. This group includes functions to create, destroy, wait and signal based upon specified variable values. Functions to set/query condition variable attributes are also included. 4. Synchronization: Routines that manage read/write locks and barriers. Naming conventions: All identifiers in the threads library begin with pthread_. Some examples are shown below. The concept of opaque objects pervades the design of the API. The basic calls work to create or modify opaque objects - the opaque objects can be modified by calls to attribute functions, which deal with opaque attributes. The Pthreads API contains around 100 subroutines. This tutorial will focus on a subset of these - specifically, those which are most likely to be immediately useful to the beginning Pthreads programmer. For portability, the pthread.h header file should be included in each source file using the Pthreads library. The current POSIX standard is defined only for the C language. Fortran programmers can use wrappers around C function calls. Some Fortran compilers (like IBM AIX Fortran) may provide a Fortram pthreads API. A number of excellent books about Pthreads are available. Several of these are listed in the References section of this tutorial. Several examples of compile commands used for pthreads codes are listed in the table below. Creating and Terminating Threads Routines: Routine Prefix Functional Group pthread_ Threads themselves and miscellaneous subroutines pthread_attr_ Thread attributes objects pthread_mutex_ Mutexes pthread_mutexattr_ Mutex attributes objects. pthread_cond_ Condition variables pthread_condattr_ Condition attributes objects pthread_key_ Thread-specific data keys pthread_rwlock_ Read/write locks pthread_barrier_ Synchronization barriers Compiling Threaded Programs Compiler / Platform Compiler Command Description IBM AIX xlc_r / cc_r C (ANSI / non-ANSI) xlC_r C++ xlf_r -qnosave xlf90_r -qnosave Fortran - using IBM's Pthreads API (non-portable) INTEL Linux icc -pthread C icpc -pthread C++ PathScale Linux pathcc -pthread C pathCC -pthread C++ PGI Linux pgcc -lpthread C pgCC -lpthread C++ GNU Linux, AIX gcc -pthread GNU C g++ -pthread GNU C++ Thread Management pthread_create (thread,attr,start_routine,arg) pthread_exit (status) Page 6 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ Creating Threads: Initially, your main() program comprises a single, default thread. All other threads must be explicitly created by the programmer. pthread_create creates a new thread and makes it executable. This routine can be called any number of times from anywhere within your code. pthread_create arguments: thread: An opaque, unique identifier for the new thread returned by the subroutine. attr: An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values. start_routine: the C routine that the thread will execute once it is created. arg: A single argument that may be passed to start_routine. It must be passed by reference as a pointer cast of type void. NULL may be used if no argument is to be passed. The maximum number of threads that may be created by a process is implementation dependent. Once created, threads are peers, and may create other threads. There is no implied hierarchy or dependency between threads. Thread Attributes: By default, a thread is created with certain attributes. Some of these attributes can be changed by the programmer via the thread attribute object. pthread_attr_init and pthread_attr_destroy are used to initialize/destroy the thread attribute object. Other routines are then used to query/set specific attributes in the thread attribute object. Some of these attributes will be discussed later. Terminating Threads: There are several ways in which a Pthread may be terminated: The thread returns from its starting routine (the main routine for the initial thread). The thread makes a call to the pthread_exit subroutine (covered below). The thread is canceled by another thread via the pthread_cancel routine (not covered here). The entire process is terminated due to a call to either the exec or exit subroutines. pthread_exit is used to explicitly exit a thread. Typically, the pthread_exit() routine is called after a thread has completed its work and is no longer required to exist. If main() finishes before the threads it has created, and exits with pthread_exit(), the other threads will continue to execute. Otherwise, they will be automatically terminated when main() finishes. The programmer may optionally specify a termination status, which is stored as a void pointer for any thread that may join the calling thread. Cleanup: the pthread_exit() routine does not close files; any files opened inside the thread will remain open after the thread is terminated. Discussion: In subroutines that execute to completion normally, you can often dispense with calling pthread_exit() - unless, of course, you want to pass a return code back. However, in main(), there is a definite problem if main() completes before the threads it spawned. If you don't call pthread_exit() explicitly, when main() completes, the process (and all threads) will be terminated. By calling pthread_exit() in main(), the process and all of its threads will be kept alive even though all of the code in main() has been executed. Example: Pthread Creation and Termination This simple example code creates 5 threads with the pthread_create() routine. Each thread prints a "Hello World!" message, and then terminates with a call to pthread_exit(). pthread_attr_init (attr) pthread_attr_destroy (attr) Question: After a thread has been created, how do you know when it will be scheduled to run by the operating system? Answer Example Code - Pthread Creation and Termination #include #include #define NUM_THREADS 5 void *PrintHello(void *threadid) { long tid; tid = (long)threadid; printf("Hello World! It's me, thread #%ld!\n", tid); pthread_exit(NULL); } Page 7 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ Passing Arguments to Threads The pthread_create() routine permits the programmer to pass one argument to the thread start routine. For cases where multiple arguments must be passed, this limitation is easily overcome by creating a structure which contains all of the arguments, and then passing a pointer to that structure in the pthread_create() routine. All arguments must be passed by reference and cast to (void *). int main (int argc, char *argv[]) { pthread_t threads[NUM_THREADS]; int rc; long t; for(t=0; tthread_id; sum = my_data->sum; hello_msg = my_data->message; ... } int main (int argc, char *argv[]) { ... Page 8 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ Joining and Detaching Threads Routines: Joining: "Joining" is one way to accomplish synchronization between threads. For example: The pthread_join() subroutine blocks the calling thread until the specified threadid thread terminates. The programmer is able to obtain the target thread's termination return status if it was specified in the target thread's call to pthread_exit(). A joining thread can match one pthread_join() call. It is a logical error to attempt multiple joins on the same thread. Two other synchronization methods, mutexes and condition variables, will be discussed later. Joinable or Not? When a thread is created, one of its attributes defines whether it is joinable or detached. Only threads that are created as joinable can be joined. If a thread is created as detached, it can never be joined. The final draft of the POSIX standard specifies that threads should be created as joinable. thread_data_array[t].thread_id = t; thread_data_array[t].sum = sum; thread_data_array[t].message = messages[t]; rc = pthread_create(&threads[t], NULL, PrintHello, (void *) &thread_data_array[t]); ... } Example 3 - Thread Argument Passing (Incorrect) This example performs argument passing incorrectly. It passes the address of variable t, which is shared memory space and visible to all threads. As the loop iterates, the value of this memory location changes, possibly before the created threads can access it. int rc; long t; for(t=0; t #include #include #define NUM_THREADS 4 void *BusyWork(void *t) { int i; long tid; double result=0.0; tid = (long)t; printf("Thread %ld starting...\n",tid); for (i=0; i<1000000; i++) { result = result + sin(i) * tan(i); } printf("Thread %ld done. Result = %e\n",tid, result); pthread_exit((void*) t); } int main (int argc, char *argv[]) { pthread_t thread[NUM_THREADS]; pthread_attr_t attr; int rc; long t; void *status; /* Initialize and set thread detached attribute */ pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); for(t=0; t #include #define NTHREADS 4 #define N 1000 #define MEGEXTRA 1000000 pthread_attr_t attr; void *dowork(void *threadid) { double A[N][N]; int i,j; long tid; size_t mystacksize; tid = (long)threadid; pthread_attr_getstacksize (&attr, &mystacksize); printf("Thread %ld: stack size = %li bytes \n", tid, mystacksize); for (i=0; i #include #include /* The following structure contains the necessary information to allow the function "dotprod" to access its input data and place its output into the structure. */ typedef struct { double *a; double *b; double sum; int veclen; } DOTDATA; /* Define globally accessible variables and a mutex */ #define NUMTHRDS 4 #define VECLEN 100 DOTDATA dotstr; pthread_t callThd[NUMTHRDS]; pthread_mutex_t mutexsum; /* The function dotprod is activated when the thread is created. All input to this routine is obtained from a structure of type DOTDATA and all output from this function is written into this structure. The benefit of this approach is apparent for the multi-threaded program: when a thread is created we pass a single argument to the activated function - typically this argument is a thread number. All the other information required by the function is accessed from the globally accessible structure. */ void *dotprod(void *arg) { /* Define and use local variables for convenience */ int i, start, end, len ; long offset; double mysum, *x, *y; offset = (long)arg; len = dotstr.veclen; start = offset*len; end = start + len; x = dotstr.a; Page 14 of 22POSIX Threads Programming 29.3.2010https://computing.llnl.gov/tutorials/pthreads/ y = dotstr.b; /* Perform the dot product and assign result to the appropriate variable in the structure. */ mysum = 0; for (i=start; i #include #include #define NUM_THREADS 3 #define TCOUNT 10 #define COUNT_LIMIT 12 int count = 0; int thread_ids[3] = {0,1,2}; pthread_mutex_t count_mutex; pthread_cond_t count_threshold_cv; void *inc_count(void *t) { int i; long my_id = (long)t; for (i=0; i