Pthreads
Introduction
One of the earliest parallelization techniques was through the use of POSIX threads, usually shortened to just pthreads. Like OpenMP, pthreads parallelization relies on the assumption of a shared memory environment and is, therefore, typically used only on a single node with the number of active threads limited by the number of available CPU cores on the node. While pthreads can be used with a variety of programming languages, in practice the main target language is C. To parallelize a Fortran program using threads, OpenMP is almost certainly a better idea while C++ programmers would probably find the constructs in the Boost threading library to be more valuable.
As one of the earliest forms of parallelization, pthreads have also served as the basis for later approaches to shared memory parallelization like OpenMP and can be thought of as forming a toolkit of threading primitives that permit the most general and low-level parallelization, at the price of sacrificing much of the simplicity and ease of use of a high level API like OpenMP. The essential model for pthreads is the dynamic spawning of lightweight sub-processes (threads) that asynchronously carry out operations and then are extinguished by rejoining the program's master process. As all the threads of a program reside in the same memory space, sharing data among them through global variables isn't difficult in comparison with a distributed approach like MPI but any modifications of this shared data have to be managed with care to avoid race conditions.
Compilation
To use the various functions and data structures associated with pthreads in your C program, you will need to include the header file pthread.h and compile your program with a special flag so that it is linked with the pthread library.
[name@server ~]$ gcc -pthread -o test threads.c
The number of threads to be used in your program can be hard-coded into the source file, a less ideal solution, or set to an integer variable in the source code whose value is specified at runtime via a command line argument or through a user-defined environment variable that your program reads.
Creation and Destruction of Pthreads
When parallelizing an existing serial program using pthreads, we use a programming model where threads are created by a parent, which may be the serial master thread or another worker thread, then carry out some work, and finally reach the end of their lifecycle and get reabsorbed (or joined) back into the parent. While there are various ways for a thread to come to an end, a single function is used to create a new thread, pthread_create. This function has four arguments: a unique identifier for the newly created thread, a set of attributes for this thread, a C function that the thread will execute upon initiation and finally an argument for this function.
#include <stdio.h>
#include <pthread.h>
const long NT = 12;
void* task(void* thread_id)
{
long tnumber = (long) thread_id;
printf("Hello World from thread %ld\n",1+tnumber);
}
int main(int argc,char** argv)
{
int success;
long i;
pthread_t threads[NT];
for(i=0; i<NT; ++i) {
success = pthread_create(&threads[i],NULL,task,(void*)i);
if (success != 0) {
printf("ERROR: Unable to create worker thread %ld successfully\n",i);
return 1;
}
}
for(i=0; i<NT; ++i) {
pthread_join(threads[i],NULL);
}
return 0;
}
This simple program creates twelve threads, each one executing the function task with the argument consisting of the thread's index, from 0 to 11. Note that the call of pthread_create is non-blocking, i.e. the root or master thread, which is executing the main function, continues to execute after each of the twelve worker threads is created. After creating the twelve threads, the master thread then goes into the second for loop and calls pthread_join, a blocking function where the master thread waits for the twelve workers to finish executing the function task and rejoin the master thread. While trivial, this program illustrates the basic lifecycle of a POSIX thread: the master thread creates a thread by assigning it a function to run and then waits it for the thread to finish the execution of the function and return and coalesce or join back into the execution of the master thread.
If you run this test program several times in a row you'll likely notice that the order in which you see the various worker threads saying hello varies, which is what we would expect since they are now running in an asynchronous manner. Each time the program is executed, the twelve threads compete for access to the standard output during the printf call and from one execution of the program to another the winners of this competition will change.
Synchronizing Data Access
In a more realistic program, the various worker threads will need to read and eventually modify certain data in order to accomplish their tasks. These data normally consist of a set of global variables of different types and dimensions, and with multiple threads reading from and writing to these data, we need to ensure that the access to these data is synchronized in some fashion to avoid the so called race conditions, i.e. situations in which the program's output could depend on the (essentially random) order in which the asynchronous threads access the data. Typically, we want the parallel version of our program to produce results identical to what we would obtain when running it serially, so the race conditions are unacceptable.
The simplest and most common form of access control that is used to serialize the reading and writing of data shared among threads is the mutex, derived from the expression 'mutual exclusion'. In pthreads, a mutex is a kind of variable may be locked by only one thread and can later be released or unlocked once the global data has been read or modified. The code that lies between the call to lock a mutex and the call to unlock it will only be executed by a single thread at a time, eliminating the risk of race conditions. To create a mutex in a pthreads program, we need to declare a global variable of type pthread_mutex_t which must be initialized before it is used by calling pthread_mutex_init and at the program's end we release the resources associated with the mutex by calling pthread_mutex_destroy.
#include <stdio.h>
#include <pthread.h>
const long NT = 12;
pthread_mutex_t mutex;
void* task(void* thread_id)
{
long tnumber = (long) thread_id;
pthread_mutex_lock(&mutex);
printf("Hello World from thread %ld\n",1+tnumber);
pthread_mutex_unlock(&mutex);
}
int main(int argc,char** argv)
{
int success;
long i;
pthread_t threads[NT];
pthread_mutex_init(&mutex,NULL);
for(i=0; i<NT; ++i) {
success = pthread_create(&threads[i],NULL,task,(void*)i);
if (success != 0) {
printf("ERROR: Unable to create worker thread %ld successfully\n",i);
pthread_mutex_destroy(&mutex);
return 1;
}
}
for(i=0; i<NT; ++i) {
pthread_join(threads[i],NULL);
}
pthread_mutex_destroy(&mutex);
return 0;
}
In this example, based on the previous code, access to the standard out is serialized - as it normally should be - using a mutex. The call to pthread_mutex_lock is blocking, i.e. the thread will continue to wait indefinitely for the mutex to become available, so care has to be used in ensuring that no deadlocks occur in your code, which is particularly problematic in a more realistic example where you may have many different mutexes designed to control access to different global data structures that have to be managed among a host of threads. There is also a non-blocking alternative, pthread_mutex_trylock, which if it fails to obtain the mutex lock returns immediately with a non-zero value indicating that the mutex is busy. You should also ensure that no extraneous code appears inside the serialized code block; since this code will be executed in a serial manner, you want it to be as minimalist as possible - consistent with the goal of program correctness - to enhance your program's parallel performance.
A more subtle form of data synchronization is possible with the read/write lock, pthread_rwlock_t. With this construct, multiple threads can simultaneously read the value of a variable but for write access, the read/write lock behaves like the standard mutex, i.e. no other thread may have have any access (read or write) to the variable. Like with a mutex, a pthread_rwlock_t must be initialized before its first use and destroyed when it is no longer needed during the program. Individual threads can obtain either a read lock, pthread_wrlock_rdlock, or a write lock pthread_rwlock_wrlock, and either one is released using the same function, pthread_rwlock_unlock.
Another construct is used to allow multiple threads to wait on a particular condition, for example waiting for work to become available for the worker threads. For this reason this construct is called a condition variable with the datatype pthread_cond_t. Similar to a mutex or read/write lock, a condition variable must be initialized before its first use and destroyed when it is no longer needed. The use of a condition variable also requires a mutex, needed to control access to the variable(s) that are the basis for the condition that is being tested. A thread that needs to wait on a condition will lock the mutex and can then call the function pthread_cond_wait with two arguments: the condition variable datatype and the mutex. The mutex will be released atomically with the creation of the condition variable that the thread is now waiting upon, so that other threads can lock the mutex to either wait on the same condition or modify one or more variables, thereby changing the condition.
#include <stdio.h>
#include <pthread.h>
const long NT = 2;
pthread_mutex_t mutex;
pthread_cond_t ticker;
int workload;
void* task(void* thread_id)
{
long tnumber = (long) thread_id;
if (tnumber == 0) {
pthread_mutex_lock(&mutex);
while(workload <= 25) {
pthread_cond_wait(&ticker,&mutex);
}
printf("Thread %ld: incrementing workload by 15\n",1+tnumber);
workload += 15;
pthread_mutex_unlock(&mutex);
}
else {
int done = 0;
do {
pthread_mutex_lock(&mutex);
workload += 3;
printf("Thread %ld: current workload is %d\n",1+tnumber,workload);
if (workload > 25) {
done = 1;
pthread_cond_signal(&ticker);
}
pthread_mutex_unlock(&mutex);
} while(!done);
}
}
int main(int argc,char** argv)
{
int success;
long i;
pthread_t threads[NT];
workload = atoi(argv[1]);
if (workload > 25) {
printf("Initial workload must be <= 25, exiting...\n");
return 0;
}
pthread_mutex_init(&mutex,NULL);
pthread_cond_init(&ticker,NULL);
for(i=0; i<NT; ++i) {
success = pthread_create(&threads[i],NULL,task,(void*)i);
if (success != 0) {
printf("ERROR: Unable to create worker thread %ld successfully\n",i);
pthread_mutex_destroy(&mutex);
return 1;
}
}
for(i=0; i<NT; ++i) {
pthread_join(threads[i],NULL);
}
printf("Final workload is %d\n",workload);
pthread_cond_destroy(&ticker);
pthread_mutex_destroy(&mutex);
return 0;
}
In the above example we have just two worker threads which are both modifying the value of the integer workload, whose initial value we assume is less than or equal to 25. The first thread locks the mutex and then waits on the condition that workload <= 25, which releases the mutex so that the second thread can perform a loop that increments the value of workload by three at each iteration. After incrementing workload, the loop checks if the value is greater than 25 in which case it uses the function pthread_cond_signal to alert at least one other waiting thread that the condition is now satisfied. We can also use pthread_cond_broadcast to notify all waiting threads that the condition is satisfied. With the first thread signalled, the second thread sets the exit condition for the loop and releases the mutex to disappear in the pthread_join. Having been woken up, the first thread increments workload by 15 and exits the function task itself. After the worker threads have been absorbed, the master thread prints out the final value of workload and the program exits.
Further Reading
This page is only intended to provide a very brief overview of what is in fact a complex and demanding subject. Individuals who are interested in a more in-depth discussion of pthreads, the various optional arguments that are available for many function calls - where we have used the default NULL argument for such parameters in this page - and advanced topics can consult sources like David Butenhof's Programming with POSIX Threads (Addison-Wesley, 1993) or the excellent LANL tutorial.