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Table of Contents
1. Pthreads Overview
1. What is a Thread?
2. What are Pthreads?
3. Why Pthreads?
4. Designing Threaded Programs
2. The Pthreads API
3. Thread Management
1. Creating Threads
2. Terminating Thread Execution
3. Example: Pthread Creation and Termination
4. Passing Arguments to Threads
5. Thread Identifiers
6. Joining Threads
7. Detaching / Joining Threads
8. Example: Joining Threads
4. Mutex Variables
1. Mutex Variables Overview
2. Creating / Destroying Mutexes
3. Locking / Unlocking Mutexes
4. Example: Using Mutexes
5. Condition Variables
1. Condition Variables Overview
2. Creating/Destroying Condition Variables
3. Waiting / Signalling on Condition Variables
4. Example: Using Condition Variables
6. LLNL
7. Pthread Library Routines Reference
8. References and More Information
9. Exercise
10. WorkshopHome
Specific Information and Recommendations
Pthreads Overview
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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?
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
May share the process resources with other threads that act equally independently (and
dependently)
Dies if the parent process dies – or something similar
To the software developer, the concept of a “procedure” that runs independently from its main program may best describe a thread.
Understanding what a thread means knowing the relationship between a process and a thread. A process is created by the operating system. 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
Registers
Stack
Heap
File descriptors
Signal actions
Shared libraries
Inter-process communication tools (such as message queues, pipes, semaphores, or shared
memory).
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Threads use and exist within these process resources, yet are able to be scheduled by the operating system and run as independent entities within a process.
A thread can possess an independent flow of control and be schedulable because it maintains its own:
Stack pointer
Registers
Scheduling properties (such as policy or priority) Set of pending and blocked signals
Thread specific data.
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A process can have multiple threads, all of which share the resources within a process and all of which execute within the same address space. Within a multi-threaded program, there are at any time multiple points of execution.
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 which adhere to this standard are referred to as POSIX threads, or Pthreads. Most hardware vendors now offer Pthreads in addition to their proprietary API’s.
Pthreads Overview
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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 the this library may be part of anotherlibrary, suchaslibc.
There are several drafts of the POSIX threads standard. It is important to be aware of the draft number of a given implementation, because there are differences between drafts that can cause problems.
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 timex utility, and units are in seconds.
Pthreads Overview
IBM Architecture
fork()
pthread_create()
real
user
sys
real
user
sys
332 MHz 604e
4 CPUs/node 512 MB Memory
92.42
2.66
105.29
8.72
4.97
3.93
222 MHz POWER3 8 CPU/node
4 GB Memory
80.05
3.71
82.30
8.64
3.74
5.84
375 MHz POWER3 16 CPUs/node
16 GB Memory
173.62 13.86
172.13
9.58
3.78
6.74
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 inter -process 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
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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.
Multi-threaded applications will work on a uniprocessor system, yet naturally take advantage of a multiprocessor system, without recompiling.
In a multiprocessor environment, the most important reason for using Pthreads is to take advantage of potential parallelism. This will be the focus of the remainder of this tutorial.
Designing Threaded Programs
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.
Tasks that may be suitable for threading include tasks that:
Block for potentially long waits
Use many CPU cycles
Must respond to asynchronous events
Are of lesser or greater importance than other tasks Are able to be performed in parallel with other tasks
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.
Pthreads Overview
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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.
The Pthreads API is defined in the ANSI/IEEE POSIX 1003.1 – 1995 standard. Unlike MPI, this standard is not freely available on the Web – it must be purchased from IEEE.
ThesubroutineswhichcomprisethePthreadsAPIcanbeinformally groupedintothreemajor classes:
1. Thread management: The first class of functions work directly on threads – creating, detaching, joining, etc. They include functions to set/query thread attributes (joinable, scheduling etc.)
2. Mutexes: The second class of functions deal with synchronization, called a “mutex”, which is an abbreviation for “mutual exclusion”. Mutex functions provide for creating, destroying, locking and unlocking mutexes. They are also supplemented by mutex attribute functions that set or modify attributes associated with mutexes.
3. Condition variables: The third class of functions address communications between threads that share a mutex. They are based upon programmer specified conditions. This class includes functions to create, destroy, wait and signal based upon specified variable values. Functions to set/query condition variable attributes are also included.
Naming conventions: All identifiers in the threads library begin with pthread_
The Pthreads API
Routine Prefix
Functional Group
pthread_
Threads themselves and miscellaneous subroutines
pthread_attr_
Thread attributes objects
pthread_mutex_
Mutexes
pthread_mutexattr_
Mutex attributes objects.
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pthread_cond_ Condition variables
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 over 60 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.
The pthread.h header file must be included in each source file using the Pthreads library.
ThecurrentPOSIX standardisdefinedonlyfortheClanguage.Fortranprogrammerscanuse wrappers around C function calls. Also, the IBM Fortran compiler provides a Pthreads API. See the XLF Language Reference, located with IBM’s Fortran documentation for more information.
A number of excellent books about Pthreads are available. Several of these are listed in the References section of this tutorial.
Creating Threads
Initially, your main() program comprises a single, default thread. All other threads must be explicitly createdbytheprogrammer.
Routines:
This routine creates a new thread and makes it executable. Typically, threads are first created from within main() inside a single process. Once created, threads are peers, and may create other threads.
The pthread_create subroutine returns the new thread ID via the thread argument. The caller can use this thread ID to perform various operations on the thread. This ID should be checked to ensure that the thread was successfully created.
The attr parameter is used to set thread attributes. You can specify a thread attributes object, or NULL for the default values. Thread attributes are discussed later.
The start_routine is the C routine that the thread will execute once it is created. http://www.llnl.gov/computing/tutorials/workshops/workshop/pthreads/MAIN.html 2002/09/10
pthread_condattr_
Condition attributes objects
pthread_key_
Thread-specific datakeys
Thread Management
pthread_create (thread,attr,start_routine,arg)
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A single argument may be passed to start_routine via arg. It must be passed by reference as a pointer cast of type void.
The maximum number of threads that may be created by a process is implementation dependent.
Question: After a thread has been created, how do you know when it will be scheduled to run by the operating system…especially on an SMP machine?
Answer
Terminating Thread Execution
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.
Routines:
This routine 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.
Recommendation:Use pthread_exit() toexit fromall threads…especiallymain().
Example: Pthread Creation and Termination
Thread Management
pthread_exit (status)
Thread Management
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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() .
Example Code – Pthread Creation and Termination
#include
void *PrintHello(void *threadid) {
printf(“\n%d: Hello World!\n”, threadid);
pthread_exit(NULL);
}
int main() {
pthread_t threads[NUM_THREADS];
int rc, t;
for(t=0;t < NUM_THREADS;t++){
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1); }
}
pthread_exit(NULL);
}
Thread Management
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 *).
Question: How can you safely pass data to newly created threads, given their non-deterministic start-up and scheduling?
Answer
Three examples of argument passing to newly created threads follow:
This code fragment demostrates how to pass a simple integer to each thread. The calling
Thread Argument Passing - Example 1
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thread uses a unique data structure for each thread, insuring that each thread's argument remains intact throughout the program.
int *taskids[NUM_THREADS];
for(t=0;t < NUM_THREADS;t++)
{
taskids[t] = (int *) malloc(sizeof(int)); *taskids[t] = t;
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, ... (void *) taskids[t]);
}
Thread Argument Passing - Example 2
This example shows how to setup/pass multiple arguments via a structure.
struct
thread_data{ int thread_id; int sum;
char *message;
};
struct thread_data thread_data_array[NUM_THREADS];
void *PrintHello(void *threadarg)
{
struct thread_data *my_data;
...
my_data = (struct thread_data *) threadarg; taskid = my_data->thread_id;
sum = my_data->sum;
hello_msg = my_data – >message;
…
}
int main() { …
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]);
}
Thread Argument Passing – Example 3 (Incorrect)
This example performs argument passing incorrectly. The loop which creates threads modifies the contents of the address passed as an argument, possibly before the created threads can access it.
int rc, t;
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for(t=0;t < NUM_THREADS;t++)
{
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, ... (void *) &t);
}
Thread Identifiers
Routines:
The pthread_self() routine returns the unique, system assigned thread ID of the calling thread.
The pthread_equal() routine compares two thread IDs. If the two IDs are different 0 is returned,
otherwise a non-zero value is returned.
Note that for both of these routines, the thread identifier objects are opaque and can not be easily inspected. Because thread IDs are opaque objects, the C language equivalence operator == should not be used to compare two thread IDs against each other, or to compare a single thread ID against another value.
Joining Threads
"Joining" is one way to accomplish synchronization between threads. Two other ways, mutexes and condition variables will be discussed later.
Routines:
Thread Management
pthread_self ()
pthread_equal (thread1,thread2)
Thread Management
pthread_join (threadid,status)
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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 specified through pthread_exit(), in the status parameter.
It is impossible to join a detached thread (discussed next)
Detaching / Joining Threads
When a thread is created, one of its attributes defines whether it is joinable or detached. Detached means it can never be joined.
Routines:
Thread Management
pthread_attr_init (attr) pthread_attr_setdetachstate (attr,detachstate) pthread_attr_getdetachstate (attr,detachstate) pthread_attr_destroy (attr)
pthread_detach (threadid,status)
Toexplicitly createathreadasjoinableordetached,theattrargumentinthepthread_create() routine is used. The typical 4 step process is:
1. Declare a pthread attribute variable of the pthread_attr_t data type
2. Initialize the attribute variable with pthread_attr_init()
3. Settheattributedetachedstatuswith pthread_attr_setdetachstate()
4. When done, free library resources used by the attribute with pthread_attr_destroy()
Thepthread_detach()routinecanbeusedtoexplicitly detachathreadeventhoughitwascreated as joinable. There is no converse routine.
The final draft of the POSIX standard specifies that threads should be created as joinable. However, not all implementations may follow this.
Recommendations:
If a thread requires joining, consider explicitly creating it as joinable. This provides
portability as not all implementations may create threads as joinable by default.
If you know in advance that a thread will never need to join with another thread, consider
creating it in a detached state. Some system resources may be able to be freed.
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Thread Management
Example: Pthread Joining
This example demonstrates how to "wait" for thread completions by using the Pthread join routine. Since not all current implementations of Pthreads create threads in a joinable state, the threads in
this example are explicitly created in a joinable state so that they can be joined later.
#include
#define NUM_THREADS 3
void *BusyWork(void *null)
{
int i;
double result=0.0;
for (i=0; i < 1000000; i++)
{
result = result + (double)random();
}
printf("result = %d\n",result); pthread_exit((void *) 0);
}
void main() {
pthread_t thread[NUM_THREADS]; pthread_attr_t attr;
int rc, t, status;
/* Initialize and set thread detached attribute */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr , PTHREAD_CREATE_JOINABLE);
for(t=0;t < NUM_THREADS;t++)
{
printf("Creating thread %d\n", t);
rc = pthread_create(&thread[t], &attr, BusyWork, NULL); if (rc)
{
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1); }
}
/* Free attribute and wait for the other threads */
pthread_attr_destroy(&attr);
for(t=0;t < NUM_THREADS;t++) {
rc = pthread_join(thread[t], (void **)&status);
if (rc) {
printf("ERROR; return code from pthread_join() is %d\n", rc);
exit(-1); }
printf("Completedjoin with thread %d status= %d\n",t, status); }
pthread_exit(NULL);
}
Example Code - Pthread Joining
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Mutex Variables
Overview
Mutex is an abbreviation for "mutual exclusion". Mutex variables are one of the primary means of implementing thread synchronization and for protecting shared data when multiple writes occur.
A mutex variable acts like a "lock" protecting access to a shared data resource. The basic concept of a mutex as used in Pthreads is that only one thread can lock (or own) a mutex variable at any given time. Thus, even if several threads try to lock a mutex only one thread will be successful. No other thread can own that mutex until the owning thread unlocks that mutex. Threads must "take turns" accessing protected data.
Mutexes can be used to prevent "race" conditions. An example of a race condition involving a bank transaction is shown below:
Thread 1
Thread 2
Balance
Read balance: $1000
$1000
Read balance: $1000
$1000
Deposit $200
$1000
Deposit $200
$1000
Update balance $1000+$200
$1200
Update balance $1000+$200
$1200
In the above example, a mutex should be used to lock the "Balance" while a thread is using this shared data resource.
Very often the action performed by a thread owning a mutex is the updating of global variables.
This is a safe way to ensure that when several threads update the same variable, the final value is the same as what it would be if only one thread performed the update. The variables being updated belongtoa"critical section".
A typical sequence in the use of a mutex is as follows:
Create and initialize a mutex variable
Several threads attempt to lock the mutex
Only one succeeds and that thread owns the mutex The owner thread performs some set of actions
The owner unlocks the mutex
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Another thread acquires the mutex and repeats the process Finally themutexisdestroyed
When several threads compete for a mutex, the losers block at that call - an unblocking call is available with "trylock" instead of the "lock" call.
When protecting shared data, it is the programmer's responsibility to make sure every thread that needs to use a mutex does so. For example, if 4 threads are updating the same data, but only one uses a mutex, the data can still be corrupted.
Creating / Destroying Mutexes
Routines:
Mutex Variables
pthread_mutex_init (mutex,attr) pthread_mutex_destroy (mutex) pthread_mutexattr_init (attr) pthread_mutexattr_destroy (attr)
Mutex variables must be declared with type pthread_mutex_t, and must be initialized before they can be used. There are two ways to initialize a mutex variable:
1. Statically, when it is declared. For example:
pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
2. Dynamically, with the pthread_mutex_init() routine. This method permits setting mutex
object attributes, attr.
The mutex is initially unlocked.
The attr object is used to establish properties for the mutex object, and must be of type pthread_mutexattr_t if used (may be specified as NULL to accept defaults). The Pthreads standard defines three optional mutex attributes:
Protocol: Specifies the protocol used to prevent priority inversions for a mutex. Prioceiling: Specifies the priority ceiling of a mutex.
Process-shared: Specifies the process sharing of a mutex.
Note that not all implementations may provide the three optional mutex attributes.
The pthread_mutexattr_init() and pthread_mutexattr_destroy() routines are used to create and
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destroy mutex attribute objects respectively.
pthread_mutex_destroy() should be used to free a mutex object which is no longer needed.
Locking / Unlocking Mutexes
Routines:
The pthread_mutex_lock() routine is used by a thread to acquire a lock on the specified mutex variable. If the mutex is already locked by another thread, this call will block the calling thread until the mutex is unlocked.
pthread_mutex_trylock() will attempt to lock a mutex. However, if the mutex is already locked, the routine will return immediately with a "busy" error code. This routine may be useful in preventing deadlock conditions, as in a priority-inversion situation.
pthread_mutex_unlock() will unlock a mutex if called by the owning thread. Calling this routine is required after a thread has completed its use of protected data if other threads are to acquire the mutex for their work with the protected data. An error will be returned if:
If the mutex was already unlocked
If the mutex is owned by another thread
Question: When more than one thread is waiting for a locked mutex, which thread will be granted the lock first after it is released?
Answer
Example: Using Mutexes
This example program illustrates the use of mutex variables in a threads program that performs a dot product. The main data is made available to all threads through a globally accessible structure. Each thread works on a different part of the data. The main thread waits for all the threads to
Mutex Variables
pthread_mutex_lock (mutex) pthread_mutex_trylock (mutex) pthread_mutex_unlock (mutex)
Mutex Variables
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complete their computations, and then it prints the resulting sum.
#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. This structure is unchanged from the sequential version.
*/
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, offset, len ; double mysum, *x, *y;
offset = (int)arg;
len = dotstr.veclen; start = offset*len; end = start + len; x = dotstr.a;
y = dotstr.b;
/*
Perform the dot product and assign result
to the appropriate variable in the structure. */
mysum = 0;
for (i=start; i < end ; i++)
{
mysum += (x[i] * y[i]);
}
/*
Lock a mutex prior to updating the value in the shared structure, and unlock it upon updating.
*/
pthread_mutex_lock (&mutexsum);
dotstr.sum += mysum;
pthread_mutex_unlock (&mutexsum);
Example Code - Using Mutexes
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pthread_exit((void*) 0);
}
/*
The main program creates threads which do all the work and then
print out result upon completion. Before creating the threads,
the input data is created. Since all threads update a shared structure, we need a mutex for mutual exclusion. The main thread needs to wait for all threads to complete, it waits for each one of the threads. We specify a thread attribute value that allow the main thread to join with the threads
it creates. Note also that we free up handles no longer needed.
*/
void main (int argc, char *argv[])
{
int i;
double *a, *b; int status; pthread_attr_t attr;
when they are
/* Assign storage and initialize values */
a = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double)); b = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double));
for (i=0; i < VECLEN*NUMTHRDS; i++)
{ a[i]=1;
b[i]=a[i]; }
dotstr.veclen = VECLEN; dotstr.a = a;
dotstr.b = b; dotstr.sum=0;
pthread_mutex_init(&mutexsum, NULL);
/* Create threads to perform the dotproduct */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr , PTHREAD_CREATE_JOINABLE);
for(i=0;i < NUMTHRDS;i++)
{
/*
Each thread works on a different set of data. The offset is specified by 'i'. The size of
the data for each thread is indicatedby VECLEN. */
pthread_create( &callThd[i], &attr, dotprod, (void *)i);
}
pthread_attr_destroy(&attr);
/* Wait on the other threads */
for(i=0;i < NUMTHRDS;i++)
{
}
/* After joining, print out the results and cleanup */ printf ("Sum = %f \n", dotstr.sum);
free (a); free (b);
pthread_mutex_destroy(&mutexsum);
pthread_exit(NULL);
}
Serial version Pthreads version
pthread_join( callThd[i], (void **)&status);
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Condition Variables
Overview
Condition variables provide yet another way for threads to synchronize. While mutexes implement synchronization by controlling thread access to data, condition variables allow threads to synchronize basedupontheactualvalueofdata.
Without condition variables, the programmer would need to have threads continually polling (possibly in a critical section), to check if the condition is met. This can be very resource consuming since the thread would be continuously busy in this activity. A condition variable is a way to
achieve the same goal without polling.
A condition variable is always used in conjunction with a mutex lock.
A representative sequence for using condition variables is shown below.
Main Thread
Declare and initialize global data/variables which require synchronization (such as
"count")
Declare and initialize a condition variable object Declare andinitialize anassociatedmutex
Create threads A and B to do work
Thread A
Thread B
Do work up to the point where a certain condition must occur (such as "count" must reach a specified value)
Lock associated mutex and check value of a global variable
Call pthread_cond_wait() to perform a blocking wait for signal from Thread- B. Note that a call to pthread_cond_wait() automatically and atomically unlocks the associated mutex variable so that it can be used by Thread-B.
When signalled, wake up. Mutex is automatically andatomically locked.
Explicitly unlock mutex
Continue
Do work
Lock associated mutex
Change the value of the global variable
that Thread-A is waiting upon.
Check value of the global Thread-A
waitvariable.Ifitfulfills thedesired
condition, signal Thread-A.
Unlock mutex.
Continue
Main Thread
Join /Continue
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Condition Variables
Creating / Destroying Condition Variables
Routines:
pthread_cond_init (condition,attr) pthread_cond_destroy (condition) pthread_condattr_init (attr) pthread_condattr_destroy (attr)
Condition variables must be declared with type pthread_cond_t , and must be initialized before they can be used. There are two ways to initialize a condition variable:
1. Statically, when it is declared. For example:
pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER;
2. Dynamically, with the pthread_cond_init() routine. The ID of the created condition variable is returned to the calling thread through the condition parameter. This method permits setting condition variable object attributes, attr.
The optional attr object is used to set condition variable attributes. There is only one attribute defined for condition variables: process-shared, which allows the condition variable to be seen by threads in other processes. The attribute object, if used, must be of type pthread_condattr_t (may be specified as NULL to accept defaults).
Note that not all implementations may provide the process -shared attribute.
The pthread_condattr_init() and pthread_condattr_destroy() routines are used to create and destroy condition variable attribute objects.
pthread_cond_destroy() should be used to free a condition variable that is no longer needed.
Waiting / Signalling on Condition Variables
Condition Variables
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Routines:
pthread_cond_wait() blocks the calling thread until the specified condition is signalled. This routine should be called while mutex is locked, and it will automatically release the mutex while it waits. Should also unlock mutex after signal has been received.
The pthread_cond_signal() routine is used to signal (or wake up) another thread which is waiting on the condition variable. It should be called after mutex is locked, and must unlock mutex in order for pthread_cond_wait() routine to complete.
The pthread_cond_broadcast() routine should be used instead of pthread_cond_signal() if more than one thread is in a blocking wait state.
It is a logical error to call pthread_cond_signal() before calling pthread_cond_wait(). Proper locking and unlocking of the associated mutex variable is essential when using these
routines. For example:
Failing to lock the mutex before calling pthread_cond_wait() may cause it NOT to block.
Failing to unlock the mutex after calling pthread_cond_signal() may not allow a matching pthread_cond_wait() routine to complete (it will remain blocked).
Example: Using Condition Variables
This simple example code demonstrates the use of several Pthread condition variable routines. The main routine creates three threads. Two of the threads perform work and update a "count" variable. The third thread waits until the count variable reaches a specified value.
#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_wait (condition,mutex) pthread_cond_signal (condition) pthread_cond_broadcast (condition)
Condition Variables
Example Code – Using Condition Variables
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pthread_cond_t count_threshold_cv;
void *inc_count(void *idp)
{
int j,i;
double result=0.0; int *my_id = idp;
for (i=0; i < TCOUNT; i++)
{ pthread_mutex_lock(&count_mutex); count++;
/*
Check the reached. */
i f
value of count and signal waiting thread when condition is Note that this occurs while mutex is locked.
== COUNT_LIMIT) {
(count
pthread_cond_signal(&count_threshold_cv);
printf("inc_count(): thread %d, count = %d Threshold reached.\n",
*my_id, count); }
printf("inc_count(): thread %d, count = %d, unlocking mutex\n", *my_id, count);
pthread_mutex_unlock(&count_mutex);
/* Do some work so threads can alternate on mutex lock */ for (j=0; j < 1000; j++)
result = result + (double)random(); }
pthread_exit(NULL);
}
void *watch_count(void *idp)
{
int *my_id = idp;
printf("Starting watch_count(): thread %d\n", *my_id);
/*
Lock mutex and wait for signal. Note that the pthread_cond_wait routine will automatically and atomically unlock mutex while it waits. Also, note that if COUNT_LIMIT is reached before this routine is run by the waiting thread, the loop will be skipped to prevent pthread_cond_wait from never returning.
*/
pthread_mutex_lock(&count_mutex);
while (count < COUNT_LIMIT) {
pthread_cond_wait(&count_threshold_cv, &count_mutex);
printf("watch_count(): thread %d Condition signal
received.\n", *my_id); }
pthread_mutex_unlock(&count_mutex);
pthread_exit(NULL);
}
void main()
{
inti, rc;
pthread_t threads[3]; pthread_attr_t attr;
/* Initialize mutex and condition variable objects */
pthread_mutex_init(&count_mutex, NULL); pthread_cond_init (&count_threshold_cv, NULL);
/*
For portability, explicitly create threads in a joinable state
so that they can be joined later.
*/
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr , PTHREAD_CREATE_JOINABLE); pthread_create(&threads[0], &attr, inc_count, (void *)&thread_ids[0]); pthread_create(&threads[1], &attr, inc_count, (void *)&thread_ids[1]); pthread_create(&threads[2], &attr, watch_count, (void *)&thread_ids[2]);
/* Wait for all threads to complete */ for (i = 0; i < NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
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printf ("Main(): Waited on %d threads. Done.\n", NUM_THREADS);
/* Clean up and exit */
pthread_attr_destroy(&attr); pthread_mutex_destroy(&count_mutex); pthread_cond_destroy(&count_threshold_cv); pthread_exit(NULL);
}
This section describes details specific to LLNL's ASCI IBM systems.
Implementations:
All ASCI IBM systems include IBM's Pthreads implementation, which follows draft 10 (final) of the POSIX standard. This is the preferred implementation.
For compatibility with earlier implementations, the IBM compilers provide a means to use a draft 7 version of Pthreads.
Why Use Pthreads on the ASCI IBM Machines?
The primary motivation for considering the use of Pthreads on any IBM SMP architecture is to achieve optimum performance. In particular, if an application is using MPI for on-node communications, thereisapotentialthatperformancecouldbegreatlyimprovedbyusingPthreads for on-node data transfer instead.
IBM's MPI provides the MP_SHARED_MEMORY environment variable to direct the use of shared memory for on-node MPI communications. By default, this is set to "yes" by LLNL in
the /etc/environment file on all ASCI IBM machines. Without this, on-node MPI communications demonstrate serious performance degradation as the number of tasks increase.
Even with MP_SHARED_MEMORY set to "yes", on-node MPI communications can not compete with Pthreads:
ASCI Blue (604e) shared memory MPI bandwidth: @80 MB/sec per MPI task
ASCI White (POWER3) shared memory MPI bandwidth: @275 MB/sec per MPI task
Pthreads worst case: Every data reference by a thread requires a memory read. In this case, a
thread's "bandwidth" is limited by the machine's memory-to-CPU bandwidth: 604e: 1.3 GB/sec
POWER3 NH-2: 16 GB/sec
Pthreads best case: Data is local to a thread's cache offering much greater cache-CPU bandwidths.
Mixing MPI with Pthreads:
LLNL Specific Information and Recommendations
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Programs that contain both MPI and Pthreads are common and easy to develop on all ASCI IBM systems.
When compiling such hybrid applications, use the thread-safe MPI compiler command.
An example code that uses both MPI and Pthreads is available below. The serial, threads-only, MPI-only and MPI-with-threads versions demonstrate one possible progression.
Serial
Pthreads only
MPI only
MPI with pthreads makefile
Compiling:
Livermore Computing maintains a number of compilers, and usually a couple different versions of each. These are documented on the LC's Supported Compilers web page.
The more commonly used thread-safe, Pthreads enabled IBM compiler commands are listed below. For more information see the man page (click on the compiler command) and the relevant IBM
compiler documentation.
Compiler Command
Description
xlc_r cc_r
C-ANSI /non-ANSI
xlC_r
C++
xlf_r xlf90_r
Fortran-usingIBM's PthreadsAPI(non-portable)
mpxlc_r mpcc_r
C with MPI - ANSI / non_ANSI
mpCC_r
C++ with MPI
mpxlf_r mpxlf90_r
Fortran with MPI - using IBM's Pthreads API (non-portable)
IBM Implementation Notes:
The three mutex attributes, protocol, prioceiling and process-shared are not supported
The process-shared condition variable attribute is not supported
AIX versions differ in how pthreads are created. Prior to AIX 4.3 threads were created in a detached state. Since AIX 4.3 threads are created in a joinable state.
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Pthread Library Routines Reference
Pthread Functions
Thread Management pthread_create
pthread_exit
pthread_join
pthread_kill
pthread_self
pthread_equal
pthread_yield
pthread_detach
Thread Specific Data
pthread_once
pthread_key_cre
ate
pthread_key_delete
Thread Cancellation
pthread_getspecific
pthread_setspecific
pthread_cancel
pthread_cleanup_pop
pthread_cleanup_push
pthread_setcancelstate
pthread_getcancelstate
pthread_testcancel
pthread_getschedparam
Thread Scheduling
pthread_setschedparam
Signals
pthread_sigmask
Pthread Attribute Functions
Basic Management
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pthread_attr_init
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pthread_attr_destroy
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Detachable or pthread_attr_setdetachstate Joinable
Thread Scheduling Attributes
pthread_attr_getdetachstate
pthread_attr_getstackaddr
Specifying Stack Information
pthread_attr_getstacksize
pthread_attr_setstackaddr
pthread_attr_setstacksize
pthread_attr_getschedparam
pthread_attr_setschedparam
pthread_attr_getschedpolicy
pthread_attr_setschedpolicy
pthread_attr_setinheritsched
pthread_attr_getinheritsched
pthread_attr_setscope
pthread_attr_getscope
Mutex Functions
Mutex Management
pthread_mutex_init
pthread_mutex_destroy
pthread_mutex_lock
pthread_mutex_unlock
pthread_mutex_trylock
Priority Management
pthread_mutex_setprioceiling
pthread_mutex_getprioceiling
Mutex Attribute Functions
Basic Management
pthread_mutexattr_init
pthread_mutexattr_destroy
Sharing
pthread_mutexattr_getpshared
pthread_mutexattr_setpshared
Protocol Attributes
pthread_mutexattr_getprotocol
pthread_mutexattr_setprotocol
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Priority Management pthread_mutexattr_setprioceiling
pthread_mutexattr_getprioceiling
Condition Variable Functions
Basic Management
pthread_cond_init
pthread_cond_destroy
pthread_cond_signal
pthread_cond_broadcast
pthread_cond_wait
pthread_cond_timedwait
Condition Variable Attribute Functions
Basic Management
pthread_condattr_init
pthread_condattr_destroy
Sharing
pthread_condattr_getpshared
pthread_condattr_setpshared
References and More Information
"Pthreads Programming". B. Nichols et al. O'Reilly and Associates.
"Threads Primer". B. Lewis and D. Berg. Prentice Hall
"Programming With POSIX Threads". D. Butenhof. Addison Wesley www.awl.com/cseng/titles/0-201-63392-2
"Programming With Threads". S. Kleiman et al. Prentice Hall
IBM Fortran Compiler Documentation www-4.ibm.com/software/ad/fortran
IBM C/C++ Compiler Documentation
www-4.ibm.com/software/ad/caix
http://www.llnl.gov/computing/tutorials/workshops/workshop/pthreads/MAIN.html Last Modified: 05/24/2002 02:51:45 blaiseb@llnl.gov
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