Max number of pthreads in one process?

19 May 2008

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Thread-topic: Max number of pthreads in one process?
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Hi all, just to see what would happen, I wrote up a small program that
spawns as many threads as it can, and then tries to cancel all of them
politely (see pasted code below).  On my machine, I spawn 7051 threads
before pthread_create() returns EAGAIN.  This part is OK, I'm happy with it;
its when I start canceling threads that I run into problems.  Only the first
2559 threads are canceled.  At that point, my main thread gets stuck in
pthread_join() waiting for the other threads to cancel.  Looking at it with
Shark, I find I'm stuck in semaphore_wait_trap().  I don't have control over
this, and as near as I can tell, I don't have any race conditions.  So,
where is the bug?

uname -a returns Darwin anvil.arl.army.mil 8.11.1 Darwin Kernel Version
8.11.1: Wed Oct 10 18:23:28 PDT 2007; root:xnu-792.25.20~1/RELEASE_I386 i386
i386.  This system is a fully patched 10.4.11 system.

The code was compiled with gcc -Wall -Wextra -pedantic -std=c99 -ggdb

Thanks,
Cem Karan

#include <assert.h>
#include <errno.h>
#include <inttypes.h>
#include <limits.h>
#include <pthread.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <unistd.h>

pthread_mutex_t myLock = PTHREAD_MUTEX_INITIALIZER;
bool dieNow = false;

void *threadFunc(void *arg)
{
    printf("About to start loop.\n");

    while (true)
    {
        assert(pthread_mutex_lock(&myLock) == 0);
            if (dieNow)
            {
                assert(pthread_mutex_unlock(&myLock) == 0);
                break;
            }
        assert(pthread_mutex_unlock(&myLock) == 0);
        usleep(100);
    }

    printf("Got message to quit.\n");

    return NULL;
}

int main(void)
{
    pthread_t ID[10000];
    uintmax_t counter, counter2;
    int result;
    bool noMoreThreads = false;

    bzero(ID, 10000 * sizeof(pthread_t));

    for (counter = 0; counter < UINT_MAX; counter++)
    {
        result = pthread_create(&(ID[counter]), NULL, threadFunc, NULL);
        switch (result)
        {
            case 0:
                break;
            case EAGAIN:
                printf("Maximum number of threads is %" PRIuMAX "\n",
counter);
                noMoreThreads = true;
                break;
            case EINVAL:
                printf("How in the world did I get an EINVAL error???\n");
                exit(1);
                break;
            default:
                printf("Unknown return value! %i \n", result);
                exit(1);
                break;
        }

        if (noMoreThreads)
        {
            break;
        }
    }

    sleep(1);

    assert(pthread_mutex_lock(&myLock) == 0);
        dieNow = true;
    assert(pthread_mutex_unlock(&myLock) == 0);

    for (counter2 = 0; counter2 < counter; counter2++)
    {
        printf("Quitting thread number %" PRIuMAX "\n", counter2);
        assert(pthread_join(ID[counter2], NULL) == 0);
    }

    return 0;
}

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Max number of pthreads in one process?

Army Research Lab