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Re: Sarge, kernel, threads and limits

On Tue, Feb 08, 2005 at 09:39:17AM +0100, Ryszard Lach wrote:
> * how to determine if my system/application is running linux or POSIX
>   threads

Your system can be "running" both libraries at the same time,
depending on flags to the dynamic loader, etc.  The only sane way to
check is a runtime one

#include <stdio.h>
#include <unistd.h>
#include <alloca.h>
#include <string.h>

int isnptl (void)
        size_t n = confstr (_CS_GNU_LIBPTHREAD_VERSION, NULL, 0);
        if (n > 0)
                char *buf = alloca (n);
                confstr (_CS_GNU_LIBPTHREAD_VERSION, buf, n);
                if (strstr (buf, "NPTL"))
                        return 1;
        return 0;

int main(void)
        printf("NPTL: %s\n", isnptl() ? "yes" : "no");
        return 0;

> * how to determine limits of threads per system, threads per process and
>   threads per user (hard compiled or set by PAM etc.)

I believe this is set by the ulimit of the number of processes.  This
can be changed by ulimit -u or in something like /etc/security/limits.conf
> * how is related amount of available stack to number of threads, which
>   process can create

Did you mean : how is the amount of available system memory for stacks
related to the number of threads?  If you use the default stack size
that you get when you call pthread_create() you're going to run out of
memory pretty quickly.  If you change down with
pthread_attr_setstacksize() you're going to be able to run many more
threads.  However, you're still going to hit a kernel limit, see

  * The default maximum number of threads is set to a safe
  * value: the thread structures can take up at most half
  * of memory.
  max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);

> * I'd like also really understand what 'ps' displays. Don't say 'read
>   manual', because manual assumes that you really know what's going on
>   in kernel and tells you only how to display it.

ps should only display the main thread.  You need to pass something
like '-m' to show the sub-threads.


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