pthread_create() blocks if maxthreads reached?

[Julian Elischer]

On Sun, 1 Feb 2004, Craig Rodrigues wrote:

> Hi,
> 
> I wrote the following small test program to spawn
> a large number of threads.
> 
> ==================================================================
> 
> #include <sys/types.h>
> #include <sys/select.h>
> #include <pthread.h>
> #include <stdio.h>
> 
> #define NUM 100000
> 
> void *thr(void *p)
> {
>   select(0, NULL, NULL, NULL, NULL);  
>   return p;
> }
> 
> 
> int
> main()
> {
>   pthread_t t[NUM];
> 
>   int ret=0;
>   int i;
>   
>   for(i=0; ; ++i)
>   {
>      printf("Entering pthread_create...\n");
>      ret = pthread_create(&t[i], NULL, thr, NULL);
>      printf("Left pthread_create...\n");
> 
>      if( ret != 0 ) {
>          printf("%d %s\n", ret, strerror(ret)); 
>          break;
>      }
>   }
>   printf("Max threads reached at: %d\n", i);
> 
>   return 0;
> }
> ==============================================================
> 
> 
> If I link this program with -lthr or -lc_r, the
> program will terminate.

Successfully? 
that's interesting.. libthr should theoretically b elimited to about
8000 threads.. (on x86).


> 
> If I link with -lpthread, the program seems to
> block in the pthread_create() call, instead of
> returning EAGAIN (to indicate that a new thread cannot
> be created due to hitting some system limit). 

is that what you expected?


> 
> Any ideas on this?

yes
to stop processes running away and making the system unusable while we
are developing the therad package we made a limit on how many 
threads we allow to ente the kernel at one time per process..
If you trust your program, you could over-ride the limit.

see:
kern.threads.max_threads_per_proc: 150
kern.threads.max_groups_per_proc: 50

Your threads are all waiting in the kernel.
If you used a blocked semaphore or some other non-kernel
method of blocking then you would discover that you could do more
or you could allow more threads to enter the kernel at a time
but that would be a poor use of resources.. it is much more
efficient to keep blocked threads in userland than to block them in the
kernel where they use precious kernel resources.
I'm not sure but I think 'sleep()' is intecepted for example to 
keep the sleeping threads in userland. (I may be wrong.
I don't know if Dan did that yet).


BTW the limit is not on making more threads, its that the last select()
REALLY blocked and did not return to the userland program to allow you
to make a new  thread.


> 
> Thanks.
> -- 
> Craig Rodrigues        
> http://crodrigues.org
> rodrigc at crodrigues.org
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