Date: Sun, 1 Feb 2004 10:21:05 -0500 From: Craig Rodrigues <rodrigc@crodrigues.org> To: Julian Elischer <julian@elischer.org> Cc: freebsd-threads@freebsd.org Subject: Re: pthread_create() blocks if maxthreads reached? Message-ID: <20040201152105.GA2370@crodrigues.org> In-Reply-To: <Pine.BSF.4.21.0401312317590.38031-100000@InterJet.elischer.org> References: <20040201061212.GA6349@crodrigues.org> <Pine.BSF.4.21.0401312317590.38031-100000@InterJet.elischer.org>
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On Sat, Jan 31, 2004 at 11:27:39PM -0800, Julian Elischer wrote:
> 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 -lthr, I get:
Max threads reached at: 109
If I link with -lc_r, I get:
Max threads reached at: 14118
If I link with -lpthread, I get about 161 threads created,
and then pthread_create() blocks and I never get
to the "Max threads" message.
>
>
> >
> > 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?
No that is not what I expected. I expected
that pthread_create() should either:
- if it creates the thread successfully, return 0
- if it cannot create the thread, return EAGAIN
if a resource limit is it, or EINVAL if bad parameters
were passed down into the pthread_create() call
I never expected that pthread_create() would block.
> 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
Understood. Keep in mind, I am trying to "kick the tires"
on libpthread, to see how it reacts on crazy corner cases.
I can certainly increase the values of these sysctls,
but I think that pthread_create() should not block if
it hits a limit.
>
> 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.
Well, this may be so, but I don't think pthread_create() should block.
For kicks, I ran this code on Solaris 2.7, and pthread_create()
returned EAGAIN after spawning 3399 threads.
--
Craig Rodrigues
http://crodrigues.org
rodrigc@crodrigues.org
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