Re: missing madvise functionality

Andrew Morton
2007-04-03T21:50:11Z

On Tue, 3 Apr 2007 16:29:37 -0400
Jakub Jelinek <jakub@redhat.com> wrote:

> On Tue, Apr 03, 2007 at 01:17:09PM -0700, Ulrich Drepper wrote:
> > Andrew Morton wrote:
> > > Ulrich, could you suggest a little test app which would demonstrate this
> > > behaviour?
> >
> > It's not really reliably possible to demonstrate this with a small
> > program using malloc. You'd need something like this mysql test case
> > which Rik said is not hard to run by yourself.
> >
> > If somebody adds a kernel interface I can easily produce a glibc patch
> > so that the test can be run in the new environment.
> >
> > But it's of course easy enough to simulate the specific problem in a
> > micro benchmark. If you want that let me know.
>
> I think something like following testcase which simulates what free
> and malloc do when trimming/growing a non-main arena.
>
> My guess is that all the page zeroing is pretty expensive as well and
> takes significant time, but I haven't profiled it.
>
> #include <pthread.h>
> #include <stdlib.h>
> #include <sys/mman.h>
> #include <unistd.h>
>
> void *
> tf (void *arg)
> {
> (void) arg;
> size_t ps = sysconf (_SC_PAGE_SIZE);
> void *p = mmap (NULL, 128 * ps, PROT_READ | PROT_WRITE,
> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> if (p == MAP_FAILED)
> exit (1);
> int i;
> for (i = 0; i < 100000; i++)
> {
> /* Pretend to use the buffer. */
> char *q, *r = (char *) p + 128 * ps;
> size_t s;
> for (q = (char *) p; q < r; q += ps)
> *q = 1;
> for (s = 0, q = (char *) p; q < r; q += ps)
> s += *q;
> /* Free it. Replace this mmap with
> madvise (p, 128 * ps, MADV_THROWAWAY) when implemented. */
> if (mmap (p, 128 * ps, PROT_NONE,
> MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0) != p)
> exit (2);
> /* And immediately malloc again. This would then be deleted. */
> if (mprotect (p, 128 * ps, PROT_READ | PROT_WRITE))
> exit (3);
> }
> return NULL;
> }
>
> int
> main (void)
> {
> pthread_t th[32];
> int i;
> for (i = 0; i < 32; i++)
> if (pthread_create (&th[i], NULL, tf, NULL))
> exit (4);
> for (i = 0; i < 32; i++)
> pthread_join (th[i], NULL);
> return 0;
> }
>

whee. 135,000 context switches/sec on a slow 2-way. mmap_sem, most
likely. That is ungood.

Did anyone monitor the context switch rate with the mysql test?

Interestingly, your test app (with s/100000/1000) runs to completion in 13
seocnd on the slow 2-way. On a fast 8-way, it took 52 seconds and
sustained 40,000 context switches/sec. That's a bit unexpected.

Both machines show ~8% idle time, too :(