Re: missing madvise functionality

[Eric Dumazet]

Andrew Morton a écrit :
> On Tue, 3 Apr 2007 16:29:37 -0400
> Jakub Jelinek <jakub@xxxxxxxxxx> 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 :(

Yes... then add to this some futex work, and you get the picture.

I do think such workloads might benefit from a vma_cache not shared by all 
threads but private to each thread. A sequence could invalidate the cache(s).

ie instead of a mm->mmap_cache, having a mm->sequence, and each thread having 
a current->mmap_cache and current->mm_sequence

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