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Date: Tue, 3 Apr 2007 16:02:31 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Jakub Jelinek <jakub@redhat.com>, Ulrich Drepper <drepper@redhat.com>,
       Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
       Hugh Dickins <hugh@veritas.com>
Subject: Re: missing madvise functionality
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On Tue, 3 Apr 2007 14:49:48 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> > 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 :(

All of which indicates that if we can remove the down_write(mmap_sem) from
this glibc operation, things should get a lot better - there will be no
additional context switches at all.

And we can surely do that if all we're doing is looking up pageframes,
putting pages into fake-swapcache and moving them around on the page LRUs.

Hugh?  Sanity check?

That difference between the 2-way and the 8-way sure is odd.