linux-kernel - Re: missing madvise functionality

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Message-Id: <20070403164455.83ae7b36.akpm@linux-foundation.org>
Date:	Tue, 3 Apr 2007 16:44:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jakub Jelinek <jakub@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: missing madvise functionality

On Tue, 3 Apr 2007 14:49:48 -0700
Andrew Morton <akpm@...ux-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 :(

Rohit solved this puzzle.

The 2-way is a single package, hyperthreaded.

The 8-way is two-package, four cores in each.

So on the 8-way, that lock is getting transferred between the two packages
like crazy.  Running the benchmark on just cpus 0 and 1 (taskset -c 0,1)
took the runtime down to eight seconds (from 52!) and the context switch
rate went up to 200,000/sec (from 45,000).


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