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Date: Thu, 5 Apr 2007 01:32:25 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Rik van Riel <riel@redhat.com>
Cc: Jakub Jelinek <jakub@redhat.com>, Ulrich Drepper <drepper@redhat.com>,
       Andi Kleen <andi@firstfloor.org>,
       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 Thu, 05 Apr 2007 03:39:29 -0400 Rik van Riel <riel@redhat.com> wrote:

> Rik van Riel wrote:
> 
> > MADV_DONTNEED, unpatched, 1000 loops
> > 
> > real    0m13.672s
> > user    0m1.217s
> > sys     0m45.712s
> > 
> > 
> > MADV_DONTNEED, with patch, 1000 loops
> > 
> > real    0m4.169s
> > user    0m2.033s
> > sys     0m3.224s
> 
> I just noticed something fun with these numbers.
> 
> Without the patch, the system (a quad core CPU) is 10% idle.
> 
> With the patch, it is 66% idle - presumably I need Nick's
> mmap_sem patch.
> 
> However, despite being 66% idle, the test still runs over
> 3 times as fast!

Please quote the context switch rate when testing this stuff (I use vmstat 1).
I've seen it vary by a factor of 10,000 depending upon what's happening.