linux-kernel - Re: missing madvise functionality

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Date:	Thu, 05 Apr 2007 04:31:55 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Jakub Jelinek <jakub@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: missing madvise functionality

Eric Dumazet wrote:

> Could you please add this patch and see if it helps on your machine ?
> 
> [PATCH] VM : mm_struct's mmap_cache should be close to mmap_sem
> 
> Avoids cache line dirtying

I could, but I already know it's not going to help much.

How do I know this?  I already have 66% idle time when running
with my patch (and without Nick Piggin's patch to take the
mmap_sem for reading only).  Interestingly, despite the idle
time increasing from 10% to 66%, throughput triples...

Saving some CPU time will probably only increase the idle time,
I see no reason your patch would reduce contention and increase
throughput.

I'm not saying your patch doesn't make sense - it probably does.
I just suspect it would have zero impact on this particular
scenario, because of the already huge idle time.

-- 
Politics is the struggle between those who want to make their country
the best in the world, and those who believe it already is.  Each group
calls the other unpatriotic.
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