linux-kernel - Re: missing madvise functionality

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Message-Id: <20070405110601.8ef4b601.dada1@cosmosbay.com>
Date:	Thu, 5 Apr 2007 11:06:01 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Rik van Riel <riel@...hat.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

On Thu, 05 Apr 2007 04:31:55 -0400
Rik van Riel <riel@...hat.com> wrote:

> 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.

I know your cpus have idle time, that not the question.

But *when* your cpus are not idle, they might be slowed down because of cache line transferts between them. This patch doesnt reduce contention, just latencies (and overall performance)

I dont currently have SMP test machine, so I couldnt test it myself.

On x86_64, I am pretty sure the patch would help, because offsetof(mmap_sem) = 0x60
On i386, offsetof(mmap_sem)=0x34, so this patch wont help.

As you said, throughput can raise and idle time raise too.

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