linux-kernel - Re: [PATCH] vmscan: improve reclaim throuput to bail out patch

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Date:	Wed, 03 Dec 2008 08:46:23 -0500
From:	Rik van Riel <riel@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, mel@....ul.ie
Subject: Re: [PATCH] vmscan: improve reclaim throuput to bail out patch

KOSAKI Motohiro wrote:
> Hi
> 
> I evaluate rvr bailout and skip-freeing patch in this week conteniously.
> I'd like to dump first output here.
> 
> 
> 
> Rik, could you please review following?
> ==
> vmscan bail out patch move nr_reclaimed variable to struct scan_control.
> Unfortunately, indirect access can easily happen cache miss.
> More unfortunately, Some architecture (e.g. ia64) don't access global
> variable so fast.

That is amazing.  Especially considering that the scan_control
is a local variable on the stack.

> if heavy memory pressure happend, that's ok.
> cache miss already plenty. it is not observable.
> 
> but, if memory pressure is lite, performance degression is obserbable.

> about 4-5% degression.
> 
> Then, this patch introduce temporal local variable.

> OK. the degression is disappeared.

I can't argue with the numbers, though :)

Maybe all the scanning we do ends up evicting the cache lines
with the scan_control struct in it from the fast part of the
CPU cache?

> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>

Acked-by: Rik van Riel <riel@...hat.com>

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