Message-ID: <45B86930.2090709@linux.vnet.ibm.com>
Date: Thu, 25 Jan 2007 13:54:16 +0530
From: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Organization: IBM
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To: Al Boldi <a1426z@gawab.com>
CC: linux-kernel@vger.kernel.org
Subject: Re: [RFC] Limit the size of the pagecache
References: <200701250942.47321.a1426z@gawab.com>
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Al Boldi wrote:
> Rik van Riel wrote:
>> Christoph Lameter wrote:
>>> This is a patch using some of Aubrey's work plugging it in what is IMHO
>>> the right way. Feel free to improve on it. I have gotten repeatedly
>>> requests to be able to limit the pagecache.
>> IMHO it's a bad hack.
>>
>> It would be better to identify the problem this "feature" is
>> trying to fix, and then fix the root cause.
> 
> Ok, here is the problem:  kswapd.
> 
> Limiting the page-cache memory inhibits invoking kswapd needlessly, aiding 
> performance and easing OOM pressures.

Apart from kswapd, limiting pagecache helps performance of
applications by not eating away their ANON pages or other parts of its
resident data set.  When there is enough free memory, then there is no
performance issue.  However memory is always utilized to the max.
Hence every pagecache page that is allocated should come from some
application's RSS, or from cold pagecache page.  If that page was
stolen from some application, then that application pays the price for
swapping or reading the page back to memory.  This scenario is what we
 want to avoid.  All that we are trying to achieve is that pagecache
eats a (unmapped) pagecache page and not steal memory from other
important application's resident set.

Certainly this should be a configurable option and kernel's behavior
should not be changed in general.

> I tried the patch; it works.

:)

> But it needs a bit of debugging.  Setting pagecache_ratio = 1 either 
> deadlocks or reduces thru-put to < 1mb/s.

Yes, going below 5% on my 1GB RAM machine causes severe performance
problems.  We need to hard wire a reasonable lower limit and not
provide a noose for the end user to tie around!

--Vaidy

> 
> Thanks!
> 
> --
> Al
> 
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