linux-kernel - Re: [PATCH] vmscan: limit concurrent reclaimers in shrink

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Date:	Mon, 14 Dec 2009 09:40:38 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	lwoodman@...hat.com, kosaki.motohiro@...fujitsu.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, aarcange@...hat.com
Subject: Re: [PATCH] vmscan: limit concurrent reclaimers in shrink_zone

On 12/14/2009 08:08 AM, Andi Kleen wrote:
> Rik van Riel<riel@...hat.com>  writes:
>
>> +max_zone_concurrent_reclaim:
>> +
>> +The number of processes that are allowed to simultaneously reclaim
>> +memory from a particular memory zone.
>> +
>> +With certain workloads, hundreds of processes end up in the page
>> +reclaim code simultaneously.  This can cause large slowdowns due
>> +to lock contention, freeing of way too much memory and occasionally
>> +false OOM kills.
>> +
>> +To avoid these problems, only allow a smaller number of processes
>> +to reclaim pages from each memory zone simultaneously.
>> +
>> +The default value is 8.
>
> I don't like the hardcoded number. Is the same number good for a 128MB
> embedded system as for as 1TB server?  Seems doubtful.
>
> This should be perhaps scaled with memory size and number of CPUs?

The limit is per _zone_, so the number of concurrent reclaimers
is automatically scaled by the number of memory zones in the
system.

Scaling up the per-zone value as well looks like it could lead
to the kind of lock contention we are aiming to avoid in the
first place.

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