linux-kernel - Re: [PATCH v2] vmscan: limit concurrent reclaimers in shrink

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Date:	Fri, 18 Dec 2009 09:09:46 -0500
From:	Rik van Riel <riel@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	lwoodman@...hat.com, LKML <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v2] vmscan: limit concurrent reclaimers in shrink_zone

On 12/18/2009 05:27 AM, KOSAKI Motohiro wrote:
>> KOSAKI Motohiro wrote:

>> Finally, having said all that, the system still struggles reclaiming
>> memory with
>> ~10000 processes trying at the same time, you fix one bottleneck and it
>> moves
>> somewhere else.  The latest run showed all but one running process
>> spinning in
>> page_lock_anon_vma() trying for the anon_vma_lock.  I noticed that there
>> are
>> ~5000 vma's linked to one anon_vma, this seems excessive!!!
>>
>> I changed the anon_vma->lock to a rwlock_t and page_lock_anon_vma() to use
>> read_lock() so multiple callers could execute the page_reference_anon code.
>> This seems to help quite a bit.
>
> Ug. no. rw-spinlock is evil. please don't use it. rw-spinlock has bad
> performance characteristics, plenty read_lock block write_lock for very
> long time.
>
> and I would like to confirm one thing. anon_vma design didn't change
> for long year. Is this really performance regression? Do we strike
> right regression point?

In 2.6.9 and 2.6.18 the system would hit different contention
points before getting to the anon_vma lock.  Now that we've
gotten the other contention points out of the way, this one
has finally been exposed.

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