linux-kernel - Re: [PATCH 1/4] vmscan: delegate pageout io to flusher thread if current is kswapd

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Date:	Thu, 15 Apr 2010 10:27:09 -0700
From:	Suleiman Souhlal <ssouhlal@...ebsd.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, suleiman@...gle.com
Subject: Re: [PATCH 1/4] vmscan: delegate pageout io to flusher thread if current is kswapd


On Apr 15, 2010, at 2:32 AM, Dave Chinner wrote:

> On Thu, Apr 15, 2010 at 01:05:57AM -0700, Suleiman Souhlal wrote:
>>
>> On Apr 14, 2010, at 9:11 PM, KOSAKI Motohiro wrote:
>>
>>> Now, vmscan pageout() is one of IO throuput degression source.
>>> Some IO workload makes very much order-0 allocation and reclaim
>>> and pageout's 4K IOs are making annoying lots seeks.
>>>
>>> At least, kswapd can avoid such pageout() because kswapd don't
>>> need to consider OOM-Killer situation. that's no risk.
>>>
>>> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
>>
>> What's your opinion on trying to cluster the writes done by pageout,
>> instead of not doing any paging out in kswapd?
>
> XFS already does this in ->writepage to try to minimise the impact
> of the way pageout issues IO. It helps, but it is still not as good
> as having all the writeback come from the flusher threads because
> it's still pretty much random IO.

Doesn't the randomness become irrelevant if you can cluster enough
pages?

> And, FWIW, it doesn't solve the stack usage problems, either. In
> fact, it will make them worse as write_one_page() puts another
> struct writeback_control on the stack...

Sorry, this patch was not meant to solve the stack usage problems.

-- Suleiman
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