linux-kernel - Re: [PATCH 1/3] mm: vmscan: Do use use PF_SWAPWRITE from zone

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Date:	Tue, 12 Jul 2011 18:55:58 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	mgorman@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	cl@...ux.com
Subject: Re: [PATCH 1/3] mm: vmscan: Do use use PF_SWAPWRITE from zone_reclaim

Hi KOSAKi,

On Tue, Jul 12, 2011 at 6:40 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
> (2011/07/12 18:27), Minchan Kim wrote:
>> Hi Mel,
>>
>> On Mon, Jul 11, 2011 at 10:01 PM, Mel Gorman <mgorman@...e.de> wrote:
>>> Zone reclaim is similar to direct reclaim in a number of respects.
>>> PF_SWAPWRITE is used by kswapd to avoid a write-congestion check
>>> but it's set also set for zone_reclaim which is inappropriate.
>>> Setting it potentially allows zone_reclaim users to cause large IO
>>> stalls which is worse than remote memory accesses.
>>
>> As I read zone_reclaim_mode in vm.txt, I think it's intentional.
>> It has meaning of throttle the process which are writing large amounts
>> of data. The point is to prevent use of remote node's free memory.
>>
>> And we has still the comment. If you're right, you should remove comment.
>> "         * and we also need to be able to write out pages for RECLAIM_WRITE
>>          * and RECLAIM_SWAP."
>>
>>
>> And at least, we should Cc Christoph and KOSAKI.
>
> Of course, I'll take full ack this. Do you remember I posted the same patch
> about one year ago. At that time, Mel disagreed me and I'm glad to see he changed
> the mind. :)


I remember that but I don't know why Mel didn't ack at that time.
http://lkml.org/lkml/2010/8/5/44

Anyway, Hannes's bd2f6199cf is to introduce lumpy reclaim of
zone_reclaim so it's natural to increase latency for getting big order
pages(ie, it's a trade-off).

And as I read about zone_reclaim_mode in Documentation/sysctl/vm.txt,
I think big latency(ie, throttling of the process) is intentional to
prevent stealing pages for other nodes.

If I am not against this patch, at least, we need agreement of
Christoph and others and if we agree this change, we changes vm.txt,
too.


-- 
Kind regards,
Minchan Kim
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