linux-kernel - Re: [PATCH -mm 2/2] mm: kswapd carefully invoke compaction

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Date:	Wed, 11 Jan 2012 08:34:06 -0500
From:	Rik van Riel <riel@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	linux-mm@...ck.org, aarcange@...hat.com,
	linux-kernel@...r.kernel.org, Mel Gorman <mel@....ul.ie>,
	akpm@...ux-foundation.org, Johannes Weiner <hannes@...xchg.org>,
	hughd@...gle.com
Subject: Re: [PATCH -mm 2/2] mm: kswapd carefully invoke compaction

On 01/11/2012 02:25 AM, KOSAKI Motohiro wrote:
>> With CONFIG_COMPACTION enabled, kswapd does not try to free
>> contiguous free pages, even when it is woken for a higher order
>> request.
>>
>> This could be bad for eg. jumbo frame network allocations, which
>> are done from interrupt context and cannot compact memory themselves.
>> Higher than before allocation failure rates in the network receive
>> path have been observed in kernels with compaction enabled.
>>
>> Teach kswapd to defragment the memory zones in a node, but only
>> if required and compaction is not deferred in a zone.
>>
>> Signed-off-by: Rik van Riel<riel@...hat.com>
>
> I agree with we need asynchronous defragmentations feature. But, do we
> really need to use kswapd for compaction? While kswapd take a
> compaction work, it can't work to make free memory.

I believe we do need some background compaction, especially
to help allocations from network interrupts.

If you believe the compaction is better done from some
other thread, I guess we could do that, but truthfully, if
kswapd spends a lot of time doing compaction, I made a
mistake somewhere :)

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