linux-kernel - Re: khugepaged: gets stuck when writing to USB flash, 2.6.38-rc2

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Date:	Thu, 3 Feb 2011 22:16:17 +0100
From:	Jindřich Makovička <makovick@...il.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: khugepaged: gets stuck when writing to USB flash, 2.6.38-rc2

On Thu, Feb 3, 2011 at 20:06, Andrea Arcangeli <aarcange@...hat.com> wrote:
> On Thu, Feb 03, 2011 at 01:24:08PM +0000, Mel Gorman wrote:
>> This is very likely. Compaction calls into migration which will wait on
>> dirty pages after a time. With a large number of dirty pages backed by a
>> slow drive such as a USB stick, it could be getting stalled there for a
>> long period of time.
>>
>> Whether migration sleeps or not can be controlled by the sync parameter
>> passed into try_to_compact_memory which could be always forced to false
>> if GFP_NO_KSWAPD?
>
> I would expect that to hide any regression we could have because of
> more dirty cache in the system, yes.
>
> However Jindřich reported not being able to reproduce anything anymore
> in -rc3, so I'm unsure if we should make that change anymore. I asked
> to try again cause it should too easy that got fixed magically

I tried again and reproduced with -rc3 too, sorry for misinformation.

I also tried echo never >  /sys/kernel/mm/transparent_hugepage/defrag
, and it seems preventing the system freeze, but still the copying
itself sometimes comes to almost complete stop (GkrellM shows short
spikes of tens of kB/s on the USB /dev/sdX). In this case, khugepaged
is also in DN. I still didn't observe this problem when disabling THP
completely by echo never > enabled .

-- 
Jindrich Makovicka
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