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

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Message-ID: <k2hd26f1ae01004151641l371404d9sb0fd36c5d7ff3388@mail.gmail.com>
Date:	Thu, 15 Apr 2010 16:41:44 -0700
From:	Suleiman Souhlal <suleiman@...gle.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Suleiman Souhlal <ssouhlal@...ebsd.org>,
	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
Subject: Re: [PATCH 1/4] vmscan: delegate pageout io to flusher thread if 
	current is kswapd

On Thu, Apr 15, 2010 at 4:33 PM, Dave Chinner <david@...morbit.com> wrote:
> On Thu, Apr 15, 2010 at 10:27:09AM -0700, Suleiman Souhlal wrote:
>>
>> 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?
>
> No. If you are doing full disk seeks between random chunks, then you
> still lose a large amount of throughput. e.g. if the seek time is
> 10ms and your IO time is 10ms for each 4k page, then increasing the
> size ito 64k makes it 10ms seek and 12ms for the IO. We might increase
> throughput but we are still limited to 100 IOs per second. We've
> gone from 400kB/s to 6MB/s, but that's still an order of magnitude
> short of the 100MB/s full size IOs with little in way of seeks
> between them will acheive on the same spindle...

What I meant was that, theoretically speaking, you could increase the
maximum amount of pages that get clustered so that you could get
100MB/s, although it most likely wouldn't be a good idea with the
current patch.

-- Suleiman
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