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

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Date:	Fri, 16 Apr 2010 10:50:02 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
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, suleiman@...gle.com
Subject: Re: [PATCH 1/4] vmscan: delegate pageout io to flusher thread if
 current is kswapd

> 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...

The usual armwaving numbers for ops/sec for an ATA disk are in the 200
ops/sec range so that seems horribly credible.

But then I've never quite understood why our anonymous paging isn't
sorting stuff as best it can and then using the drive as a log structure
with in memory metadata so it can stream the pages onto disk. Read
performance is goig to be similar (maybe better if you have a log tidy
when idle), write ought to be far better.

Alan
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