Date: Fri, 16 Apr 2010 10:50:02 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Dave Chinner <david@fromorbit.com>
Cc: Suleiman Souhlal <ssouhlal@freebsd.org>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Mel Gorman <mel@csn.ul.ie>, Chris Mason <chris.mason@oracle.com>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       linux-fsdevel@vger.kernel.org, suleiman@google.com
Subject: Re: [PATCH 1/4] vmscan: delegate pageout io to flusher thread if
 current is kswapd
Message-ID: <20100416105002.191adeb1@lxorguk.ukuu.org.uk>
In-Reply-To: <20100415233339.GW2493@dastard>
References: <20100415013436.GO2493@dastard>
	<20100415130212.D16E.A69D9226@jp.fujitsu.com>
	<20100415131106.D174.A69D9226@jp.fujitsu.com>
	<64BE60A8-EEF9-4AC6-AF0A-0ED3CB544726@freebsd.org>
	<20100415093214.GV2493@dastard>
	<85DB7083-8E78-4884-9E76-5BD803C530EF@freebsd.org>
	<20100415233339.GW2493@dastard>
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> 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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