linux-kernel - Re: How can we make page replacement smarter (was: swap-prefetch)

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Date:	Sun, 29 Jul 2007 00:07:51 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Al Boldi <a1426z@...ab.com>
CC:	Chris Snook <csnook@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: How can we make page replacement smarter (was: swap-prefetch)

Al Boldi wrote:
> Chris Snook wrote:

>> At best, reads can be read-ahead and cached, which is why
>> sequential swap-in sucks less.  On-demand reads are as expensive as I/O
>> can get.
> 
> Which means that it should be at least as fast as swap-out, even faster 
> because write to disk is usually slower than read on modern disks.  But 
> linux currently shows a distinct 2x slowdown for sequential swap-in wrt 
> swap-out. 

That's because writes are faster than reads in moderate
quantities.

The disk caches writes, allowing the OS to write a whole
bunch of data into the disk cache and the disk can optimize
the IO a bit internally.

The same optimization is not possible for reads.

-- 
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