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

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Message-Id: <200707281411.57823.a1426z@gawab.com>
Date:	Sat, 28 Jul 2007 14:11:57 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: How can we make page replacement smarter (was: swap-prefetch)

Chris Snook wrote:
> Al Boldi wrote:
> > Because it is hard to quantify the expected swap-in speed for random
> > pages, let's first tackle the swap-in of consecutive pages, which should
> > be at least as fast as swap-out.  So again, why is swap-in so slow?
>
> If I'm writing 20 pages to swap, I can find a suitable chunk of swap and
> write them all in one place.  If I'm reading 20 pages from swap, they
> could be anywhere.  Also, writes get buffered at one or more layers of
> hardware.

Ok, this explains swap-in of random pages.  Makes sense, but it doesn't 
explain the awful tmpfs performance degradation of consecutive read-in runs 
from swap, which should have at least stayed constant

> 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.  And to prove this point, just try suspend to disk where you can 
see sequential swap-out being reported at about twice the speed of 
sequential swap-in on resume.  Why is that?

Thanks!

--
Al

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