linux-kernel - Re: Improvement on memory subsystem

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Date:	Wed, 19 Jul 2006 10:18:44 +0100
From:	Ian Stirling <tandra@...ve.plus.com>
To:	Valdis.Kletnieks@...edu
CC:	yunfeng zhang <zyf.zeroos@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Improvement on memory subsystem

Valdis.Kletnieks@...edu wrote:
> On Tue, 18 Jul 2006 18:03:54 +0800, yunfeng zhang said:
> 
> 
>>2. Read-ahead process during page-in/out (page fault or swap out) should be
>>based on its VMA to enhance IO efficiency instead of the relative physical pages
>>in swap space.
> 
> 
> But wouldn't that end up causing a seek storm, rather than handling the pages
> in the order that minimizes the total seek distance, no matter where they are
> in memory? Remember - if you have a 2Ghz processor, and a disk that seeks in 1
> millisecond, every seek is (*very* roughly) about 2 million instructions.  So
> if we can burn 20 thousand instructions finding a read order that eliminates
> *one* seek, we're 1.98M instructions ahead.

To paraphrase shakespear - all the world is not a P4 - and all the swap 
devices are not hard disks.

For example - I've got a 486/33 laptop with 12M RAM that I sometimes use 
, with swapping to a 128M PCMCIA RAM card that I got from somewhere.

20K instructions wasted on a device with no seek time is just annoying.

And on my main laptop - I have experimented with swap-over-wifi to a 
large ramdisk on my server - which works quite well. (until the wifi 
connection falls over).
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