linux-kernel - Re: Improvement on memory subsystem

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Message-Id: <200607191456.k6JEuW0x004945@turing-police.cc.vt.edu>
Date:	Wed, 19 Jul 2006 10:56:32 -0400
From:	Valdis.Kletnieks@...edu
To:	Ian Stirling <tandra@...ve.plus.com>
Cc:	yunfeng zhang <zyf.zeroos@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Improvement on memory subsystem

On Wed, 19 Jul 2006 10:18:44 BST, Ian Stirling said:

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

Been there, done that.  I used to admin a net of Sun 3/50s where /dev/swap
was a symlink to a file on an NFS server, because the "shoebox" local hard
drives for those were so slow that throwing it across the ethernet to a
3/280 with Fujitsu Super-Eagles was faster...

> 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.

If we go to the effort of writing code that tries to be smart about grouping
swap reads/writes by cost, it's easy enough to flag any sort of ram-disk device
as a 'zero seek time' device.  Remember that I suggested making it dependent
on "how long until the next pass of the elevator" - for a ramdisk that basically
is zero, so the algorithm easily degenerates into "just queue the requests in
expected order you'll need the results".

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

On the other hand, how long does it take to move a 4K page across the
PCMCIA interface?  If you're seeing deep queues on it, you may *still*
want to optimize the order of requests...

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