linux-kernel - Re: [ck] Re: [PATCH] mm: swap prefetch improvements

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Message-Id: <20070522214222.8b197247.thatistosayiseenem@gawab.com>
Date:	Tue, 22 May 2007 21:42:22 +0100
From:	Ash Milsted <thatistosayiseenem@...ab.com>
To:	ck@....kolivas.org
Cc:	linux-kernel@...r.kernel.org, ck list <ck@....kolivas.org>
Subject: Re: [ck] Re: [PATCH] mm: swap prefetch improvements

On Tue, 22 May 2007 20:37:54 +1000
Con Kolivas <kernel@...ivas.org> wrote:

> On Tuesday 22 May 2007 20:25, Ingo Molnar wrote:
> > * Con Kolivas <kernel@...ivas.org> wrote:
> > > > > there was nothing else running on the system - so i suspect the
> > > > > swapin activity flagged 'itself' as some 'other' activity and
> > > > > stopped? The swapins happened in 4 bursts, separated by 5 seconds
> > > > > total idleness.
> > > >
> > > > I've noted burst swapins separated by some seconds of pause in my
> > > > desktop system too (with sp_tester and an idle gnome).
> > >
> > > That really is expected, as just about anything, including journal
> > > writeout, would be enough to put it back to sleep for 5 more seconds.
> >
> > note that nothing like that happened on my system - in the
> > swap-prefetch-off case there was _zero_ IO activity during the sleep
> > period.
> 
> Ok, granted it's _very_ conservative. I really don't want to risk its presence 
> being a burden on anything, and the iowait it induces probably makes it turn 
> itself off for another PREFETCH_DELAY (5s). I really don't want to cross the 
> line to where it is detrimental in any way. Not dropping out on a 
> cond_resched and perhaps making the delay tunable should be enough to make it 
> a little less "sleepy".
> 
> -- 
> -ck

Hi. I just did some video encoding on my desktop and I was noticing
(for the first time in a while) that running apps had to hit swap quite
a lot when I switched to them (the encoding was going at full blast for
most of the day, and most of the time other running apps were
idle). Now, a good half of my RAM appeared to be free during all this,
so I was thinking at the time that it would be nice if swap prefetch
could be tunably more aggressive. I guess it would be ideal in this
case if it could kick in during tunably low disk-IO periods, even if
the CPU is rather busy. I'm sure you've considered this, so I only butt
in here to cast a vote for it. :)

Of course, I could be completely wrong about the possibility.. and I
seem to remember that the disk cache can take up about half the ram by
default without this showing up in 'gnome-system-monitor'... which I
guess might happen during heavy encoding.. but even if it did, I could
have set the limit lower, and would then have still appreciated
prefetching.

Ash
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