linux-kernel - Re: sluggish system responsiveness under higher IO load

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Date:	Mon, 9 Oct 2006 01:50:26 +0200
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Jens Axboe" <jens.axboe@...cle.com>
Cc:	Christian <christiand59@....de>, linux-kernel@...r.kernel.org
Subject: Re: sluggish system responsiveness under higher IO load

On 08/10/06, Jens Axboe <jens.axboe@...cle.com> wrote:
> On Sun, Oct 08 2006, Christian wrote:
> > Am Freitag, 6. Oktober 2006 17:58 schrieb Paolo Ornati:
> > > On Sun, 13 Aug 2006 18:15:12 +0200
> > >
> > > Matthias Dahl <mlkernel@...tal-soul.de> wrote:
> > > > Just let me know once you got them, so I can safely delete them again.
> > > >
> > > > At the moment, I am trying without preemption but for example doing a
> > > > untar kernel sources still results in sluggish system responsiveness. :-(
> > >
> > > I used to have this type of problem and 2.6.19-rc1 looks much better
> > > than 2.6.18.
> > >
> > > I'm using CONFIG_PREEMPT + CONFIG_PREEMPT_BKL, CFQ i/o scheduler
> > > and /proc/sys/vm/swappiness = 20.
> >
> >
> > Which change in the new kernel has made it better? I was following the lkml
> > very close and didn't see any change that could have fixed that problem.
>
> There is a substantial CFQ update, so it could be that. Or it could be
> something unrelated of course, I didn't check if eg the cpu scheduler
> changed much. Or vm :-)
>
> --
> Jens Axboe
>
I want to chime in here and let you know that I've experienced
something similar.

I'm using CFQ as my default I/O schedular.
Since 2.6.18-git<something_I'm_not_sure_of> I've experienced that when
doing heavy (or even not so heave) disk I/O my system gets very
sluggish. Observable by the fact that my mouse pointer in X "jumps"
which it never did before, and switching windows I can see the new
window repaint slowly whereas earlier it would just snap onto the
screen.

<serious hand-waving enabled>
This is quite unreliable, but I *seem* to have observed a slightly
higher overall memory use for my system since 2.6.18+ as well as the
software interrupt rate (as observable by 'top' oscilating between 1.5
& 5 % with 2.6.18+ where with older kernels it would seem to mostly
stay below 1%.
I've also observed that when rebooting, unmounting my local
filesystems takes significantly longer (previously just a second or
two, recently up to a minute or not at all, that is, the system just
hangs) - may or may not be related to this.
<serious hand-waving disabled>

In any case, disk I/O seems to have a large negative impact on system
performance recently compared to pre-2.6.18* kernels.

-- 
Jesper Juhl <jesper.juhl@...il.com>
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