Subject: Re: Poor desktop responsiveness with background I/O-operations
From: Mike Galbraith <efault@gmx.de>
To: "Justin P. Mattock" <justinmattock@gmail.com>
Cc: Ulrich Lukas <stellplatz-nr.13a@datenparkplatz.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Date: Mon, 21 Sep 2009 06:23:54 +0200
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On Sun, 2009-09-20 at 17:22 -0700, Justin P. Mattock wrote:
> Wow, at first I did not notice anything
> from your *.sh script, but then a few moments
> later, the system became vary sluggish. firefox
> takes about 15 seconds to load. OS is mint linux
> with the latest HEAD on an imac9,1.

CFQ inherits IO priority from the parent on fork(), so nicing a shell or
the dd should (and does) help a LOT.  It also inherits the CPU scheduler
class.  I just checked, and IO does indeed inherit SCHED_IDLE as well as
the RT classes (SCHED_BATCH does nothing).  Setting the dd to SCHED_IDLE
makes a big difference, as it should, so there's a bigger hammer than
nice + ionice.  SCHED_IDLE is CPU_adorable + IO_nicest.

Anyway, the problem seems to have to have at least two facets:
1.  streaming IO causing pagecache reclaim.  That problem doesn't seem
to want to go away.  Fixing that won't help 2 one bit though.

2.  reads are sync, more heavily affected by seek latency than writes.
If the disk your binaries live on is seeking a lot, you pay a LOT.  

Hohum, IO + VM = haaaaard ;-)

	-Mike

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