linux-kernel - Re: [Alsa-user] new source of MIDI playback slow-down identified - 5a03b051ed87e72b959f32a86054e1142ac4cf55 thp: use compaction in kswapd for GFP

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Date:	Tue, 22 Feb 2011 08:37:23 +0100
From:	Clemens Ladisch <cladisch@...glemail.com>
To:	Arthur Marsh <arthur.marsh@...ernode.on.net>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	alsa-user@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [Alsa-user] new source of MIDI playback slow-down identified
 - 5a03b051ed87e72b959f32a86054e1142ac4cf55 thp: use compaction in kswapd
 for GFP_ATOMIC order > 0

Arthur Marsh wrote:
> I'm experiencing MIDI playback slow-downs when I'm observing kswapd0 
> active (a few percent of cpu in top output) in recent kernels.
> 
> I git-bisected the problem down to:
> 
> commit 5a03b051ed87e72b959f32a86054e1142ac4cf55
> Author: Andrea Arcangeli <aarcange@...hat.com>
> Date:   Thu Jan 13 15:47:11 2011 -0800
> 
>      thp: use compaction in kswapd for GFP_ATOMIC order > 0
> 
>      This takes advantage of memory compaction to properly generate pages of
>      order > 0 if regular page reclaim fails and priority level becomes more
>      severe and we don't reach the proper watermarks.
> 
>      Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
>      Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
>      Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> 
> I ran git-bisect over the weekend, building and installing ALSA 1.0.24 
> with each kernel. After identifying the above commit, I rebuilt the 2.6 
> head with that commit reverted and verified that the problem was no 
> longer present.

Apparently, huge page compaction disables interrupts for much too long.

> MIDI playback was via aplaymidi -p 17:0 to a Soundblaster Audigy 2 ZS 
> (SB0350) wavetable.

The ALSA sequencer uses either the system timer or an HR timer at 1 kHz
to deliver MIDI commands (notes); the wavetable driver requires its own
interrupts in regular 5.3 ms intervals.


Regards,
Clemens
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