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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Message-ID: <4D6369E8.7080708@internode.on.net>
Date:	Tue, 22 Feb 2011 18:16:48 +1030
From:	Arthur Marsh <arthur.marsh@...ernode.on.net>
To:	Clemens Ladisch <cladisch@...glemail.com>
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



Clemens Ladisch wrote, on 22/02/11 18:07:
> 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
>

Hi, Andrea Arcangeli's "z1" patch (attached) solved the problem for me, 
even with significant swap activity.

Regards,

Arthur.

View attachment "z1" of type "text/plain" (1413 bytes)