linux-ext4 - Re: High latency with ext4 / jbd2 kernel thread

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Date:	Tue, 12 Jul 2011 18:48:14 +0800
From:	Tao Ma <tm@....ma>
To:	Andy Campbell <andycampbell.uk@...il.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: High latency with ext4 / jbd2 kernel thread

Hi Andy,
On 07/12/2011 08:07 AM, Andy Campbell wrote:
> Hi ext4 mailing list members,
> 
> I hope you can help me with a problem.
> 
> I recently upgraded my main audio PC to Fedora FC15 and, at the same
> time, reformatted by root drive to ext4.  Since this upgrade this, my
> system has high system latency which is causing my audio hardware
> (firewire) to experience buffer under-runs (XRUNs).  This causes
> "unhandled xrun" errors and the audio software fails.
> 
> My audio software (jackd) is running in realtime mode with SCHED_FIFO priority.
> 
> I have been using latencytop (http://www.latencytop.org/) to try to
> understand what is causing the system latency.
> Typically my system runs with a max latency around 5ms.  If i run a
> disk heavy operation (such as updatedb) I see a max latency of around
> 20ms.
> 
> Around the time that jackd fails, I can see events like this with a
> latency of 40 - 120 ms:
> "writing buffer to disk (synchronous)"  = "jbd2_journal_commit_transaction"
> Screenshot here
> http://dl.dropbox.com/u/3289905/latencytop.png
I guess the reason may be some regression.
I have sent a patch to linux-ext4 with the subject
"jbd/2[stable only]: Use WRITE_SYNC_PLUG in journal_commit_transaction."

Can you try it to see if it helps.
> 
> My kernel version is
> Linux speedy.local 2.6.38.8-32.fc15.i686.PAE #1 SMP Mon Jun 13
> 19:55:27 UTC 2011 i686 i686 i386 GNU/Linu
> 
> The ext4 drive is mounted with these options:
> /dev/sda4 on / type ext4 (rw,relatime,seclabel,barrier=1,data=ordered)
> I also tried mounting using using noatime and also using
> data=writeback options.  Unfortunately neither of these parameters
> resolved the problem

Thanks
Tao
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