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

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Date:	Fri, 15 Jul 2011 11:00:03 +0100
From:	Andy Campbell <andycampbell.uk@...il.com>
To:	Theodore Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: High latency with ext4 / jbd2 kernel thread

Hi Ted, thanks very much for getting back to me

The problem started when I upgraded from Fedora 14 to Fedora 15.
Previously I was using ext3.  However, I was also using a different
firewire stack (the "old stack", instead of the new "Juju stack").
So, there have been several changes around the time that the problem
started, not only a change to Ext4.

I have tried many different buffer sizes on the audio software.
Previously this hardware worked perfectly with 3 * 256 buffers.  I've
tried up to 3 * 1024 buffers, but I still get the same problem.

I tried using barrier=0.
This stopped the "writing buffer to disk (synchronous)"  =
"jbd2_journal_commit_transaction" latency events from showing, which
is great.  However, I still get high latency from other events in the
system including "kworker/1:1 Executing raw SCSI command" and
"kworker/u:7 - Fork() system call".

These still cause drop-outs on my system.  It looks like either the
new stack is more sensitive to latency, or  something else changed in
the kernel between Fedora 14 and Fedora 15.

I think my conclusion is that my problems are not caused by Ext4, but
are something else... I will keep looking :O)

Thanks for your help!

A

On Tue, Jul 12, 2011 at 12:17 PM, Theodore Tso <tytso@....edu> wrote:
>
> On Jul 11, 2011, at 8:07 PM, 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.
>
> What were you using before?  ext3 ?   It may be that ext4's default use of
> barriers (for safety reasons) is causing a difference.   Try mounting your
> file systems with barrier=0.   For the root filesystem, add "rootflags=barrier=0".
> This is an unsafe way to run, in that if you have a power failure while you
> are writing to the disk, your file system(s) could get corrupted.   Ext3
> defaulted to this for historical reasons (although the enterprise Linux systems,
> RHEL and SuSE changed the default for safety reasons).
>
> If this solves the problem, I'd suggest that you look into finding ways to
> increase the buffer size of your audio system (were you recording or
> doing playback at the time).   You might also try to see if there was
> some amount of disk activity (especially write activity) that could be
> avoided or find some way of avoiding the use of fsync() so that you
> can re-enable write barriers without causing your latency skips.
>
> Regards,
>
> -- Ted
>
>
>
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