Re: [FFADO-user] FFADO-user Digest, Vol 52, Issue 9

[Stefan R. <st...@s5...>]

On Jul 12 Arnold Krille wrote:
> On Tuesday 12 July 2011 00:09:34 Stefan Richter wrote:
> > On Jul 11 Andy Campbell wrote:
> > > I believe that every time I refresh latencytop after an
> > > unhandled XRUN, I see a "process" named jbd2/sda4-8 with >40 ms
> > > latency.
[...]
> > jbd2 is a kernel thread; it deals with the journal on sda4.  jbd2 is used
> > by the ext4 and ocfs2 filesystems.
> > 
> > Its development mailinglist is lin...@vg....  It's probably
> > best to let the developers know that the journalling process appears to
> > heavily impact audio latency.
> 
> Better not write the kernel devs about that without good prove. ext[2-4] are 
> working very well for audio for a lot of people. It would be surprising if 
> this was really the cause for problems no one with similar setups and usage 
> encountered before...

I suppose if Andy switches to CONFIG_PREEMPT=y, any latency in the jbd2
thread should become a non-issue.  AFAIU jackd/ffado userspace threads are
going to be able to preempt the jbd2 kernel thread with CONFIG_PREEMPT=y.

If a few common distributions install "/boot/config-$(uname -r)" --- 
or "/lib/modules/$(uname -r)/{build,source}/.config" ? --- it would be
nice to have ffado-diag check for CONFIG_PREEMPT=y or CONFIG_PREEMPT_RT=y
there.  Perhaps I'll send a patch.
-- 
Stefan Richter
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http://arcgraph.de/sr/