Re: Excessive stall times on ext4 in 3.9-rc2

[Dave Chinner <david@fromorbit.com>]

On Fri, Apr 12, 2013 at 11:19:52AM -0400, Theodore Ts'o wrote:
> On Fri, Apr 12, 2013 at 02:50:42PM +1000, Dave Chinner wrote:
> > > If that is the case, one possible solution that comes to mind would be
> > > to mark buffer_heads that contain metadata with a flag, so that the
> > > flusher thread can write them back at the same priority as reads.
> > 
> > Ext4 is already using REQ_META for this purpose.
> 
> We're using REQ_META | REQ_PRIO for reads, not writes.
> 
> > I'm surprised that no-one has suggested "change the IO elevator"
> > yet.....
> 
> Well, testing to see if the stalls go away with the noop schedule is a
> good thing to try just to validate the theory.

Exactly.

> The thing is, we do want to make ext4 work well with cfq, and
> prioritizing non-readahead read requests ahead of data writeback does
> make sense.  The issue is with is that metadata writes going through
> the block device could in some cases effectively cause a priority
> inversion when what had previously been an asynchronous writeback
> starts blocking a foreground, user-visible process.

Here's the historic problem with CFQ: it's scheduling algorithms
change from release to release, and so what you tune the filesystem
to for this release is likely to cause different behaviour
in a few releases time.

We've had this problem time and time again with CFQ+XFS, so we
stopped trying to "tune" to a particular elevator long ago.  The
best you can do it tag the Io as appropriately as possible (e.g.
metadata with REQ_META, sync IO with ?_SYNC, etc), and then hope CFQ
hasn't been broken since the last release....

> At least, that's the theory; we should confirm that this is indeed
> what is causing the data stalls which Mel is reporting on HDD's before
> we start figuring out how to fix this problem.

*nod*.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com