Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 2 Oct 2008 10:22:13 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:

> On Thu, Oct 02 2008, Andrew Morton wrote:
> > On Thu, 2 Oct 2008 09:45:24 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:
> > 
> > > > So.  Where are these atime updaters getting blocked?
> > > 
> > > Behind other IO activity I suppose, since it's marked async. A more
> > > appropriate fix may be to mark atime updates as sync IO.
> > 
> > No, they might be getting blocked at a higher level.
> > 
> > An async atime update gets recorded into the current transaction. 
> > kjournald is working on the committing transaction.  We try to keep
> > those separated, to prevent user processes from getting blocked behind
> > kjournald activity.
> > 
> > But sometimes that doesn't work (including the place where I knowingly
> > broke it).  If we can find and fix the offending piece of jbd logic (a
> > big if) then all is peachy.
> > 
> > If the above theory turns out to be true then diddling IO priorities
> > is but a workaround.
> 
> If diddling with io priorities makes a big difference (and Arjan said it
> does), it's clearly stuck inside the io scheduler waiting to be
> dispatched. If it was marked sync, it would be going through much
> faster. As async, it'll get mixed in with other async writeout and that
> doesn't get a lot of attention in case of sync activity.
> 

Maybe.  And maybe marking all journal writeout and all commit activity
as sync would have the same effect with less downside.

But it'd be better to not wait on this IO at all, if poss..