Re: regression in page writeback

[Dave Chinner <david@fromorbit.com>]

On Thu, Sep 24, 2009 at 08:38:20PM -0400, Chris Mason wrote:
> On Fri, Sep 25, 2009 at 10:11:17AM +1000, Dave Chinner wrote:
> > On Thu, Sep 24, 2009 at 11:15:08AM +0800, Wu Fengguang wrote:
> > > On Wed, Sep 23, 2009 at 10:00:58PM +0800, Chris Mason wrote:
> > > > The only place that actually honors the congestion flag is pdflush.
> > > > It's trivial to get pdflush backed up and make it sit down without
> > > > making any progress because once the queue congests, pdflush goes away.
> > > 
> > > Right. I guess that's more or less intentional - to give lowest priority
> > > to periodic/background writeback.
> > 
> > IMO, this is the wrong design. Background writeback should
> > have higher CPU/scheduler priority than normal tasks. If there is
> > sufficient dirty pages in the system for background writeback to
> > be active, it should be running *now* to start as much IO as it can
> > without being held up by other, lower priority tasks.
> 
> I'd say that an fsync from mutt or vi should be done at a higher prio
> than a background streaming writer.

I don't think you caught everything I said - synchronous IO is
un-throttled. Background writeback should dump async IO to the
elevator as fast as it can, then get the hell out of the way. If
you've got a UP system, then the fsync can't be issued at the same
time pdflush is running (same as right now), and if you've got a MP
system then fsync can run at the same time. On the premise that sync
IO is unthrottled and given that elevators queue and issue sync IO
sperately to async writes, fsync latency would be entirely derived
from the elevator queuing behaviour, not the CPU priority of
pdflush.

Look at it this way - it is the responsibility of pdflush to keep
the elevator full of background IO. It is the responsibility of
the elevator to ensure that background IO doesn't starve all other
types of IO. If pdflush doesn't run because it can't get CPU time,
then background IO does not get issued, and system performance
suffers as a result.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com