Re: regression in page writeback

Chris Mason
2009-09-25T00:38:47Z

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.

-chris