Re: regression in page writeback

[Chris Mason <chris.mason@oracle.com>]

On Mon, Sep 28, 2009 at 11:07:00AM +1000, Dave Chinner wrote:
> On Fri, Sep 25, 2009 at 02:45:03PM +0800, Wu Fengguang wrote:
> > On Fri, Sep 25, 2009 at 01:04:13PM +0800, Dave Chinner wrote:
> > > 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.
> > 
> > O_SYNC writes may be un-throttled in theory, however it seems to be
> > throttled in practice:
> > 
> >   generic_file_aio_write
> >     __generic_file_aio_write
> >       generic_file_buffered_write
> >         generic_perform_write
> >           balance_dirty_pages_ratelimited
> >     generic_write_sync
> > 
> > Do you mean some other code path?
> 
> In the context of the setup I was talking about, I meant is that sync
> IO _should_ be unthrottled because it is self-throttling by it's
> very nature. The current code makes no differentiation between the
> two.

This isn't entirely true anymore.  WB_SYNC_ALL is turned into a sync
bio, which is sent down with higher priority.  There may be a few spots
that still need to be changed for it, but it is much better than it was.

re: pageout() being the worst way to do IO, definitely agree.

-chris