Re: regression in page writeback

[Wu Fengguang <fengguang.wu@intel.com>]

On Fri, Sep 25, 2009 at 08:11:17AM +0800, 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.
> 
> Cleaning pages is important to keeping the system running smoothly.
> Given that IO takes time to clean pages, it is therefore important
> to issue as much as possible as quickly as possible without delays
> before going back to sleep. Delaying issue of the IO or doing
> sub-optimal issue simply reduces performance of the system because
> it takes longer to clean the same number of dirty pages.
> 
> > > Nothing stops other procs from keeping the queue congested forever.
> > > This can only be fixed by making everyone wait for congestion, at which
> > > point we might as well wait for requests.
> > 
> > Yes. That gives everyone somehow equal opportunity, this is a policy change
> > that may lead to interesting effects, as well as present a challenge to
> > get_request_wait(). That said, I'm not against the change to a wait queue
> > in general.
> 
> If you block all threads doing _writebehind caching_ (synchronous IO
> is self-throttling) to the same BDI on the same queue as the bdi
> flusher then when congestion clears the higher priority background
> flusher thread should run first and issue more IO.  This should
> happen as a natural side-effect of our scheduling algorithms and it
> gives preference to efficient background writeback over in-efficient
> foreground writeback. Indeed, with this approach we can even avoid
> foreground writeback altogether...

I don't see how balance_dirty_pages() writeout is less efficient than
pdflush writeout.

They all called the same routines to do the job.
balance_dirty_pages() sets nr_to_write=1536 at least for ext4 and xfs
(unless memory is tight; btrfs is 1540), which is in fact 50% bigger
than the 1024 pages used by pdflush. And it won't back off on congestion.
The s_io/b_io queues are shared, so a balance_dirty_pages() will just
continue from where the last sync thread exited. So it does not make
much difference who initiates the IO. Did I missed something?

Thanks,
Fengguang