Re: + mm-balance_dirty_pages-reduce-calls-to-global_page_state-to-reduce-c ache-references.patch added to -mm tree

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

On Wed, Sep 02, 2009 at 04:31:40PM +0800, Peter Zijlstra wrote:
> On Sat, 2009-08-22 at 20:11 +0200, Peter Zijlstra wrote:
> > > > +           /* always throttle if over threshold */
> > > > +           if (nr_reclaimable + nr_writeback < dirty_thresh) {
> > > 
> > > That 'if' is a big behavior change. It effectively blocks every one
> > > and canceled Peter's proportional throttling work: the less a process
> > > dirtied, the less it should be throttled.
> > 
> > Hmm, I think you're right, I had not considered that, thanks for
> > catching that.
> 
> So in retrospect I think I might have been wrong here.
> 
> The per task thing causes the bdi limit to be lower than the bdi limit
> based on writeback speed alone. That is, the more a task dirties, the
> lower the bdi limit is as seen for that task.

Right. If I understand it right, there will be a safety margin of about
(1/8) * dirty_limit for 1 heavy dirtier case, and that gap scales down
when there are more concurrent heavy dirtiers.

In principle, the ceiling will be a bit higher for a light dirtier to
make it easy to pass in the presence of more heavy dirtiers.

> So if we get a task that generates tons of dirty pages (dd) then it
> won't ever actually hit the full dirty limit, even if its the only task
> on the system, and this outer if() will always be true.

Right, we have the safety margin :)

> Only when we actually saturate the full dirty limit will we fall through
> and throttle, but that is ok -- we want to enforce the full limit.
> 
> In short, a very aggressive dirtier will have a bdi limit lower than the
> total limit (at all times) leaving a little room at the top for the
> occasional dirtier to make quick progress.
> 
> Wu, does that cover the scenario you had in mind?

Yes thanks! Please correct me if wrong:
- the lower-ceiling-for-heavier-dirtier algorithm in task_dirty_limit()
  is elegant enough to prevent heavy dirtier to block light ones
- the test (nr_reclaimable + nr_writeback < dirty_thresh) is not
  relevant in normal, but can be kept for safety in the form of

          if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
              nr_reclaimable + nr_writeback < dirty_thresh)
                  break;

- clip_bdi_dirty_limit() could be removed: we have been secured by the
  above test

Thanks,
Fengguang