Re: [PATCH 3/5] writeback: dirty rate control

[Wu Fengguang]

On Wed, Aug 10, 2011 at 12:19:32AM +0800, Peter Zijlstra wrote:
> On Tue, 2011-08-09 at 18:16 +0200, Peter Zijlstra wrote:
> > On Tue, 2011-08-09 at 11:50 -0400, Vivek Goyal wrote:
> > > 
> > > So IIUC, bdi->dirty_ratelimit is the dynmically adjusted desired rate
> > > limit (based on postion ratio, dirty_bw and write_bw). But this seems
> > > to be overall bdi limit and does not seem to take into account the
> > > number of tasks doing IO to that bdi (as your comment suggests). So
> > > it probably will track write_bw as opposed to write_bw/N. What am
> > > I missing? 
> > 
> > I think the per task thing comes from him using the pages_dirtied
> > argument to balance_dirty_pages() to compute the sleep time. Although
> > I'm not quite sure how he keeps fairness in light of the sleep time
> > bounding to MAX_PAUSE.
> 
> Furthermore, there's of course the issue that current->nr_dirtied is
> computed over all BDIs it dirtied pages from, and the sleep time is
> computed for the BDI it happened to do the overflowing write on.
> 
> Assuming an task (mostly) writes to a single bdi, or equally to all, it
> should all work out.

Right. That's one pitfall I forgot to mention, sorry.

If _really_ necessary, the above imperfection can be avoided by adding
tsk->last_dirty_bdi and tsk->to_pause, and to do so when switching to
another bdi:

        to_pause += nr_dirtied / task_ratelimit
        if (to_pause > reasonable_large_pause_time) {
                sleep(to_pause)
                to_pause = 0
        }
        nr_dirtied  = 0

Thanks,
Fengguang
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