Re: [PATCH 06/13] writeback: bdi write bandwidth estimation

[Wu Fengguang]

On Wed, Nov 24, 2010 at 09:14:37PM +0800, Wu Fengguang wrote:
> On Wed, Nov 24, 2010 at 08:50:47PM +0800, Peter Zijlstra wrote:
> > On Wed, 2010-11-24 at 20:10 +0800, Wu Fengguang wrote:
> > > > > +       /*
> > > > > +        * When there lots of tasks throttled in balance_dirty_pages(), they
> > > > > +        * will each try to update the bandwidth for the same period, making
> > > > > +        * the bandwidth drift much faster than the desired rate (as in the
> > > > > +        * single dirtier case). So do some rate limiting.
> > > > > +        */
> > > > > +       if (jiffies - bdi->write_bandwidth_update_time < elapsed)
> > > > > +               goto snapshot;
> > > >
> > > > Why this goto snapshot and not simply return? This is the second call
> > > > (bdi_update_bandwidth equivalent).
> > > 
> > > Good question. The loop inside balance_dirty_pages() normally run only
> > > once, however wb_writeback() may loop over and over again. If we just
> > > return here, the condition
> > > 
> > >         (jiffies - bdi->write_bandwidth_update_time < elapsed)
> > > 
> > > cannot be reset, then future bdi_update_bandwidth() calls in the same
> > > wb_writeback() loop will never find it OK to update the bandwidth.
> > 
> > But the thing is, you don't want to reset that, it might loop so fast
> > you'll throttle all of them, if you keep the pre-throttle value you'll
> > eventually pass, no?
> 
> It (let's name it A) only resets the _local_ vars bw_* when it's sure
> by the condition
> 
>         (jiffies - bdi->write_bandwidth_update_time < elapsed)

this will be true if someone else has _done_ overlapped estimation,
otherwise it will equal:

        jiffies - bdi->write_bandwidth_update_time == elapsed

Sorry the comment needs updating.

Thanks,
Fengguang

> that someone else (name B) has updated the _global_ bandwidth in the
> time range we planned. So there may be some time in A's range that is
> not covered by B, but sure the range is not totally bypassed without
> updating the bandwidth.
> 
> > > It does assume no races between CPUs.. We may need some per-cpu based
> > > estimation. 
> > 
> > But that multi-writer race is valid even for the balance_dirty_pages()
> > call, two or more could interleave on the bw_time and bw_written
> > variables.
> 
> The race will only exist in each task's local vars (their bw_* will
> overlap). But the update bdi->write_bandwidth* will be safeguarded
> by the above check. When the task is scheduled back, it may find
> updated write_bandwidth_update_time and hence give up his estimation.
> This is rather tricky..
> 
> Thanks,
> Fengguang
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