Re: [PATCH 02/35] writeback: safety margin for bdi stat error

[Jan Kara]

On Thu 13-01-11 12:14:40, Wu Fengguang wrote:
> On Thu, Jan 13, 2011 at 05:59:49AM +0800, Jan Kara wrote:
> > > So the root cause is, the bdi_dirty is well under the global nr_dirty
> > > due to accounting errors. This can be fixed by using bdi_stat_sum(),
> >   So which statistic had the big error? I'd just like to understand
> > this (and how come your patch improves the situation)...
> 
> bdi_stat_error() = nr_cpu_ids * BDI_STAT_BATCH
>                  = 8 * (8*(1+ilog2(8)))
>                  = 8 * 8 * 4
>                  = 256 pages
>                  = 1MB
  Yes, my question was more aiming at on which statistics the error happens
so that it causes problems for you. Thinking about it now I suppose you
observe that bdi_nr_writeback + bdi_nr_reclaimable < bdi_thresh but in fact
the number of pages is higher than bdi_thresh because of accounting errors.
And thus we are able to reach global dirty limit and the tasks get
throttled heavily. Am I right?

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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