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

Jan Kara
2011-01-13T10:38:42Z

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@suse.cz>
SUSE Labs, CR