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SubjectRe: [PATCH] writeback: fix writeback cache thrashing
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On Sat, 2013-01-05 at 11:26 +0800, Fengguang Wu wrote:
> > > > Hi Namjae,
> > > >
> > > > Why use bdi_stat_error here? What's the meaning of its comment "maximal
> > > > error of a stat counter"?
> > > Hi Simon,
> > >
> > > As you know bdi stats (BDI_RECLAIMABLE, BDI_WRITEBACK …) are kept in
> > > percpu counters.
> > > When these percpu counters are incremented/decremented simultaneously
> > > on multiple CPUs by small amount (individual cpu counter less than
> > > threshold BDI_STAT_BATCH),
> > > it is possible that we get approximate value (not exact value) of
> > > these percpu counters.
> > > In order, to handle these percpu counter error we have used
> > > bdi_stat_error. bdi_stat_error is the maximum error which can happen
> > > in percpu bdi stats accounting.
> > >
> > > bdi_stat(bdi, BDI_RECLAIMABLE);
> > > -> This will give approximate value of BDI_RECLAIMABLE by reading
> > > previous value of percpu count.
> > >
> > > bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> > > ->This will give exact value of BDI_RECLAIMABLE. It will take lock
> > > and add current percpu count of individual CPUs.
> > > It is not recommended to use it frequently as it is expensive. We
> > > can better use “bdi_stat” and work with approx value of bdi stats.
> > >
> >
> > Hi Namjae, thanks for your clarify.
> >
> > But why compare error stat count to bdi_bground_thresh? What's the
>
> It's not comparing bdi_stat_error to bdi_bground_thresh, but rather,
> in concept, comparing bdi_stat (with error bound adjustments) to
> bdi_bground_thresh.
>
> > relationship between them? I also see bdi_stat_error compare to
> > bdi_thresh/bdi_dirty in function balance_dirty_pages.
>

Hi Fengguang,

> Here, it's trying to use bdi_stat_sum(), the accurate (however more
> costly) version of bdi_stat(), if the error would possibly be large:

Why error is large use bdi_stat_sum and error is few use bdi_stat?

>
> if (bdi_thresh < 2 * bdi_stat_error(bdi)) {
> bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> //...
> } else {
> bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> //...
> }
>
> Here the comment should have explained it well:
>
> * In theory 1 page is enough to keep the comsumer-producer
> * pipe going: the flusher cleans 1 page => the task dirties 1
> * more page. However bdi_dirty has accounting errors. So use

Why bdi_dirty has accounting errors?

> * the larger and more IO friendly bdi_stat_error.
> */
> if (bdi_dirty <= bdi_stat_error(bdi))
> break;
>
>
> Thanks,
> Fengguang


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