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