On Fri, Jan 04, 2013 at 11:26:43PM -0600, Simon Jeons wrote:
> 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?
It's the opposite. Please check this per-cpu counter routine to get an idea:
/*
* Add up all the per-cpu counts, return the result. This is a more accurate
* but much slower version of percpu_counter_read_positive()
*/
s64 __percpu_counter_sum(struct percpu_counter *fbc)
> >
> > 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?
Because it typically uses bdi_stat() to get the rough sum of the per-cpu
counters.
Thanks,
Fengguang
> > * the larger and more IO friendly bdi_stat_error.
> > */
> > if (bdi_dirty <= bdi_stat_error(bdi))
> > break;
> >
> >
> > Thanks,
> > Fengguang
>
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
