Re: [PATCH] writeback: fix writeback cache thrashing

[Fengguang Wu <fengguang.wu@intel.com>]

> > > 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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