Re: [PATCH] lib/flex_proportions.c: Use abs() when percpu_counter is negative.

[chi wu]

Jan Kara <jack@xxxxxxx> 于2021年5月18日周二 下午6:39写道：
>
> On Tue 18-05-21 18:22:05, chi wu wrote:
> > Jan Kara <jack@xxxxxxx> 于2021年5月18日周二 下午4:59写道：
> > >
> > > On Tue 18-05-21 11:42:53, chi wu wrote:
> > > > Chi Wu <wuchi.zero@xxxxxxxxx> 于2021年5月17日周一 下午11:53写道：
> > > > >
> > > > > The value of percpu_counter_read() may become negative after
> > > > > running percpu_counter_sum() in fprop_reflect_period_percpu().
> > > > > The value of variable 'num' will be zero in fprop_fraction_percpu()
> > > > > when using percpu_counter_read_positive(), but if using the abs of
> > > > > percpu_counter_read() will be close to the correct value.
> > > > >
> > > >
> > > > I realized that I was wrong as follow:
> > > > (a) the decay rule is broken, the negative means the difference in
> > > > decay here.
> > > > (b) as the target event increasing, proportion of the event will
> > > > decrease to 0 firstly and then it will increase. The logic is bad.
> > > > 1. abs(-50) / abs(100) = 50%       //+50 to 2
> > > > 2. abs(0) / abs(150) = 0 %           //+50 to 3
> > > > 3. abs(50)/abs(200) = 25%
> > > >
> > > > Anyway, the percpu_counter_sum() had cost a lost performance,
> > > > may be we could get a little benefits from that. So could we add a
> > > > variable to stroe the decay value, we will get the value when
> > > > percpu_counter_read() is negative?
> > >
> > > The result of percpu_counter_read() is inherently inexact (but fast! ;). It
> > > can be upto number_of_cpus * counter_batch away from the real counter
> > > value. But do you observe any practical problems with this inaccuracy on
> > > your system? Sure, cache memory won't be split among devices exactly
> > > according to writeout proportion but that usually does not matter.
> > >
> > >                                                                 Honza
> > >
> >
> > Thanks, Got it.
> > Just try to optimize the fuse (with strictlimit feature)performance
> > issue: The writing thread will be paused and runs slowly, when the
> > proportion of fuse-bdi is 0.
> > The issue is normal,and one of reasons is the characteristics of
> > percpu_counter batch. Even the pages are writeout, we may be could not
> > get the real proportion value due to side effects of counter
> > performance. It's just a slight disappointment.
>
> Well, you can tune 'min_ratio' of the fuse-bdi to avoid problems with these
> near-zero states... To always give the bdi some breathing room for ramping
> up.
>

Thanks, it is useful to me.

>                                                                 Honza
>
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR