Re: [PATCH] block: per-cpu counters for in-flight IO accounting

[Jens Axboe]

On 2014-06-04 20:33, Shaohua Li wrote:
> On Wed, Jun 04, 2014 at 08:16:32PM -0600, Jens Axboe wrote:
> > On 2014-06-04 20:09, Shaohua Li wrote:
> > > On Wed, Jun 04, 2014 at 02:08:46PM -0600, Jens Axboe wrote:
> > > > On 06/04/2014 05:29 AM, Matias Bjørling wrote:
> > > > > It's in
> > > > >
> > > > > blk_io_account_start
> > > > >    part_round_stats
> > > > >      part_round_state_single
> > > > >        part_in_flight
> > > > >
> > > > > I like the granularity idea.
> > > >
> > > > And similarly from blk_io_account_done() - which makes it even worse,
> > > > since it at both ends of the IO chain.
> > >
> > > But part_round_state_single is supposed to only call part_in_flight every
> > > jiffery. Maybe we need something below:
> > > 1. set part->stamp immediately
> > > 2. fixed granularity
> > > Untested though.
> > >
> > >
> > > diff --git a/block/blk-core.c b/block/blk-core.c
> > > index 40d6548..5f0acaa 100644
> > > --- a/block/blk-core.c
> > > +++ b/block/blk-core.c
> > > @@ -1270,17 +1270,19 @@ static void part_round_stats_single(int cpu, struct hd_struct *part,
> > >   				    unsigned long now)
> > >   {
> > >   	int inflight;
> > > +	unsigned long old_stamp;
> > >
> > > -	if (now == part->stamp)
> > > +	if (time_before(now, part->stamp + msecs_to_jiffies(10)))
> > >   		return;
> > > +	old_stamp = part->stamp;
> > > +	part->stamp = now;
> > >
> > >   	inflight = part_in_flight(part);
> > >   	if (inflight) {
> > >   		__part_stat_add(cpu, part, time_in_queue,
> > > -				inflight * (now - part->stamp));
> > > -		__part_stat_add(cpu, part, io_ticks, (now - part->stamp));
> > > +				inflight * (now - old_stamp));
> > > +		__part_stat_add(cpu, part, io_ticks, (now - old_stamp));
> > >   	}
> > > -	part->stamp = now;
> > >   }
> > >
> > >   /**
> >
> > It'd be a good improvement, and one we should be able to do without
> > screwing anything up. It'd be identical to anyone running at HZ==100
> > right now.
> >
> > So the above we can easily do, and arguably should just do. We wont
> > see real scaling in the IO stats path before we fixup the hd_struct
> > referencing as well, however.
>
> That's true. maybe a percpu_ref works here.

Maybe, but it would require more than a direct replacement. The 
hd_struct stuff currently relies on things like atomic_inc_not_zero(), 
which would not be cheap to do. And this does happen for every new IO, 
so can't be amortized over time like the part stats rounding.

--
Jens Axboe

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