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

[Shaohua Li <shli@kernel.org>]

On Fri, May 09, 2014 at 10:41:27AM -0600, Jens Axboe wrote:
> On 05/09/2014 08:12 AM, Jens Axboe wrote:
> > On 05/09/2014 03:17 AM, Matias Bjørling wrote:
> >> With multi-million IOPS and multi-node workloads, the atomic_t in_flight
> >> tracking becomes a bottleneck. Change the in-flight accounting to per-cpu
> >> counters to elevate.
> > 
> > The part stats are a pain in the butt, I've tried to come up with a
> > great fix for them too. But I don't think the percpu conversion is
> > necessarily the right one. The summing is part of the hotpath, so percpu
> > counters aren't necessarily the right way to go. I don't have a better
> > answer right now, otherwise it would have been fixed :-)
> 
> Actual data point - this slows my test down ~14% compared to the stock
> kernel. Also, if you experiment with this, you need to watch for the
> out-of-core users of the part stats (like DM).

I had a try with Matias's patch. Performance actually boost significantly.
(there are other cache line issue though, eg, hd_struct_get). Jens, what did
you run? part_in_flight() has 3 usages. 2 are for status output, which are cold
path. part_round_stats_single() uses it too, but it's a cold path too as we
simple data every jiffy. Are you using HZ=1000? maybe we should simple the data
every 10ms instead of every jiffy?

Thanks,
Shaohua