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

[Shaohua Li <shli@kernel.org>]

On Fri, May 30, 2014 at 07:49:52AM -0600, Jens Axboe wrote:
> On 2014-05-30 06:11, Shaohua Li wrote:
> >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?
> 
> I ran peak and normal benchmarks on a p320, on a 4 socket box (64
> cores). The problem is the one hot path of part_in_flight(), summing
> percpu for that is too expensive. On bigger systems than mine, it'd
> be even worse.

I run a null_blk test with 4 sockets, Matias has improvement. And I didn't find
part_in_flight() is called in any hot path.

Thanks,
Shaohua