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

Jens Axboe
2014-06-05T02:16:37Z

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.

--
Jens Axboe