Messages in this thread | | | Date | Wed, 04 Jun 2014 20:42:59 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH] block: per-cpu counters for in-flight IO accounting |
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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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