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

From: Jens Axboe
Date: Wed Jun 04 2014 - 22:16:40 EST

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


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)))
+ 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

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