linux-kernel - Re: [patch 4/4] blk-mq: switch to percpu-ida for tag menagement

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Date:	Sat, 12 Oct 2013 08:49:05 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org, kmo@...erainc.com
Subject: Re: [patch 4/4] blk-mq: switch to percpu-ida for tag menagement

On Fri, Oct 11, 2013 at 08:28:54AM -0600, Jens Axboe wrote:
> On 10/11/2013 01:18 AM, Shaohua Li wrote:
> > Using percpu-ida to manage blk-mq tags. the percpu-ida has similar algorithm
> > like the blk-mq-tag. The difference is when a cpu can't allocate tags
> > blk-mq-tag uses ipi to purge remote cpu cache and percpu-ida directly purges
> > remote cpu cache. In practice (testing null_blk), the percpu-ida approach is
> > much faster when total tags aren't enough.
> 
> I'm not surprised it's a lot faster the the pathological case of needing
> to prune tags, the IPI isn't near ideal for that. I'm assuming the
> general performance is the same for the non-full case?

Yep. My test is done in a 2 sockets machine, 12 process cross the 2 sockets. So
if there is lock contention or ipi, should be stressed heavily. Testing is done
for null-blk.

hw_queue_depth	nopatch iops	patch iops
64		~800k/s		~1470k/s
2048		~4470k/s	~4340k/s

In the 2048 case, perf doesn't should any percpu-ida function is hot (no one
use > 1% cpu time), so the small difference should be drift. So yes, the
general performance is the same.

Thanks,
Shaohua
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