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

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Date:	Sun, 13 Oct 2013 12:26:40 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Shaohua Li <shli@...nel.org>
Cc:	linux-kernel@...r.kernel.org, kmo@...erainc.com
Subject: Re: [patch 4/4] blk-mq: switch to percpu-ida for tag menagement

On Sat, Oct 12 2013, Shaohua Li wrote:
> 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.

Yep, that definitely looks a lot prettier. Thanks! I've had this on the
TODO for a while, since the ida got fixed up.

-- 
Jens Axboe

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