linux-kernel - Re: blk-mq: bitmap tag: performance degradation?

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Date:	Thu, 5 Jun 2014 16:01:27 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org, Ming Lei <tom.leiming@...il.com>
Subject: Re: blk-mq: bitmap tag: performance degradation?

On Wed, Jun 04, 2014 at 08:18:42AM -0600, Jens Axboe wrote:
> A null_blk test is the absolute best case for percpu_ida, since
> there are enough tags and everything is localized. The above test is
> more useful for testing blk-mq than any real world application of
> the tagging.
> 
> I've done considerable testing on both 2 and 4 socket (32 and 64
> CPUs) and bitmap tagging is better in a much wider range of
> applications. This includes even high tag depth devices like nvme,
> and more normal ranges like mtip32xx and scsi-mq setups.

Just for the record: bitmap tags on a 48 CPU box with NVMe device
indeed shows almost the same performance/cache rate as the stock
kernel.

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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