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

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Date:	Thu, 5 Jun 2014 22:16:16 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Alexander Gordeev <agordeev@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: blk-mq: bitmap tag: performance degradation?

On Thu, Jun 5, 2014 at 10:03 PM, Jens Axboe <axboe@...nel.dk> wrote:
> On 2014-06-05 08:01, Alexander Gordeev wrote:
>>
>> 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.
>
>
> Thanks for confirming. It's one of the dangers of null_blk, it's not always
> a very accurate simulation of what a real device will do. I think it's
> mostly a completion side thing, would be great with a small device that
> supported msi-x and could be used as an irq trigger :-)

Maybe null_blk at IRQ_TIMER mode is more close to
a real device, and I guess the result may be different with
mode IRQ_NONE/IRQ_SOFTIRQ.

Thanks,
-- 
Ming Lei
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