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

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Message-ID: <53911FA6.1030505@kernel.dk>
Date:	Thu, 05 Jun 2014 19:55:50 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <tom.leiming@...il.com>
CC:	Alexander Gordeev <agordeev@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: blk-mq: bitmap tag: performance degradation?

On 2014-06-05 17:33, Ming Lei wrote:
> On Fri, Jun 6, 2014 at 1:17 AM, Jens Axboe <axboe@...nel.dk> wrote:
>> On 06/05/2014 08:16 AM, Ming Lei wrote:
>>> 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.
>>
>> It'd be closer in behavior, but the results might then be skewed by
>> hitting the timer way too hard. And it'd be a general slowdown, again
>> possibly skewing it. But I haven't tried with the timer completion, to
>> see if that yields more accurate modelling for this test, so it might
>> actually be a lot better.
>
> My test on a 16core VM(host: 2 sockets, 16core):
>
> 1, bitmap tag allocation(3.15-rc7-next):
> - softirq mode: 759K IOPS
> - timer mode: 409K IOPS
>
> 2, percpu_ida allocation(3.15-rc7)
> - softirq mode: 1116K IOPS
> - timer mode: 411K IOPS

It's hard to say if this is close, or whether we are just timer bound at 
that point.

What other parameters did you load null_blk with (unmber of queues, 
queue depth)?


-- 
Jens Axboe

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