Date:	Thu, 05 Jun 2014 20:40:11 -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 20:35, Ming Lei wrote:
> On Fri, Jun 6, 2014 at 9:55 AM, Jens Axboe <axboe@...nel.dk> wrote:
>> 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.
>
> You are right, my previous test should be timer bound, but it
> should be eased by increasing timer period.
>
> I do the test again with increasing parameter of completion_nsec
> to 235000 from default 10000:
>
> 1, nullblk(timer mode)3.15-rc7:
> - each fio cpu utilization: 80% ~ 90%
> - 860K IOPS
>
> 2, nullbk(timer mode)3.15-rc7-next
> - each fio cpu utilization: 70~80%
> - 940K IOPS
>
> Then bitmap based allocation can be observed to be a bit
> better than percpu ida.

That's more inline with the real device testing I did. If tags are 
plenty, it's a wash between the two. But once you exceed 50% 
utilization, percpu_ida starts to degrade, and in some cases very badly. 
This is especially apparent on bigger 2 socket, or 4 socket boxes.

-- 
Jens Axboe

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