linux-kernel - Re: [Xen-devel] [PATCH RFC v2 0/5] Multi-queue support for xen-blkfront and xen-blkback

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Message-ID: <55C8C8CE.7020301@fb.com>
Date:	Mon, 10 Aug 2015 09:52:46 -0600
From:	Jens Axboe <axboe@...com>
To:	Rafal Mielniczuk <rafal.mielniczuk@...rix.com>,
	Marcus Granado <Marcus.Granado@...rix.com>,
	Bob Liu <bob.liu@...cle.com>,
	Arianna Avanzini <avanzini.arianna@...il.com>
CC:	Felipe Franciosi <felipe.franciosi@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	David Vrabel <david.vrabel@...rix.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
	Jonathan Davies <Jonathan.Davies@...rix.com>
Subject: Re: [Xen-devel] [PATCH RFC v2 0/5] Multi-queue support for xen-blkfront
 and xen-blkback

On 08/10/2015 05:03 AM, Rafal Mielniczuk wrote:
> On 01/07/15 04:03, Jens Axboe wrote:
>> On 06/30/2015 08:21 AM, Marcus Granado wrote:
>>> Hi,
>>>
>>> Our measurements for the multiqueue patch indicate a clear improvement
>>> in iops when more queues are used.
>>>
>>> The measurements were obtained under the following conditions:
>>>
>>> - using blkback as the dom0 backend with the multiqueue patch applied to
>>> a dom0 kernel 4.0 on 8 vcpus.
>>>
>>> - using a recent Ubuntu 15.04 kernel 3.19 with multiqueue frontend
>>> applied to be used as a guest on 4 vcpus
>>>
>>> - using a micron RealSSD P320h as the underlying local storage on a Dell
>>> PowerEdge R720 with 2 Xeon E5-2643 v2 cpus.
>>>
>>> - fio 2.2.7-22-g36870 as the generator of synthetic loads in the guest.
>>> We used direct_io to skip caching in the guest and ran fio for 60s
>>> reading a number of block sizes ranging from 512 bytes to 4MiB. Queue
>>> depth of 32 for each queue was used to saturate individual vcpus in the
>>> guest.
>>>
>>> We were interested in observing storage iops for different values of
>>> block sizes. Our expectation was that iops would improve when increasing
>>> the number of queues, because both the guest and dom0 would be able to
>>> make use of more vcpus to handle these requests.
>>>
>>> These are the results (as aggregate iops for all the fio threads) that
>>> we got for the conditions above with sequential reads:
>>>
>>> fio_threads  io_depth  block_size   1-queue_iops  8-queue_iops
>>>       8           32       512           158K         264K
>>>       8           32        1K           157K         260K
>>>       8           32        2K           157K         258K
>>>       8           32        4K           148K         257K
>>>       8           32        8K           124K         207K
>>>       8           32       16K            84K         105K
>>>       8           32       32K            50K          54K
>>>       8           32       64K            24K          27K
>>>       8           32      128K            11K          13K
>>>
>>> 8-queue iops was better than single queue iops for all the block sizes.
>>> There were very good improvements as well for sequential writes with
>>> block size 4K (from 80K iops with single queue to 230K iops with 8
>>> queues), and no regressions were visible in any measurement performed.
>> Great results! And I don't know why this code has lingered for so long,
>> so thanks for helping get some attention to this again.
>>
>> Personally I'd be really interested in the results for the same set of
>> tests, but without the blk-mq patches. Do you have them, or could you
>> potentially run them?
>>
> Hello,
>
> We rerun the tests for sequential reads with the identical settings but with Bob Liu's multiqueue patches reverted from dom0 and guest kernels.
> The results we obtained were *better* than the results we got with multiqueue patches applied:
>
> fio_threads  io_depth  block_size   1-queue_iops  8-queue_iops  *no-mq-patches_iops*
>       8           32       512           158K         264K         321K
>       8           32        1K           157K         260K         328K
>       8           32        2K           157K         258K         336K
>       8           32        4K           148K         257K         308K
>       8           32        8K           124K         207K         188K
>       8           32       16K            84K         105K         82K
>       8           32       32K            50K          54K         36K
>       8           32       64K            24K          27K         16K
>       8           32      128K            11K          13K         11K
>
> We noticed that the requests are not merged by the guest when the multiqueue patches are applied,
> which results in a regression for small block sizes (RealSSD P320h's optimal block size is around 32-64KB).
>
> We observed similar regression for the Dell MZ-5EA1000-0D3 100 GB 2.5" Internal SSD
>
> As I understand blk-mq layer bypasses I/O scheduler which also effectively disables merges.
> Could you explain why it is difficult to enable merging in the blk-mq layer?
> That could help closing the performance gap we observed.
>
> Otherwise, the tests shows that the multiqueue patches does not improve the performance,
> at least when it comes to sequential read/writes operations.

blk-mq still provides merging, there should be no difference there. Does 
the xen patches set BLK_MQ_F_SHOULD_MERGE?

-- 
Jens Axboe

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