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

[Bob Liu]

On 08/10/2015 11:52 PM, Jens Axboe wrote:
> 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?
> 

Yes.
Is it possible that xen-blkfront driver dequeue requests too fast after we have multiple hardware queues?
Because new requests don't have the chance merging with old requests which were already dequeued and issued.

-- 
Regards,
-Bob
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