Re: [Xen-devel] [PATCH RFC v2 0/5] Multi-queue support for xen-blkfront and xen-blkback
From: Bob Liu
Date: Mon Aug 10 2015 - 07:15:39 EST
On 08/10/2015 07:03 PM, Rafal Mielniczuk wrote:
> On 01/07/15 04:03, Jens Axboe wrote:
>> On 06/30/2015 08:21 AM, Marcus Granado wrote:
>>> 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
>>> 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?
> 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
Which block scheduler was used in domU? Please try to "cat /sys/block/sdxxx/queue/scheduler".
How about the result if using "noop" scheduler?
> 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.
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