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

From: Rafal Mielniczuk
Date: Tue Aug 11 2015 - 05:49:14 EST

On 11/08/15 07:08, Bob Liu wrote:
> 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.
>

For some reason we don't see merges even when we set multiqueue to 1.
Below are some stats from the guest system when doing sequential 4KB reads:

$ fio --name=test --ioengine=libaio --direct=1 --rw=read --numjobs=8
--iodepth=32 --time_based=1 --runtime=300 --bs=4KB
--filename=/dev/xvdb

$ iostat -xt 5 /dev/xvdb
avg-cpu: %user %nice %system %iowait %steal %idle
0.50 0.00 2.73 85.14 2.00 9.63

Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s
avgrq-sz avgqu-sz await r_await w_await svctm %util
xvdb 0.00 0.00 156926.00 0.00 627704.00 0.00
8.00 30.06 0.19 0.19 0.00 0.01 100.48

$ cat /sys/block/xvdb/queue/scheduler
none

$ cat /sys/block/xvdb/queue/nomerges
0

Relevant bits from the xenstore configuration on the dom0:

/local/domain/0/backend/vbd/2/51728/dev = "xvdb"
/local/domain/0/backend/vbd/2/51728/backend-kind = "vbd"
/local/domain/0/backend/vbd/2/51728/type = "phy"
/local/domain/0/backend/vbd/2/51728/multi-queue-max-queues = "1"

/local/domain/2/device/vbd/51728/multi-queue-num-queues = "1"
/local/domain/2/device/vbd/51728/ring-ref = "9"
/local/domain/2/device/vbd/51728/event-channel = "60"

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