Re: [Xen-devel] [PATCH RFC v2 0/5] Multi-queue support for xen-blkfront and xen-blkback
From: Bob Liu
Date: Wed Aug 12 2015 - 06:17:25 EST
On 08/12/2015 01:32 AM, Jens Axboe wrote:
> On 08/11/2015 03:45 AM, Rafal Mielniczuk wrote:
>> 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:
>>>>> 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?
>>> 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
>> $ 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
>> $ cat /sys/block/xvdb/queue/nomerges
>> 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"
> If you add --iodepth-batch=16 to that fio command line? Both mq and non-mq relies on plugging to get
> batching in the use case above, otherwise IO is dispatched immediately. O_DIRECT is immediate.
> I'd be more interested in seeing a test case with buffered IO of a file system on top of the xvdb device,
> if we're missing merging for that case, then that's a much bigger issue.
I was using the null block driver for xen blk-mq test.
There were not merges happen any more even after patch:
(Which just converted xen block driver to use blk-mq apis)
Will try a file system soon.
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