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

[Rafal Mielniczuk]

On 12/08/15 11:17, Bob Liu wrote:
> 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:
> ...
>>>>>> 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"
>> 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: 
> https://lkml.org/lkml/2015/7/13/185
> (Which just converted xen block driver to use blk-mq apis)
>
> Will try a file system soon.
>
I have more results for the guest with and without the patch
https://lkml.org/lkml/2015/7/13/185
applied to the latest stable kernel (4.1.5).

Command line used was:
fio --name=test --ioengine=libaio --rw=read --numjobs=8 \
    --iodepth=32 --time_based=1 --runtime=300 --bs=4KB \
    --filename=/dev/xvdb --direct=(0 and 1) --iodepth_batch=16

without patch (--direct=1):
  xvdb: ios=18696304/0, merge=75763177/0, ticks=11323872/0, in_queue=11344352, util=100.00%

with patch (--direct=1):
  xvdb: ios=43709976/0, merge=97/0, ticks=8851972/0, in_queue=8902928, util=100.00%

without patch buffered (--direct=0):
  xvdb: ios=1079051/0, merge=76/0, ticks=749364/0, in_queue=748840, util=94.60

with patch buffered (--direct=0):
  xvdb: ios=1132932/0, merge=0/0, ticks=689108/0, in_queue=688488, util=93.32%

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