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

[Bob Liu]

On 08/13/2015 12:46 AM, Rafal Mielniczuk wrote:
> 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).
> 

Thank you.

> 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%
> 

So request merge can happen just more difficult to be triggered.
How about the iops of both cases?

> 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%
> 

-- 
Regards,
-Bob
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