Re: [PATCH 0/4] blk-mq: support to use hw tag for scheduling

Jens Axboe
2017-05-03T14:10:58Z

On 05/03/2017 08:08 AM, Jens Axboe wrote:
> On 05/02/2017 10:03 PM, Ming Lei wrote:
>> On Fri, Apr 28, 2017 at 02:29:18PM -0600, Jens Axboe wrote:
>>> On 04/28/2017 09:15 AM, Ming Lei wrote:
>>>> Hi,
>>>>
>>>> This patchset introduces flag of BLK_MQ_F_SCHED_USE_HW_TAG and
>>>> allows to use hardware tag directly for IO scheduling if the queue's
>>>> depth is big enough. In this way, we can avoid to allocate extra tags
>>>> and request pool for IO schedule, and the schedule tag allocation/release
>>>> can be saved in I/O submit path.
>>>
>>> Ming, I like this approach, it's pretty clean. It'd be nice to have a
>>> bit of performance data to back up that it's useful to add this code,
>>> though. Have you run anything on eg kyber on nvme that shows a
>>> reduction in overhead when getting rid of separate scheduler tags?
>>
>> I can observe small improvement in the following tests:
>>
>> 1) fio script
>> # io scheduler: kyber
>>
>> RWS="randread read randwrite write"
>> for RW in $RWS; do
>> echo "Running test $RW"
>> sudo echo 3 > /proc/sys/vm/drop_caches
>> sudo fio --direct=1 --size=128G --bsrange=4k-4k --runtime=20 --numjobs=1 --ioengine=libaio --iodepth=10240 --group_reporting=1 --filename=$DISK --name=$DISK-test-$RW --rw=$RW --output-format=json
>> done
>>
>> 2) results
>>
>> ---------------------------------------------------------
>> |sched tag(iops/lat) | use hw tag to sched(iops/lat)
>> ----------------------------------------------------------
>> randread |188940/54107 | 193865/52734
>> ----------------------------------------------------------
>> read |192646/53069 | 199738/51188
>> ----------------------------------------------------------
>> randwrite |171048/59777 | 179038/57112
>> ----------------------------------------------------------
>> write |171886/59492 | 181029/56491
>> ----------------------------------------------------------
>>
>> I guess it may be a bit more obvious when running the test on one slow
>> NVMe device, and will try to find one and run the test again.
>
> Thanks for running that. As I said in my original reply, I think this
> is a good optimization, and the implementation is clean. I'm fine with
> the current limitations of when to enable it, and it's not like we
> can't extend this later, if we want.
>
> I do agree with Bart that patch 1+4 should be combined. I'll do that.

Actually, can you do that when reposting? Looks like you needed to
do that anyway.

--
Jens Axboe