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

Jens Axboe
2017-05-03T14:08:33Z

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.

--
Jens Axboe