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

Ming Lei
2017-05-03T04:03:22Z

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,
Ming