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
