On 12/7/22 14:32, Gulam Mohamed wrote: > As per the review comment from Jens Axboe, I am re-sending this patch > against "for-6.2/block". > why is this marked as RFC ? are you waiting for something more to get resolved so this can be merged ? > > Use ktime to change the granularity of IO accounting in block layer from > milli-seconds to nano-seconds to get the proper latency values for the > devices whose latency is in micro-seconds. After changing the granularity > to nano-seconds the iostat command, which was showing incorrect values for > %util, is now showing correct values. > > We did not work on the patch to drop the logic for > STAT_PRECISE_TIMESTAMPS yet. Will do it if this patch is ok. > > The iostat command was run after starting the fio with following command > on an NVME disk. For the same fio command, the iostat %util was showing > ~100% for the disks whose latencies are in the range of microseconds. > With the kernel changes (granularity to nano-seconds), the %util was > showing correct values. Following are the details of the test and their > output: > > fio command > ----------- > [global] > bs=128K > iodepth=1 > direct=1 > ioengine=libaio > group_reporting > time_based > runtime=90 > thinktime=1ms > numjobs=1 > name=raw-write > rw=randrw > ignore_error=EIO:EIO > [job1] > filename=/dev/nvme0n1 > > Correct values after kernel changes: > ==================================== > iostat output > ------------- > iostat -d /dev/nvme0n1 -x 1 > > Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > nvme0n1 0.08 0.05 0.06 128.00 128.00 0.07 6.50 > > Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > nvme0n1 0.08 0.06 0.06 128.00 128.00 0.07 6.30 > > Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > nvme0n1 0.06 0.05 0.06 128.00 128.00 0.06 5.70 > > From fio > -------- > Read Latency: clat (usec): min=32, max=2335, avg=79.54, stdev=29.95 > Write Latency: clat (usec): min=38, max=130, avg=57.76, stdev= 3.25 > > Values before kernel changes > ============================ > iostat output > ------------- > > iostat -d /dev/nvme0n1 -x 1 > > Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > nvme0n1 0.08 0.06 0.06 128.00 128.00 1.07 97.70 > > Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > nvme0n1 0.08 0.06 0.06 128.00 128.00 1.08 98.80 > > Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > nvme0n1 0.08 0.05 0.06 128.00 128.00 1.06 97.20 > > From fio > -------- > Read Latency: clat (usec): min=33, max=468, avg=79.56, stdev=28.04 > Write Latency: clat (usec): min=9, max=139, avg=57.10, stdev= 3.79 > > Changes in V2: > 1. Changed the try_cmpxchg() to try_cmpxchg64() in function > update_io_ticks()as the values being compared are u64 which was giving > a build error on i386 and microblaze > > Signed-off-by: Gulam Mohamed <gulam.mohamed@xxxxxxxxxx> > --- I believe it has no effect on the overall performance, if so I'd document that. Based on the quantitative data present in the commit log this looks good to me, I believe you did audit all drivers and places in the block layer. Looks good. Reviewed-by: Chaitanya Kulkarni <kch@xxxxxxxxxx> -ck
