On Wed, Dec 07, 2022 at 10:32:04PM +0000, Gulam Mohamed wrote:
> As per the review comment from Jens Axboe, I am re-sending this patch
> against "for-6.2/block".
>
>
> 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.
Please add the theory behind why using nano-seconds can get correct accounting.
>
> 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
Can you explain a bit why the above %util is correct?
BTW, %util is usually not important for SSDs, please see 'man iostat':
%util
Percentage of elapsed time during which I/O requests were issued to the device (bandwidth uti‐
lization for the device). Device saturation occurs when this value is close to 100% for devices
serving requests serially. But for devices serving requests in parallel, such as RAID arrays
and modern SSDs, this number does not reflect their performance limits.
Thanks,
Ming
