Re: [RFC for-6.2/block V2] block: Change the granularity of io ticks from ms to ns

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

On 12/7/22 15:08, Jens Axboe wrote:
> On 12/7/22 3:32?PM, 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.
>>
>> 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:
> 
> My default peak testing runs at 122M IOPS. That's also the peak IOPS of
> the devices combined, and with iostats disabled. If I enabled iostats,
> then the performance drops to 112M IOPS. It's no longer device limited,
> that's a drop of about 8.2%.
> 

Wow, clearly not acceptable that's exactly I asked for perf
numbers :).

-ck