Re: [RFC] block: Change the granularity of io ticks from ms to ns

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2022/12/07 11:15, Ming Lei 写道:
> On Wed, Dec 07, 2022 at 10:19:08AM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2022/12/07 2:15, Gulam Mohamed 写道:
>>> 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.
>>
>> This patch didn't correct the counting of io_ticks, just make the
>> error accounting from jiffies(ms) to ns. The problem that util can be
>> smaller or larger still exist.
> 
> Agree.
> 
>>
>> However, I think this change make sense consider that error margin is
>> much smaller, and performance overhead should be minimum.
>>
>> Hi, Ming, how do you think?
> 
> I remembered that ktime_get() has non-negligible overhead, is there any
> test data(iops/cpu utilization) when running fio or t/io_uring on
> null_blk with this patch?

Yes, testing with null_blk is necessary, we don't want any performance
regression.

BTW, I thought it's fine because it's already used for tracking io
latency.
> 
> 
> Thanks,
> Ming
> 
> 
> .
>