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

[Ming Lei]

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?


Thanks, 
Ming