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

[Yu Kuai]

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
>
>
> .