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

[Jens Axboe]

On 12/7/22 6:09 AM, Yu Kuai wrote:
> 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.

null_blk is fine as a substitute, but I'd much rather run this on my
test bench with actual IO and devices.

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

Reading a nsec timestamp is a LOT more expensive than reading jiffies,
which is essentially free. If you look at the amount of work that's
gone into minimizing ktime_get() for the fast path in the IO stack,
then that's a testament to that.

So that's a very bad assumption, and definitely wrong.

-- 
Jens Axboe