Re: [PATCH RFC 5.13 1/2] io_uring: add support for ns granularity of io_sq_thread_idle

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/30/21 9:51 AM, Pavel Begunkov wrote:
> On 9/29/21 1:13 PM, Hao Xu wrote:
>> 在 2021/9/29 下午7:37, Pavel Begunkov 写道:
>>> On 9/29/21 10:24 AM, Hao Xu wrote:
>>>> 在 2021/9/28 下午6:51, Pavel Begunkov 写道:
>>>>> On 9/26/21 11:00 AM, Hao Xu wrote:
>>> [...]
>>>>>> I'm gonna pick this one up again, currently this patch
>>>>>> with ktime_get_ns() works good on our productions. This
>>>>>> patch makes the latency a bit higher than before, but
>>>>>> still lower than aio.
>>>>>> I haven't gotten a faster alternate for ktime_get_ns(),
>>>>>> any hints?
>>>>>
>>>>> Good, I'd suggest to look through Documentation/core-api/timekeeping.rst
>>>>> In particular coarse variants may be of interest.
>>>>> https://www.kernel.org/doc/html/latest/core-api/timekeeping.html#coarse-and-fast-ns-access
>>>>>
>>>> The coarse functions seems to be like jiffies, because they use the last
>>>> timer tick(from the explanation in that doc, it seems the timer tick is
>>>> in the same frequency as jiffies update). So I believe it is just
>>>> another format of jiffies which is low accurate.
>>>
>>> I haven't looked into the details, but it seems that unlike jiffies for
>>> the coarse mode 10ms (or whatever) is the worst case, but it _may_ be
>> Maybe I'm wrong, but for jiffies, 10ms uis also the worst case, no?
>> (say HZ = 100, then jiffies updated by 1 every 10ms)
> 
> I'm speculating, but it sounds it's updated on every call to ktime_ns()
> in the system, so if someone else calls ktime_ns() every 1us, than the
> resolution will be 1us, where with jiffies the update interval is strictly
> 10ms when HZ=100. May be not true, need to see the code.

Taking a second quick look, doesn't seem to be the case indeed. And it's
limited to your feature anyway, so the overhead of ktime_get() shouldn't
matter much.

>>> much better on average and feasible for your case, but can't predict
>>> if that's really the case in a real system and what will be the
>>> relative error comparing to normal ktime_ns().

-- 
Pavel Begunkov