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From: Pavel Begunkov <[email protected]>
To: Hao Xu <[email protected]>, Jens Axboe <[email protected]>
Cc: [email protected], Joseph Qi <[email protected]>
Subject: Re: [PATCH RFC 5.13 1/2] io_uring: add support for ns granularity of io_sq_thread_idle
Date: Thu, 30 Sep 2021 09:51:16 +0100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

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.


>> 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

  reply	other threads:[~2021-09-30  8:51 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-28 13:32 [PATCH RFC 5.13 0/2] adaptive sqpoll and its wakeup optimization Hao Xu
2021-04-28 13:32 ` [PATCH RFC 5.13 1/2] io_uring: add support for ns granularity of io_sq_thread_idle Hao Xu
2021-04-28 14:07   ` Pavel Begunkov
2021-04-28 14:16     ` Jens Axboe
2021-04-28 14:53       ` Pavel Begunkov
2021-04-28 14:54         ` Jens Axboe
2021-04-29  3:41       ` Hao Xu
2021-04-29  9:11         ` Pavel Begunkov
2021-05-05 14:07           ` Hao Xu
2021-05-05 17:40             ` Pavel Begunkov
2021-04-29  3:28     ` Hao Xu
2021-04-29 22:15       ` Pavel Begunkov
2021-09-26 10:00         ` Hao Xu
2021-09-28 10:51           ` Pavel Begunkov
2021-09-29  7:52             ` Hao Xu
2021-09-29  9:24             ` Hao Xu
2021-09-29 11:37               ` Pavel Begunkov
2021-09-29 12:13                 ` Hao Xu
2021-09-30  8:51                   ` Pavel Begunkov [this message]
2021-09-30 12:04                     ` Pavel Begunkov
2021-10-05 15:00                       ` Hao Xu
2021-04-28 13:32 ` [PATCH RFC 5.13 2/2] io_uring: submit sqes in the original context when waking up sqthread Hao Xu
2021-04-28 14:12   ` Jens Axboe
2021-04-29  4:12     ` Hao Xu
2021-04-28 14:34   ` Pavel Begunkov
2021-04-28 14:37     ` Pavel Begunkov
2021-04-29  4:37       ` Hao Xu
2021-04-29  9:28         ` Pavel Begunkov
2021-05-05 11:20           ` Hao Xu
2021-04-28 14:39     ` Jens Axboe
2021-04-28 14:50       ` Pavel Begunkov
2021-04-28 14:53         ` Jens Axboe
2021-04-28 14:56           ` Pavel Begunkov
2021-04-28 15:09             ` Jens Axboe
2021-04-29  4:43       ` Hao Xu
2021-04-29  8:44     ` Hao Xu
2021-04-29 22:10       ` Pavel Begunkov
2021-05-05 13:10         ` Hao Xu
2021-05-05 17:44           ` Pavel Begunkov
2021-04-29 22:02   ` Pavel Begunkov

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