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
