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
