> From: Morten Brørup [mailto:m...@smartsharesystems.com] > Sent: Thursday, 26 October 2023 16.50 > > > From: Thomas Monjalon [mailto:tho...@monjalon.net] > > Sent: Thursday, 26 October 2023 16.31 > > > > 26/10/2023 16:08, Morten Brørup: > > > > From: Thomas Monjalon [mailto:tho...@monjalon.net] > > > > Sent: Thursday, 26 October 2023 16.05 > > > > > > > > 26/10/2023 15:57, Morten Brørup: > > > > > > From: Morten Brørup [mailto:m...@smartsharesystems.com] > > > > > > Sent: Thursday, 26 October 2023 15.45 > > > > > > > > > > > > > From: Thomas Monjalon [mailto:tho...@monjalon.net] > > > > > > > Sent: Thursday, 26 October 2023 15.37 > > > > > > > > > > > > > > 25/10/2023 18:31, Thomas Monjalon: > > > > > > > > Real-time thread priority was been forbidden on Unix > > > > > > > > because of problems they can cause. > > > > > > > > Warnings and helpers are added to avoid deadlocks, > > > > > > > > so real-time can be allowed on all systems. > > > > > > > > > > > > > > Unit test is failing: > > > > > > > DPDK:fast-tests / threads_autotest TIMEOUT 600.01 s > > > > > > > > > > > > > > It is seen in only 1 target (maybe the failure occurence is > random): > > > > > > > Debian 11 (Buster) (ARM) | PASS > > > > > > > Fedora 37 (ARM) | PASS > > > > > > > CentOS Stream 9 (ARM) | FAIL > > > > > > > Fedora 38 (ARM) | PASS > > > > > > > Fedora 38 (ARM Clang) | PASS > > > > > > > Ubuntu 20.04 (ARM) | PASS > > > > > > > > > > > > > > I need to send a v4 with new implementation and better comments. > > > > > > > The Unix sleep will be upgraded from 1 ns to 1 us in case it makes > a > > > > > > > difference. > > > > > > > > > > > > It will not make a difference. The kernel will go through the > sleeping > > > > steps, > > > > > > then wake up again and see the real-time thread is ready to run, and > > then > > > > > > immediately schedule it. > > > > > > > > > > > > For testing purposes, consider sleeping 10 milliseconds or something > > > > > > significant like that. > > > > > > > > > > A bit more details... > > > > > > > > > > In our recent tests, nanosleep() itself took around 50 us. So you need > > to > > > > sleep longer than that for your thread not to be runnable when the > > nanosleep() > > > > wakes up again, because 50 us has already passed in "nanosleep > overhead". > > > > > 10 milliseconds provides plenty of margin, and corresponds to 10 > jiffies > > on > > > > a 1000 Hz kernel. (I don't know if it makes any difference for the > kernel > > > > scheduler if the timer crosses a jiffy border or not.) > > > > > > > > 10 ms looks like an eternity. > > > > > > Agree. It is only for functional testing, not for production! > > > > Realtime thread won't make any sense if we have to insert a long sleep. > > It seems David came to our rescue here! > > I have just tried running our test again with prctl(PR_SET_TIMERSLACK) of 1 > ns, and the nanosleep(1 ns) delay dropped from ca. 50 us to ca. 2.5 us. > > The timeout parameter to epoll_wait() is in milliseconds, which is useless for > low-latency. > Perhaps real-time threads can be used with epoll() combined with timerfd for > nanosecond resolution timeout.
Or epoll_pwait2(), which has nanosecond resolution timeout. Unfortunately, rte_epoll_wait() is not an experimental API anymore, so we cannot change its timeout parameter from milliseconds to micro- or nanoseconds. We would have to introduce a new API for this. > > > We need to find a good sleep value or give up with real-time threads. > > (note I'm not sure how much it is useful) > > Only the application developer knows how much delay is acceptable. Which is > why I mentioned that the new yield functions should document the delay. > > > > > > > I will try. > > > > (Anyway I did a mistake when sending v4) > > > > I've sent a trial with 1 ms. > >
