> 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. > 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. >
