I'm following along with day 018 (Enforcing A Video Frame Rate) using Linux. I'm using nanosleep and with the default scheduler policy SCHED_OTHER and setpriority(PRIO_PROCESS, getpid(), -20) it is unable to reliably wake up from sleep at my requested time. I thought about setting the scheduler policy to SCHED_FIFO and setting sched_get_priority_max(SCHED_FIFO) and the nanosleep would yield the cpu to other tasks. With other processes running like xorg I thought this may cause issues for the end-user. So, the only way I can see is to do the while(1) loop and just 'melt' the CPU. Does anyone have any suggestions/experiences with this on Linux? Thanks.
We are currently in the process of converting the website to the new design. Some pages, like this one, are still broken. We appreciate your patience.
One crazy idea I explored a while ago was to predict the noise on the system by sampling recent history and make an educated guess as to how much to sleep for. I was able to cancel system noise quite well this way but I think my code would go off the rails in other cases so I'm neither using it nor going to share it. If someone does a provably robust version of that idea I would love to see it.
05:49 <neo> this is a minute of timing data for abstime and a minute of timing data for my new version, note that the chart cuts off some of the abstime datapoints because the outliers for it are large
05:50 <neo> abstime min-max 4.0x10^7 to 4.3x10^7
05:50 <neo> my ver min-max 4.15x10^7 to 4.18x10^7
05:51 <neo> abstime stdev: 73518.51835
05:51 <neo> my ver stdev: 35637.30126
To more helpfully answer your question,
17:54 <Discord> <Croepha (Dave from SF)> Hey, anyone have insights on why epoll_wait uses the CLOCK_MONOTONIC clock instead of the CLOCK_MONOTONIC_RAW clock? it seems like if you are holding off some action based on some delay (like a retry or a throttle) you wouldn't want that to be affected by NTP or clock drift... I don't have much experience with adjtime, it may be that it is always only a trivial amount of drift...
04:40 <Discord> <Croepha (Dave from SF)> so, looking at this: https://github.com/ntp-project/ntp/blob/9c75327c3796ff59ac648478cd4da8b205bceb77/ntpdate/ntpdate.h#L64 it looks like adjtime is only used for sub-second adjustments, so, using CLOCK_MONOTONIC seems fine then
05:16 <neo> @Croepha what clock would you recommend for frame capping an emulator? I went with using clock_nanosleep with CLOCK_MONOTONIC and TIMER_ABSTIME but I haven't thought deeply about it, I just want to consistently present frames at a fixed interval without frame drift etc
05:16 <neo> https://gitlab.com/-/snippets/2135217#LC1057
16:02 <Discord> <Peter Fors> @neo I did a little test to see stability of nanosleep, there were some fluctuations when waiting the entire time with nanosleep, so I sleep the last ms with a poll-loop with clock_gettime https://pastebin.com/c0fcb8Ry if you are helped with that
16:04 <Discord> <Peter Fors> On my machine the furthest from 10ms is +20us or so over 10 minutes
06:53 <Discord> <Croepha (Dave from SF)> @neo sorry, wasn't ignoring you but the @ wasn't pinging me... probably has something to do with discord seeing you as a bot or something.... but I think I would use clock_nanosleep as you were doing, I think that is the best you can get in linux, as far as I can tell
TL;DR: Peter's technique is to sleep less than needed and then busy-loop to burn the remaining time e.g. last 1ms
Linux Scheduling Granularity
I think it comes from windows implementation - there Casey used timeBeginPeriod that changes sleep granularity in whole OS down to 1 millisecond. Then you can sleep down to last millisecond and then busy-loop last millisecond or less.
In newer Windows 10 versions they changed that timeBeginPeriod does not affect other processes as much - it is still not completely process-local, but affects system way less than it used to: https://randomascii.wordpress.com/2020/10/04/windows-timer-resolution-the-great-rule-change/