General to do list

[mohse-n]

• [x] Suggestion: Would calling clock_nanosleep just before ecrt_master_send reduce jitter?

Additional Details By calling clock_nanosleep before ecrt_master_receive (the current order) we ensure that the datagrams are received (ecrt_master_receive) every 1 ms.
We assume we send the datagrams (ecrt_send_master) every 1 ms. This assumption is valid only if what comes before ecrt_send_master (and after clock_nanosleep) takes the same amount of time every cycle. This includes
ecrt_master_receive , ecrt_domain_process ,ecrt_domain_queue, distributed clock function calls, and any additional processing. But there's no guarantee that those function themselves are jitter-free. The functions' jitter can be measured (similar to what's done by uncommenting MEASURE_PERF) and logged.

Result no-stress test: although the worst-case jitter didn't change (still about 37 microseconds. At this point I'd say it's probably hardware-related), the update rate vs. time graph looks much cleaner. Additionally, five-sigma jitter is reduced to 1.4 microseconds.
See commit aaa2445251c52263d0d44b2f44b14953c649d60c.

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• [ ] Suggestion: Add a userspace thread to main.c for inserting data into the queue when the main thread is sleeping (i.e. when clock_nanosleep is called).

Additional Details The thread should have a somewhat lower FIFO priority (for instance 80). The code in the main thread can give a signal to the queue thread (e.g. by setting variable flag, or a mutex) exactly before it calls clock_nanosleep. Variables that are to be inserted into the queue can be global.

Result

[mohse-n]

• [ ] Suggestion: Disable Specter and Meltdown patches, which could cause degradation in the real-time performance.

Additional Details I believe it's possible to disable them via the kernel command line.

Result

[mohse-n]

• [ ] Suggestion: Disable i915 (Intel Graphics) driver, and use CONFIG_HZ_PERIODIC instead of NO_HZ. (Also, disable anything under "Kernel Hacking"?)

Additional Details In this thread of PREEMPT_RT mailing list, the following configurations are recommended:

• Enabling CONFIG_HZ_PERIODIC instead of NO_HZ variants.
• Disabling i915 driver (via kernel command line).
• Disabling? anything under "Kernel Hacking".

Result