Re: RT linux and priorities

["Konrad, Martin via Core-talk" <core-talk at aps.anl.gov>]

Hi Michael,
> It looks like the ISR threads default to SCHED_FIFO/50, which is also what
> epicsThreadPriorityMedium currently maps to.  So eg. the 1 second scan task
> has a higher priority (63) than the NIC ISR thread (50).  This seems like
> a recipe for some unexpected priority inversions.

Looking at the output of your "ps" command the header of the first two
columns say "RTPRIO" and "PRI" (in this order). It seems like you are
comparing numbers in the "PRI" column. The man page of "ps" says:

pri         PRI       priority of the process.  Higher number means
                      lower priority.

If I understand that correctly 63 would mean _lower_ priority than _50_.
I'm not sure if the fact that SCHED_RR is active changes the meaning of
these numbers.

> Now I'm also wondering how firefox is able to set SCHED_RR/10
Hmm, IIRC setting SCHED_RR requires elevated privileges. Maybe you have
granted a parent process (or possibly even PID 1) elevated permissions?
If your machine is using systemd you might want to run

grep -ri rtprio /etc/systemd/

to check. "ulimit -r" might also provide some insight.

Martin

-- 
Martin Konrad
Facility for Rare Isotope Beams
Michigan State University
640 South Shaw Lane
East Lansing, MI 48824-1321, USA
Tel. 517-908-7253
Email: konrad at frib.msu.edu