How To Use Posix Thread Priority Scheduling under Linux
What is this about?
The linux scheduler supports the SCHED_FIFO scheduling policy defined by POSIX.1-2001. Threads scheduled with this "real-time" policy can be assigned a priority (under linux) in the range 1..99 with 99 representing the highest priority. Since ordinary, "non-real time" processes execute at priority 0 ("nice" affects a "dynamic priority" which only affects processes with real-time priority 0 and implements fair timesharing among such processes) SCHED_FIFO threads are always capable to preempt "ordinary processes" (these use the policy SCHED_OTHER).
Note, however, that unless the linux kernel is built with the RT_PREEMPT patch and measures are taken to lock the EPICS process in memory etc., no strictly deterministic latencies can be expected even under the SCHED_FIFO policy which thus is to be considered soft-real time.
How can I let epicsThreads use the SCHED_FIFO policy?
In order to let EPICS use the SCHED_FIFO real-time policy you first need to check that the following option is set to "YES" in epics-base/configure/CONFIG_SITE
# Use POSIX thread priority scheduling (YES or NO) USE_POSIX_THREAD_PRIORITY_SCHEDULING = YES
If you find that the current setting is "NO" then you need to rebuild EPICS base (make clean uninstall; make) after changing to 'YES'.
Since engaging the SCHED_FIFO policy gives any thread created under that policy a higher priority than any normal process'es using SCHED_FIFO requires special privileges. Under a reasonably recent linux equipped with the pam_security module it is not necessary to execute EPICS as root. The system administrator can define the maximum priority that may be used by specific users and/or groups in a file under /etc/security/limits.d/, see man limits.conf(5).
E.g., if a file /etc/security.limits.d/epics.conf is created with the following contents:
someuser hard rtprio 20 someuser soft rtprio 0
Then processes created by 'someuser' still have priority 0 by default (this is the 'soft' value) but 'someuser' may increase the resource limit up to 20 either from a running program (using the system call setrlimit(2)) or from a shell with the ulimit -r utility.
Note that
