Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Tue, 27 Nov 2012 15:18:36 -0600

Hi Till,

On 2012-11-26 Till Straumann wrote:
> The current implementation of posix/osiSpawnDetachedProcess()
> basically just forks and execs. The new process inherits the
> scheduling policy and priority of whoever called spawnDetachedProcess().
> 
> This means e.g., that a caRepeater spawned by a "real-time enabled"
> IOC application executes under SCHED_FIFO with priority 51.
> 
> Not sure this is a good thing. IMO osiSpawnDetachedProcess()
> should be fixed so that the new process starts out with SCHED_OTHER.

I doubt if there are many users of osiSpawnDetachedProcess() other than the 
caRepeater, but I wouldn't want to break any that do exist which need to 
inherit the SCHED_FIFO setting.  If the child process of the fork() were to 
change to SCHED_OTHER before calling execlp() the new program might not have 
the privilege necessary to switch back — is that correct?

Personally I would solve the problem of having an RT-scheduled caRepeater by 
ensuring that caRepeater always gets started when the system comes up before 
the IOCs get run, say from an /etc/init.d script.

We could add another API that switches to SCHED_OTHER before the execlp() and 
use that when starting the caRepeater, but I'm not sure about portability.

What do you think?

- Andrew
-- 
Computer science is as much about computers as astronomy is about
telescopes. -- Edsger Dijkstra

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System