Experimental Physics and
Industrial Control System

"Hill, Jeff" <[email protected]> · Tue, 27 Nov 2012 22:34:43 +0000

> 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.

I'm ok with adding a new function called osiSpawnDefaultAttributeDetachedProcess() for use with starting the ca repeater, and documenting also that the intent of osiSpawnDetachedProcess will be to create a process inheriting most of the calling processes attributes. 

Jeff

> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> On Behalf Of Andrew Johnson
> Sent: Tuesday, November 27, 2012 2:19 PM
> To: [email protected]
> Subject: Re: posix osiSpawnDetachedProcess inherits scheduling policy +
> priority
> 
> 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