[fpc-devel] Linux thread priority mess (and possible implementation)

[Jonas Maebe]

On 24 Jun 2010, at 15:05, Graeme Geldenhuys wrote:

> Also does multiple schedulers apply to other non-Linux platforms too?
> eg: OSX, FreeBSD, NetBSD, Haiku, etc.  Because currently any unix-type
> platform doesn't have Thread Priority support in FPC.

The POSIX pthread_setschedparam() api is not really appropriate in  
this case. It's mainly geared at real-time threads, which is why POSIX  
only define the behaviour of SCHED_RR and SCHED_FIFO parameters (both  
of which are real-time schedulers in this context).

For SCHED_OTHER, the OS can do whatever it wants (including treating  
it as another real-time scheduler class, or just as an alias for  
SCHED_RR or SCHED_FIFO), and there is no guarantee that the  
sched_priority field of the schedparam srtucture is used in that case  
(the parameters are completely implementation-defined). Both http://opengroup.org/onlinepubs/007908775/xsh/pthread_setschedparam.html 
  and the Linux man page for pthread_setschedparam() mention that.

I'm not aware of any portable Unix API to set the priority of threads  
scheduled by the default scheduler (unless that happens to be a  
SCHED_FIFO or SCHED_RR real-time scheduler, which will only be the  
case on real-time systems). I'm not even sure about what the low level  
APIs are on particular Unix platforms such as Linux and Mac OS X.


Jonas