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

[Hans-Peter Diettrich]

Graeme Geldenhuys schrieb:

> Looking at the rtl/unix/cthread.pp unit, I noticed two $Warning lines sayin
> that setting or getting thread priority is not implemented for any unix
> system.  WHAT???
> 
> I then thought I would go ahead and do the implementation. Seeing that
> cthreads using libc and POSIX implementation, there was lots of docs about
> it on the net, but the deeper I went the more confused I got.
> 
> In summary, this info is what I gathered so far:
> 
> * Linux thread priority is VERY confusing (compared to Windows).
> * You have Static and Dynamic thread priority.
> * You also have various Thread Scheduling priorities (different from
>   thread priorities).
[...]

There exists a broad range of scheduler algorithms and implementations, 
that can vary even with customized kernels of the same version, or with 
a user specific configuration. Some schedulers maintain a dynamic 
priority for every process or thread, that varies over time, so that 
long-running high priority jobs do not block the entire system. True 
realtime systems impose further constraints on (high priority) threads, 
e.g. that they must either finish in a given time period, or must reduce 
their own priority after the really time critical parts have been done, 
or otherwise the system will assign them a lower priority automatically.

Thus priorities should only be considered as *hints* to the scheduler, 
with unpredictable consequences on arbitrary machines. When an 
application is tuned for best performance on a specific machine, it may 
behave miserably on a similar machine with only a different scheduler, 
or on a very different platform.

DoDi