6bc
Subj : Re: Solaris real-time thread capabilities
To   : comp.unix.solaris,comp.programming.threads
From : Logan Shaw
Date : Sat Sep 03 2005 04:37 am

capnwhit@yahoo.com wrote:
> ========
> QUESTION
> ========
> How can I get 10 millisecond timing reliably on a 2 CPU UltraSPARC-II
> 450MHz system?


> (H) Using spin loop
> -------------------------------------
> In desperation I have also tried using a spin loop using "gethrtime()"
> in order to _NOT_ call nanosleep, as shown conceptually below:
> 
> ----------
> t2 = gethrtime();
> while(t2 < target_time)
> {
>   t1 = gethrtime();
>   t2 = gethrtime();
> 
>   diff = t2 - t1;
>   [...cut...]
> }
> ----------
> 
> In the above loop, the average difference between t1 and t2 is about
> 1000 nanoseconds (this is good), but once in a while the difference
> between t1 and t2 is 300,000,000 nanoseconds (this is bad)!!!

If I had to guess (and a guess is exactly what it'd be), I would say
that what's going on here is that your code is running as a user
process, and every now and then there is a kernel thread that is a
higher priority that runs and excludes your thread from the
processor.

If I recall correctly, the range of priorities for userspace threads
is a subset of the range of priorities for kernel threads, so it is
possible to have kernel threads that have a priority higher than your
userspace thread, no matter how high you make its priority, and I
believe that even applies if you make your thread a realtime thread.
I've tried to find my Solaris 8 Internals book to confirm that, but I
don't see it, and I don't have time to undertake a major excavation
on my desk right now...  ;-)

   - Logan

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