Subj : Solaris real-time thread capabilities
To   : comp.unix.solaris,comp.programming.threads
From : capnwhit
Date : Fri Sep 02 2005 08:37 pm

========
QUESTION
========
How can I get 10 millisecond timing reliably on a 2 CPU UltraSPARC-II
450MHz system?

=================
PROBLEM STATEMENT
=================
I have a Solaris application in which I need a thread to process
packages approximately every 10 milliseconds. The thread should be
woken up every 10 ms to send the queued packages and the thread should
then "sleep" until the next 10 ms interval. There is a jitter buffer at
the other end, so I don't need _EXACTLY_ 10 ms... even if I get woken
up within 20 ms it is OK... but being off by 100 ms would be bad.

=======
SUMMARY
=======
I have tried but I can't get the required timing despite using
"hires_tick", and despite creating a RT (realtime) thread, and despite
creating a processor set and allocating one entire CPU to said thread
on a 2 CPU machine. I have read everything I can get my hands on and I
have tried every trick in my book, but I have failed to maintain the
required timing... I tried using SIGALRM to get the required timing, I
tried nanosleep, and I even tried a spin loop, and nothing works... any
help would be appreciated.

===========
DESCRIPTION
===========
I have tried many, many different combinations of the different
strategies shown below... None have yielded adequate results. Anybody
have any other hints or tricks?

(A) Thread priority
-------------------------------------
I have tried increasing the priority of the given thread using
"pthread_attr_setschedparam". Even after setting the maximum priority,
the thread still does not get woken up in time.

(B) Using high resolution timers
-------------------------------------
I have tried enabling "hires_tick" in /etc/system, as follows:

----------
set hires_tick = 1
----------

Even after setting "hires_tick" my thread does not get woken up
reliably every 10 ms.

(C) Using RT (realtime) class
-------------------------------------
I have tried using a Solaris realtime thread using "priocntl"... This
type of thread will not get preempted by the operating system (and you
need root access to create one of these threads). This did not help
either.

(D) Using processor sets
-------------------------------------
I have created a processor set using "psrset", and I have allocated the
second processor (processor 2) to the processor set:

----------
root@vulcan:/> psrset
user processor set 1: processor 2
----------

I have allocated only 1 (one) thread to the processor set. In other
words, only my realtime LWP is running on processor #2 (nothing else is
running on said processor). Even with an entire CPU at it's disposal,
the thread still does not get woken up in time.

(E) Using processor NO-INTERRUPT mode
-------------------------------------
I have prevented processor 2 from being interrupted by I/O devices by
setting it to "no-interrupt" mode:

----------
root@vulcan:/> psrinfo
0       on-line   since 08/26/05 16:17:01
2       no-intr   since 08/26/05 17:19:48
----------

Even if the processor is set to "no-intr", the thread still does not
maintain reliable timing.

(F) Using SIGALRM
-------------------------------------
My initial attempt was to use SIGALRM to deliver the required timing,
but under Solaris 8 this did not work reliably.

(G) Using nanosleep
-------------------------------------
I have tried using "nanosleep" to sleep the prescribed amount of time.
Most of the time nanosleep wakes up my thread reliably, but once in a
while the thread is woken up too late (i.e. more than 100 ms have
elapsed).

(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)!!!

(H) I am out of ideas...
-------------------------------------
Anybody have any suggestions? Is there any way to get reliable 10
millisecond resolution from Solaris?


======================
HARDWARE CONFIGURATION
======================
System Configuration:  Sun Enterprise 220R
CPU: 2 X UltraSPARC-II 450MHz
System clock frequency: 113 MHz
Memory size: 2048 Megabytes

Thanks!

.