Subj : Re: why my threads do not wake up
To   : comp.programming.threads
From : David Schwartz
Date : Thu Aug 18 2005 01:04 pm


"Jingbo" <jingbo@cadence.com> wrote in message 
news:1124384707.985991.201890@g14g2000cwa.googlegroups.com...

> I created 8 threads to do some tasks in parallel. I ran my program on a
> SunFire which has 8 CPUs. I found that only 2 or 3 threads are working
> on their tasks. Other threads are waiting in pthread_cond_wait(). I
> print a message after pthread_cond_wait(), and found out they are never
> waken up.
>
> I'm sure that there are plenty of jobs on the queue and
> pthread_cond_broadcast() is called and there is no other processes
> running on the server therefore CPUs are idel and sometimes my program
> can use all 8 CPUs.
>
> My question is, why Solaris did not put my threads on CPU even if there
> are idel CPUs?

    Because the threads are waiting for a condition to be signalled. You 
called 'pthread_cond_wait' and that will cause your threads to wait until 
some other thread signals the condition variable.

    Now, if you had said, "I print a message after pthread_cond_broadcast, 
and found out they are never waking up", you'd have something. That's the 
function that's supposed to wake them up.

    Condition variables are hard to use correctly until you get the hang of 
it. If your call to 'pthread_cond_wait' doesn't look like this:

pthread_mutex_lock(...)
while(nothing_to_do()) pthread_cond_wait(...)
do_stuff();
pthread_mutex_unlock();

    Then that's your problem. Most importantly, don't wait if there's work 
to do!

    DS

.