Question about pthreads on Linux

Question about pthreads on Linux

[Jonathan Walsh]

I am attempting to set up a program that has 3 threads.  The first
thread is a receiver process that listens on a UDP port and puts a
struct on a one of two queues depending on a flag in the packet.  The
other two threads block on a pthread_cond_t (that is signaled when a
message is added to their queue), then take messages off their
corresponding queues and perform a fixed amount of computation.  I am
setting thread priorities as follows:

  pthread_t tid_processor;
  struct sched_param sparam;
  pthread_attr_t tattr;
  pthread_attr_init(&tattr);
  pthread_attr_getschedparam(&tattr, &sparam);
  sparam.sched_priority = SOME_THREAD_PRIORITY;
  pthread_attr_setschedpolicy(&tattr, SCHED_FIFO);
  pthread_attr_setschedparam(&tattr, &sparam));
  pthread_create(&tid, &tattr, &thread_function, NULL );

I set the receiver thread to the highest priority, then one of the
processor threads is higher than the other.

I have two questions about this situation:

1) Are pthread priorities reversed from Linux priorities.  In other
words, does a lower number mean a higher priority?
2) Will a higher priority thread preempt the execution of a lower
priority thread?  With the behavior I am seeing now, this seems to not
be the case.  If the lower priority processing thread begins execution
first the higher priority processing thread will not begin until it has
finished.  Though I correctly receive messages and put them into the
queue making this all the more puzzling.

Thank you for any help with my problem!  Let me know if you need more
information or detail in my questions.


Jonathan Walsh
Member Engineering Staff
Lockheed Martin Advanced Technology Laboratories

Re: Question about pthreads on Linux

[Steve Graegert]

Jonathan,

On 9/25/07, Jonathan Walsh <jwalsh@atl.lmco.com> wrote:
> I am attempting to set up a program that has 3 threads.  The first
> thread is a receiver process that listens on a UDP port and puts a
> struct on a one of two queues depending on a flag in the packet.  The
> other two threads block on a pthread_cond_t (that is signaled when a
> message is added to their queue), then take messages off their
> corresponding queues and perform a fixed amount of computation.  I am
> setting thread priorities as follows:
>
>   pthread_t tid_processor;
>   struct sched_param sparam;
>   pthread_attr_t tattr;
>   pthread_attr_init(&tattr);
>   pthread_attr_getschedparam(&tattr, &sparam);
>   sparam.sched_priority = SOME_THREAD_PRIORITY;
>   pthread_attr_setschedpolicy(&tattr, SCHED_FIFO);
>   pthread_attr_setschedparam(&tattr, &sparam));
>   pthread_create(&tid, &tattr, &thread_function, NULL );
>
> I set the receiver thread to the highest priority, then one of the
> processor threads is higher than the other.
>
> I have two questions about this situation:
>
> 1) Are pthread priorities reversed from Linux priorities.  In other
> words, does a lower number mean a higher priority?

No. SCHED_FIFO or SCHED_RR have an allowed range of 1 to 99 with lower
numbers representing lower thread priorities.

> 2) Will a higher priority thread preempt the execution of a lower
> priority thread?

Normally, this is the true for SCHED_FIFO, where threads run until
preempted by a thread of higher priority, or until blocked.

However, Linux limits the scheduling policies SCHED_RR and SCHED_FIFO
to processes with superuser privileges.  So, unless your program runs
with elevated privileges SCHED_OTHER will be selected by default which
is "based on the nice level ... and increased for each time quantum
the process [i.e. thread] is unable to run." according to
sched_setscheduler(2).

(Note, the pthread_set_schedparam() call maps to the
sched_setscheduler() call. An operation that requires root
privileges.)

For SCHED_OTHER the allowed mininmum and maximum priorities are 0.
Therefore it is not possible to change the priority.

Hope that sheds some light on your questions

	\Steve