Subj : Re: pthread priority in Linux 2.4 Kernel
To   : comp.programming.threads
From : forSale
Date : Tue Aug 02 2005 05:58 pm

Thank Dave, for your answer. Our system is designed such a way that I need
to have multiple prority threads. To prove the priority scheme is working in
LINUX pthread environment I needed to test a small program. You are right
about  STDIO blocking,  which might  give just enough time to lower prority
thread to run. In real-time system, lower prority thread never runs util the
higher priority thread is blocked .. I know Linux is not a real-time system
however pthread claims SCHED_FIFO as real-time scheduler. I have modifed my
code, in my new code the  low priorty thread prints data on the screen and
high priorty task does some number crunching. Now in real-time system the
low priority task should never get the schedule time. It doesnt seem that
way in Linux .. Let me know if you have tried thread priority test before
and how would you prove pthread priority scheme is working


include <pthread.h>
#include <stdio.h>
void * lowPriotiyThread(void *arg)
{
    while (1)) printf ("Thread--- %s \n", (unsigned char *) arg);
}

void * HighPriotiyThread(void *arg)
{
int i =0
    while (1) i++;
 }
int main ()
{
 pthread_attr_t attr;
pthread_t thread1, thread2;
 struct ched_param param;
 pthread_attr_init(&attr);
param.sched_priority =20;
pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
pthread_attr_setchedparam(&attr, &param)
pthread_create(&thread1, &attr,lowPriotiyThread, "Thread1");
param.sched_priority =22;
pthread_create(&thread2,&attr,HighPriotiyThread, "Thread2");
pthread_join ( thread1, NULL)
}


Output
Thread--- Thread1
Thread--- Thread1
Thread--- Thread1
Thread--- Thread1
Thread--- Thread1
Thread--- Thread1
..........

This output continues never stops. That tells me Linux is still assigning
time slice to low priority task.
By the way how do I know the system is running on two VIRTUAL CPUs.
         Thanks
              Ottawa


"David Schwartz" <davids@webmaster.com> wrote in message
news:dcoed9$t2i$1@nntp.webmaster.com...
>
> "forSale" <@hotmail.com> wrote in message
> news:G7qdnURgOZzhEXLfRVn-ug@magma.ca...
>
> > Now Since thread 2 has higher priority than thread1. Thread1 should
never
> > get executed once thread2 start to run.  Why is the thread1 getting
> > executing again.
>
>     Your assumption is definitely wrong. Just because one thread has
higher
> priority than another, it doesn't mean the other thread won't run! It just
> means that if the system ever has to choose which of the two threads to
run,
> it will choose the higher priority one. But maybe it doesn't have to
choose
> because it has two virtual CPUs. Or maybe the higher priority thread is
> blocked trying to output data to a full buffer.
>
>     DS
>
>

.