Real Time processes in Linux


 
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# 1  
Old 08-15-2011
Real Time processes in Linux

I was trying to experiment with SCHED_RR and SCHED_FIFO.

I tried benchmarking the SCHED_NORMAL with these two real time priorities.

What I found is strange result. SHED_RR was the slowest then comes SCHED_FIFO and SCHED_NORMAL was the fastest one.

All tests are run in same situation and independently. And these process are only CPU intensive, no IO.

I expect FIFO to run faster because no no content switching. Can anyone help understand this mystery.
# 2  
Old 08-15-2011
It would help to see your code.
# 3  
Old 08-17-2011
Hi Corona,

following is my code for FIFO and RR. And have started these process from root user to have the CAP_SYS_RESOURCE.

Code:
[hawker@localhost assign_4]$ cat ff_racehorse.c 
#include <stdio.h> 
#include <sched.h>
#include <unistd.h>
#include <sys/types.h>



int main()
{
    
    double a,b,c;
    
    long counter = 1024*1024*512;;    
    
    struct sched_param param;
    param.sched_priority = 99;
    pid_t pid = getpid();
    sched_setscheduler(pid,SCHED_FIFO,&param);
    while(counter--)
    {
        c=a*b;
    }    
    printf("%f %ld\n",c,counter);
}
[hawker@localhost assign_4]$ cat racehorse.c 
#include <stdio.h> 
#include <sched.h>
#include <unistd.h>
#include <sys/types.h>



int main()
{
    
    double a,b,c;
    
    long counter = 1024*1024*512;;    
    
    struct sched_param param;
    param.sched_priority = 99;
    pid_t pid = getpid();
    sched_setscheduler(pid,SCHED_RR,&param);
    while(counter--)
    {
        c=a*b;
    }    
    printf("%f %ld\n",c,counter);
}
[hawker@localhost assign_4]$

Normal process took 55 secs
FIFO took 1 min 25 secs
RR took 2 min 10 secs.

All of them were started one by one.
# 4  
Old 08-19-2011
I would expect SCHED_RR to be slower than SCHED_FF, since it's actually limited to time-slices when SCHED_FF is not.

There are higher-priority things pre-empting your process all the time, interrupt handlers and the like, and FIFO/RR dno't mean they're necessarily given the lion's share of time -- just guaranteed never to be starved.
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