RT thread running slower as SCHED_OTHER threads increases

[Journey Journey <journeywang123@xxxxxxxxx>]

Hi  Dear All

I had tested a program in order to calculate computing time in a
machine with 48  intel cpu cores.

1. when running ./test, it outputs like this:
MM: 324.039000 ms
MM: 324.052000 ms
MM: 324.079000 ms
MM: 324.042000 ms
MM: 324.060000 ms
MM: 324.052000 ms
MM: 324.026000 ms
MM: 324.040000 ms
MM: 324.075000 ms
MM: 324.011000 ms
MM: 324.072000 ms

2. running other programs, like sample code 2
./aa &
./aa &
....
about 30 times

3. ./test output changes:
MM: 354.601000 ms
MM: 322.289000 ms
MM: 322.250000 ms
MM: 322.348000 ms
MM: 354.700000 ms
MM: 322.556000 ms
MM: 354.834000 ms
MM: 354.816000 ms
MM: 354.777000 ms
MM: 354.812000 ms
MM: 322.451000 ms
MM: 322.507000 ms
MM: 322.473000 ms

4. For purpose to decrease computing cost time, I had tried:
(1) set test policy to SCHED_RR/SCHED_FIFO, and priority to 50/98
 chrt -f -p 98 5911
(2)  bind test to specific cpu
taskset -pc 4 5911
(3) limit aa processes to other cpus
for i in `ps -ef |grep aa|awk '{print $2}'`; do echo $i; taskset -pc
5-47 $i; done

It still not working, the result is not good.
MM: 322.316000 ms
MM: 322.307000 ms
MM: 326.739000 ms
MM: 354.399000 ms
MM: 354.662000 ms
MM: 354.740000 ms
MM: 322.275000 ms

5. But when limit aa processes to a small cpuset, it works,
for i in `ps -ef |grep aa|awk '{print $2}'`; do echo $i; taskset -pc
5-7 $i; done

the result:
MM: 322.304000 ms
MM: 322.335000 ms
MM: 322.344000 ms
MM: 322.319000 ms
MM: 322.340000 ms
MM: 322.328000 ms
MM: 322.298000 ms
MM: 322.318000 ms


It's quite amazing why such low priority processes can affect realtime
process, please help me!



Sample test code1:

#include <stdio.h>
#include <sys/time.h>

void calc() {
    int c, d, k;
    int m = 100000, q = 1, p = 1000;
    int sum;
    for (c = 0; c < m; c++) {
        for (d = 0; d < q; d++) {
            for (k = 0; k < p; k++) {
                 sum = c * m * d * q * p * k;
            }
        }

    }
}

int main() {
    struct timeval t1, t2;
    double elapsedTime;

    while(1) {
        gettimeofday(&t1, NULL);
        calc();
        gettimeofday(&t2, NULL);
        elapsedTime = (t2.tv_sec - t1.tv_sec) * 1000.0;      // sec to ms
        elapsedTime += (t2.tv_usec - t1.tv_usec) / 1000.0;   // us to ms
        printf("MM: %lf ms\n", elapsedTime);
        sleep(1);
    }
}


Sample Code 2:

#include <stdio.h>

int main() {
    while(1) {
    }
}

Thanks~
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