Question

I’m writing a small program that uses a certain percentage of CPU. The basic strategy is that I will continuously check the CPU usage, and make the process sleep if the level of usage is higher than the given value.

Moreover, since I’m using MacOS(no proc/stat like Linux, no PerformanceCounter in C#), I have to execute top command in another thread and get the CPU usage from it.

The problem is I keep getting a very high usage of CPU even I give a small value as argument. And after several experiments, it seems caused by shared field by multithreads.

Here are my code(code 1) and experiments:

(code 2)Initially I thought it is the shell commands make the usage very high, so I commented the infinite loop in run(), leaving only the getCpuUsage() running. However, the CPU usage is nearly zero.

(code 3)Then, I wrote another run() function independent from the cpuUsage, which is intended to use 50% of CPU. It works well! I think the only difference between code 1 and code 3 is the usage of cpuUsage. So I’m wondering if sharing field between threads will use CPU a lot?

code 1

const char CPU_COMMAND[] = "top -stats cpu -l 1 -n 0| grep CPU\\ usage | cut -c 12-15";

int cpuUsage; // shared field that stores the cpu usage

// thread that continuously check CPU usage
// and store it in cpuUsage
void getCpuUsage() {
    char usage[3];
    FILE *out;
    while (1) {
        out = popen(CPU_COMMAND, "r");
        if (fgets(usage, 3, out) != NULL) {
            cpuUsage = atof(usage);
        } else {
            cpuUsage = 0;
        }
        pclose(out);
    }
}

// keep the CPU usage under ratio
void run(int ratio) {
    pthread_t id;
    int ret = pthread_create(&id, NULL, (void *)getCpuUsage, NULL);
    if (ret!=0) printf("thread error!");

    while (1) {
        // if current cpu usage is higher than ration, make it asleep
        if (cpuUsage > ratio) {
            usleep(10);
        }
    }

    pthread_join(id, NULL);
}

---

code 2

// keep the CPU usage under ratio
void run(int ratio) {
    pthread_t id;
    int ret = pthread_create(&id, NULL, (void *)getCpuUsage, NULL);
    if (ret!=0) printf("thread error!");

    /*while (1) {
        // if current cpu usage is higher than ration, make it asleep
        if (cpuUsage > ratio) {
            usleep(10);
        }
    }*/

    pthread_join(id, NULL);
}

---

code 3

void run() {
    const clock_t busyTime = 10;
    const clock_t idleTime = busyTime;

    while (1) {
        clock_t startTime = clock();
        while (clock() - startTime <= busyTime);
        usleep(idleTime);
    }
}

Answer 1

Does shared field in multithreading C program use CPU a lot?

Yes, constant reads/writes to/from a shared memory locations, by multiple threads on multiple CPUs cause a cache line to constantly move between CPUs (cache bounce). IMO, it’s the single most important reason for poor scalability in naive “parallel” applications.