在多线程共享过程中，原子操作似乎比信号量操作慢

Question

Can you think on any good reason why atomic operations seems slower than semaphores, even though there is a decrease on instructions?

Sample code:

 void increment(){
     if (strcmp(type, "ATOMIC") == 0) {
         for (int i = 0; i < RUN_TIME; ++i) {
            atomic_fetch_add_explicit(&count, 1, memory_order_relaxed);
        }
    }
     if (strcmp(type, "SEMAPHORE") == 0){
        for (int i = 0; i < RUN_TIME; ++i) {
            sem_wait(sem);
            count++;
            sem_post(sem);
        }
    }
}

Output:

   time ./CMAIN "SEMAPHORE";time ./CMAIN "ATOMIC";
 [C] SEMAPHORE, count 4000000

 real    0m0.039s
 user    0m0.029s
 sys     0m0.002s
[C] ATOMIC, count 4000000

 real    0m0.092s
 user    0m0.236s
 sys     0m0.003s

Answer 1

It shouldn't because what I read is that "in semaphore When some process is trying to access semaphore which is not available, semaphore puts process on wait queue(FIFO) and puts task on sleep , it's more time consuming or more overheads for CPU rather than Atomic operations.

normally atomic operation will perform faster because it will load, update & modify instruction all together. But Atomic operation are CPU specific ie n++ will executed in single instruction (INC) or not, always can't guarantee. So it's upto CPU to decide, May be because of this reason you are getting output like this.

What I understood I wrote, suggestion will be appreciated.

Answer 2

Can't reproduce. For 10^9 iterations, I'm getting (from bash, i5, x86_64, Linux):

$ TIMEFORMAT="%RR %UU %SS"
$ gcc atomic.c -Os -lpthread && ( time ./a.out ATOMIC  ; time ./a.out  SEMAPHORE )
1.572R  1.568U  0.000S  #ATOMIC
5.542R  5.536U  0.000S  #SEMAPHORE

(About the same ratio for 4000000 iterations.)

My atomic.c (your example with the blanks filled in):

#include <stdio.h>
#include <string.h>
#include <stdatomic.h>
#include <semaphore.h>
#define RUN_TIME 100000000
char * type;
sem_t *sem;

_Atomic int count = ATOMIC_VAR_INIT(0);

 void increment(){
     if (strcmp(type, "ATOMIC") == 0) {
         for (int i = 0; i < RUN_TIME; ++i) {
            atomic_fetch_add_explicit(&count, 1, memory_order_relaxed);
        }
    }
     if (strcmp(type, "SEMAPHORE") == 0){
        for (int i = 0; i < RUN_TIME; ++i) {
            sem_wait(sem);
            count++;
            sem_post(sem);
        }
    }
}

int main(int C, char**V)
{
    sem_t s;
    sem_init(&s, 0, 1);
    sem = &s;
    type = V[1];
    increment();
}

Please post an mcve , along with your platform specs.