mem.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <sys/time.h>

// Simple routine to return absolute time (in seconds).
double Time_GetSeconds() {
    struct timeval t;
    int rc = gettimeofday(&t, NULL);
    assert(rc == 0);
    return (double) ((double)t.tv_sec + (double)t.tv_usec / 1e6);
}

// Program that allocates an array of ints of certain size,
// and then proceeds to update each int in a loop, forever.
int main(int argc, char *argv[]) {
    for(int i = 1; i < 9; i++) {
        long long int size = (long long int) i;
        long long int size_in_bytes = size * 1024 * 1024 * 1024;

        printf("allocating %lld bytes (%.2f MB)\n", 
        size_in_bytes, size_in_bytes / (1024 * 1024.0));

        // the big memory allocation happens here
        int *x = malloc(size_in_bytes);
        if (x == NULL) {
            fprintf(stderr, "memory allocation failed\n");
            exit(1);
        }

        long long int num_ints = size_in_bytes / sizeof(int);
        printf("  number of integers in array: %lld\n", num_ints);

        // now the main loop: each time through, touch each integer
        // (and increment its value by 1).
        int k = 0;
        double time_since_last_print = 2.0; 
        double t = Time_GetSeconds();
        int loop_count = 0;
        double bandwidthSum = 0;
        while (1) {
            x[k++] += 1; // main work of loop done here.

            // if we've gone through the whole loop, reset a bunch of stuff
            // and then (perhaps) print out some statistics. 
            if (k == num_ints) {
                double delta_time = Time_GetSeconds() - t;
                time_since_last_print += delta_time;
                double bandwidth = size_in_bytes / (1024.0*1024.0*delta_time);
                bandwidthSum += bandwidth;
                if (time_since_last_print >= 0.2) { // only print every .2 seconds
                    printf("loop %d in %.2f ms (bandwidth: %.2f MB/s)\n", 
                        loop_count, 1000 * delta_time, bandwidth);
                    time_since_last_print = 0;
                }

                if (loop_count > 9) {
                    break;
                } else {
                    k = 0;
                    t = Time_GetSeconds();
                    loop_count++;
                }
            }
        }
        free(x);
        printf("Average bandwidth: %.2f\n\n", bandwidthSum / loop_count);
    }
    return 0;
}