random_access_test.c

#define _GNU_SOURCE // O_DIRECT
#include <xxhash.h> // XXH64
#include <stddef.h> // size_t
#include <stdint.h> // uint64_t

typedef uint64_t u64;

void genRand(u64* a64, size_t a64Size)
{
        u64 r = XXH64(NULL, 0, a64Size);
        for (size_t n=0; n<a64Size; n++) {
                 a64[n] = r;
                 r = XXH64(&r, 8, 0);
        }
}


#include <fcntl.h>   // open
#include <stdlib.h>  // malloc
#include <time.h>    // clock_t
#include <stdio.h>   // fopen, fclose,
#include <assert.h>


u64 getFileSize(FILE* f)
{
        fseek(f, 0, SEEK_END); // seek to end of file
        u64 const size = ftell(f); // get current file pointer
        fseek(f, 0, SEEK_SET);
        return size;
}

#define NB_RAND_ACC 10000
int main(int argc, const char* argv[])
{
        assert(argc == 2);
        const char* const filename = argv[1];
        printf("opening %s ...\n", filename);
        //int const lf = open(filename, O_RDONLY | O_DIRECT); assert(lf != -1);
        //FILE* const f = fdopen(lf, "rb"); assert(f != NULL);
        FILE* const f = fopen(filename, "rb"); assert(f != NULL);
        u64 const filesize = getFileSize(f);
        printf(" size = %llu \n", (unsigned long long)filesize);

        u64* const r64 = malloc(NB_RAND_ACC * sizeof(*r64)); assert(r64 != NULL);
        genRand(r64, NB_RAND_ACC);

        u64 acc = 0;
        clock_t const start = clock();
        for (size_t n=0; n<NB_RAND_ACC; n++) {
                u64 const pos = r64[n] % filesize;
                //printf("pos = %9u \n", (unsigned)pos);
                int const ir = fseek(f, pos, SEEK_SET);
                assert(ir==0);
                uint8_t val;
                size_t const n = fread(&val, 1, 1, f);
                assert(n==1);
                acc += val;
        }
        clock_t const duration = clock() - start;

        printf("%u reads => %.3f seconds  (total=%llu) \n", NB_RAND_ACC, (double)duration / CLOCKS_PER_SEC, (unsigned long long)acc);

        free(r64);
        fclose(f);
}