Question

I have a function as follow:

int doSomething(long numLoop,long arraySize){
    int * buffer;
    buffer = (int*) malloc (arraySize * sizeof(int));
    long k;
int i;

    for (i=0;i<arraySize;i++)           
    buffer[i]=2;//write to make sure memory is allocated
    //start reading from cache
    for(k=0;k<numLoop;k++){
        int i;
        int temp
        for (i=0;i<arraySize;i++)           
        temp = buffer[i];
    }
}

What it do is to declare an array and read from the beginning to the end. The purpose is to see the effect of cache.
What I expect to see is: when I call doSomething(10000,1000), the arraySize is small so it is all stored in the cache. After that I call doSomething(100,100000), the arraySize is bigger than that of the cache. As a result, the 2nd function call should take longer than the 1st one. The latter function call involved in some memory access as the whole array cannot be stored in the cache.
However, it seems that the 2nd operation takes approximately the same time as the 1st one. So what’s wrong here? I tried to compile with -O0 and it doesnt solve the problem.
Thank you.

Update 1: these are the code with random access and it seems to work, time access with large array is ~15s while small array is ~3s

int doSomething(long numLoop,int a, long arraySize){
    int * buffer;
    buffer = (int*) malloc (arraySize * sizeof(int));
    long k;
    int i;

    for (i=0;i<arraySize;i++)           
        buffer[i]=2;//write to make sure memory is allocated
    //start reading from cache
    for(k=0;k<numLoop;k++){
        int temp;
            for (i=0;i<arraySize;i++){
                long randnum = rand();//max is 32767
                randnum = (randnum <<16) | rand();
                if (randnum < 0) randnum = -randnum;
                randnum%=arraySize;
                temp = buffer[randnum];
            }
    }
}

Answer 1

You are accessing the array in sequence,

for (i=0;i<arraySize;i++)           
    temp = buffer[i];

so the part you are accessing will always be in the cache since that pattern is trivial to predict. To see a cache-effect, you must access the array in a less predictable order, for example by generating (pseudo)random indices, so that you jump between the fron and the back of the array.