Simple test to measure cache lines size - c++

Question

Starting from this article - Gallery of Processor Cache Effects by Igor Ostrovsky - I wanted to play with his examples on my own machine.
This is my code for the first example, that looks at how touching different cache lines affect running time:
#include <iostream>
#include <time.h>
using namespace std;
int main(int argc, char* argv[])
{
int step = 1;
const int length = 64 * 1024 * 1024;
int* arr = new int[length];
timespec t0, t1;
clock_gettime(CLOCK_REALTIME, &t0);
for (int i = 0; i < length; i += step)
arr[i] *= 3;
clock_gettime(CLOCK_REALTIME, &t1);
long int duration = (t1.tv_nsec - t0.tv_nsec);
if (duration < 0)
duration = 1000000000 + duration;
cout<< step << ", " << duration / 1000 << endl;
return 0;
}
Using various values for step, I don't see the jump in the running time:
step, microseconds
1, 451725
2, 334981
3, 287679
4, 261813
5, 254265
6, 246077
16, 215035
32, 207410
64, 202526
128, 197089
256, 195154
I would expect to see something similar with:
But from 16 onwards, the running time is halved each time we double the step.
I test it on an Ubuntu13, Xeon X5450 and compiling it with: g++ -O0.
Is something flawed with my code, or the results are actually ok?
Any insight on what I'm missing would be highly appreciated.

Answer 1

As i see you want to observe effect of cache line sizes, i recommend tool cachegrind, part of valgrind tool set. Your approach is right but not close to results.
#include <iostream>
#include <time.h>
#include <stdlib.h>
using namespace std;
int main(int argc, char* argv[])
{
int step = atoi(argv[1]);
const int length = 64 * 1024 * 1024;
int* arr = new int[length];
for (int i = 0; i < length; i += step)
arr[i] *= 3;
return 0;
}
Run tool valgrind --tool=cachegrind ./a.out $cacheline-size and you should see results. After plotting this you will get desired results with accuracy. Happy Experimenting!!

Answer 2

public class CacheLine {
public static void main(String[] args) {
CacheLine cacheLine = new CacheLine();
cacheLine.startTesting();
}
private void startTesting() {
byte[] array = new byte[128 * 1024];
for (int testIndex = 0; testIndex < 10; testIndex++) {
testMethod(array);
System.out.println("--------- // ---------");
}
}
private void testMethod(byte[] array) {
for (int len = 8192; len <= array.length; len += 8192) {
long t0 = System.nanoTime();
for (int i = 0; i < 10000; i++) {
for (int k = 0; k < len; k += 64) {
array[k] = 1;
}
}
long dT = System.nanoTime() - t0;
System.out.println("len: " + len / 1024 + " dT: " + dT + " dT/stepCount: " + (dT) / len);
}
}
}
This code helps you with determining L1 data cache size. You can read about it more in detail here. https://medium.com/#behzodbekqodirov/threading-in-java-194b7db6c1de#.kzt4w8eul