Here is a piece of code that shows some very peculiar
performance. For some strange reason, sorting the data miraculously speeds up the code by almost 6x:
#include <algorithm>
#include <ctime>
#include <iostream>
int main()
{
// generate data
const unsigned arraySize = 32768;
int data[arraySize];
for (unsigned c = 0; c < arraySize; ++c)
data[c] = std::rand() % 256;
// !!! with this, the next loop runs faster
std::sort(data, data + arraySize);
// test
clock_t start = clock();
long long sum = 0;
for (unsigned i = 0; i < 100000; ++i)
{
// primary loop
for (unsigned c = 0; c < arraySize; ++c)
{
if (data[c] >= 128)
sum += data[c];
}
}
double elapsedTime = static_cast<double>(clock() - start) / CLOCKS_PER_SEC;
std::cout << elapsedTime << std::endl;
std::cout << "sum = " << sum << std::endl;
}
Without std::sort(data, data + arraySize);, the code runs in 11.54 seconds.
With the sorted data, the code runs in 1.93 seconds.
Initially I thought this might be just a language or compiler anomaly. So I tried it Java...
import java.util.Arrays;
import java.util.Random;
public class Main
{
public static void main(String[] args)
{
// generate data
int arraySize = 32768;
int data[] = new int[arraySize];
Random rnd = new Random(0);
for (int c = 0; c < arraySize; ++c)
data[c] = rnd.nextInt() % 256;
// !!! with this, the next loop runs faster
Arrays.sort(data);
// test
long start = System.nanoTime();
long sum = 0;
for (int i = 0; i < 100000; ++i)
{
// primary loop
for (int c = 0; c < arraySize; ++c)
{
if (data[c] >= 128)
sum += data[c];
}
}
System.out.println((System.nanoTime() - start) / 1000000000.0);
System.out.println("sum = " + sum);
}
}
with a similar but less extreme result.
My first thought was that sorting brings the data into cache, but my next thought was how silly that is because the array was just generated.
What is going on? Why is a sorted array faster than an unsorted array? The code is summing up some independent terms, the order should not matter.
