C# Memory optimization for large arrays

Question

Here are two code parts in c++ and c# doing absolutely the same thing:

C++
http://ideone.com/UfL5R

#include <stdio.h>
int main(int argc, char *argv[]) {
  char p[1000000];
  unsigned int i,j;
  unsigned long long s=0;
  for(i=2;i<1000000;i++) p[i]=1;
  for(i=2;i<500000;) {
    for(j=2*i;j<1000000;j+=i) p[j]=0;
    for(i++;!p[i];i++);
  }
  for(i=3,s=2;i<1000000;i+=2) if(p[i]) s+=i;
  printf ("%lld\n",s);
  return 0;
}

time: 0.01s memmory: 2576 kB

C#
http://ideone.com/baXYm

using System;

namespace ConsoleApplication4
{
    internal class Program
    {
        private  static void Main(string[] args)
        {
            var p = new byte[1000000];
            ulong i, j;
            double s = 0;
            for(i=2;i<1000000;i++) 
                p[i]=1;

            for(i=2;i<500000;) 
            {
                for(j=2*i;j<1000000;j+=i) 
                    p[j]=0;
                for(i++;p[i]==0;i++);
            }

            for(i=3,s=2;i<1000000;i+=2) 
                if(p[i]!=0) s+=i;

            Console.WriteLine(s);
        }
    }
}

time: 0.05s mem: 38288 kB

How can I improve the C# code to prove that C# can be as fast as C++ to my colleague?

As you can see the C# execution time is 5 time larger, and the memory consumption is 15 times larger.

Answer 1

Compile and run in Release mode. I get exactly 0.01s from the C# version when built and run in Release mode. As far as memory consumption is concerned you are comparing apples to oranges. A managed environment will consume more memory as it is hosting the CLR and the Garbage Collector which don't come without cost.

Answer 2

How to GREATLY Increase the Performance of your C# Code

Go "unsafe" (unmanaged) for that... every time you're doing someSortOfArray[i], the .NET framework is doing all kinds of neat-o things (such as out of bounds checking) which take up time.

That's really the whole point of going unmanaged (and then using pointers and doing myPointer++).

Just to clarify, if you go unmanaged and then still do a for-loop and do someArray[i], you've saved nothing.

Another S.O. question that may help you: True Unsafe Code Performance

Disclaimer

By the way, I'm not saying to do this all the time, but rather as an answer for THIS specific question only.