Accessing the same data from multiple threads

Question

Hi
In the following program the task5 routine takes ~3s to complete, when I uncomment the t2 lines it takes 11s (this is on a quad-core x86/64 machine). Since there is no explicit synchronization I was expecting 3s in both cases.
public static int sdata;
public static void Task5()
int acc = 0;
for (int i = 0; i < 1000000000; ++i)
sdata = i;
acc += sdata;
[STAThread]
static void Main()
Stopwatch sw = new Stopwatch();
sw.Start();
Thread t1 = new Thread(new ThreadStart(Task5));
// Thread t2 = new Thread(new ThreadStart(Task5));
t1.Start();
// t2.Start();
t1.Join();
// t2.Join();
sw.Stop();
System.Diagnostics.Debug.WriteLine(sw.ElapsedMilliseconds.ToString());
Why are these threads blocking each other?

Answer 1

This is loosely a duplicate of https://social.msdn.microsoft.com/Forums/en-US/cd00284d-3da3-457e-8926-c490e7ca6d92/atomic-loadstore?forum=vclanguage
I answered you in detail over at the other thread.
But the short version is that the threads are competing for access to system memory, specifically at the memory location of sdata.  This demonstrates how to spoil the benefits of not having to write-through from your CPU cache to system memory.  CPU
Caches are wonderful things.  CPU cache memory is WAY faster than system memory.