.NET 3.5SP1 64-bit memory model vs. 32-bit memory model
- by James Dunne
As I understand it, the .NET memory model on a 32-bit machine guarantees 32-bit word writes and reads to be atomic operations but does not provide this guarantee on 64-bit words. I have written a quick tool to demonstrate this effect on a Windows XP 32-bit OS and am getting results consistent with that memory model description.
However, I have taken this same tool's executable and run it on a Windows 7 Enterprise 64-bit OS and am getting wildly different results. Both the machines are identical specs just with different OSes installed. I would have expected that the .NET memory model would guarantee writes and reads to BOTH 32-bit and 64-bit words to be atomic on a 64-bit OS. I find results completely contrary to BOTH assumptions. 32-bit reads and writes are not demonstrated to be atomic on this OS.
Can someone explain to me why this fails on a 64-bit OS?
Tool code:
using System;
using System.Threading;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
var th = new Thread(new ThreadStart(RunThread));
var th2 = new Thread(new ThreadStart(RunThread));
int lastRecordedInt = 0;
long lastRecordedLong = 0L;
th.Start();
th2.Start();
while (!done)
{
int newIntValue = intValue;
long newLongValue = longValue;
if (lastRecordedInt > newIntValue) Console.WriteLine("BING(int)! {0} > {1}, {2}", lastRecordedInt, newIntValue, (lastRecordedInt - newIntValue));
if (lastRecordedLong > newLongValue) Console.WriteLine("BING(long)! {0} > {1}, {2}", lastRecordedLong, newLongValue, (lastRecordedLong - newLongValue));
lastRecordedInt = newIntValue;
lastRecordedLong = newLongValue;
}
th.Join();
th2.Join();
Console.WriteLine("{0} =? {2}, {1} =? {3}", intValue, longValue, Int32.MaxValue / 2, (long)Int32.MaxValue + (Int32.MaxValue / 2));
}
private static long longValue = Int32.MaxValue;
private static int intValue;
private static bool done = false;
static void RunThread()
{
for (int i = 0; i < Int32.MaxValue / 4; ++i)
{
++longValue;
++intValue;
}
done = true;
}
}
}
Results on Windows XP 32-bit:
Windows XP 32-bit
Intel Core2 Duo P8700 @ 2.53GHz
BING(long)! 2161093208 > 2161092246, 962
BING(long)! 2162448397 > 2161273312, 1175085
BING(long)! 2270110050 > 2270109040, 1010
BING(long)! 2270115061 > 2270110059, 5002
BING(long)! 2558052223 > 2557528157, 524066
BING(long)! 2571660540 > 2571659563, 977
BING(long)! 2646433569 > 2646432557, 1012
BING(long)! 2660841714 > 2660840732, 982
BING(long)! 2661795522 > 2660841715, 953807
BING(long)! 2712855281 > 2712854239, 1042
BING(long)! 2737627472 > 2735210929, 2416543
1025780885 =? 1073741823, 3168207035 =? 3221225470
Notice how BING(int) is never written and demonstrates that 32-bit reads/writes are atomic on this 32-bit OS.
Results on Windows 7 Enterprise 64-bit:
Windows 7 Enterprise 64-bit
Intel Core2 Duo P8700 @ 2.53GHz
BING(long)! 2208482159 > 2208121217, 360942
BING(int)! 280292777 > 279704627, 588150
BING(int)! 308158865 > 308131694, 27171
BING(long)! 2549116628 > 2548884894, 231734
BING(int)! 534815527 > 534708027, 107500
BING(int)! 545113548 > 544270063, 843485
BING(long)! 2710030799 > 2709941968, 88831
BING(int)! 668662394 > 667539649, 1122745
1006355562 =? 1073741823, 3154727581 =? 3221225470
Notice that BING(long) AND BING(int) are both displayed! Why are the 32-bit operations failing, let alone the 64-bit ones?
