How are you spawning the threads?  Can you show us a code snippet?

How are you determining this?  Are you looking at the CPU usage in task
manager or are you doing something more sophisticated?

Are you certain that all 4 threads are actually running simultaneously?
Have you checked to make sure they are all making progress, and that there
isn't some synchronization issue making them take it in turns?  I'd use
something like Process Explorer (a free tool from www.sysinternals.com ) to
look at the per-thread CPU usage.

That should really only affect your GC behaviour.  The workstation version
of the CLR is perfecly capable of using multiple CPUs, so long as it's your
code that's doing the actual work.

Err, no.  It sounds like that message got badly garbled somewhere along the
way.

In a Windows Forms application, it is vitally important that any access to
the UI is done on the same *thread*.  But there are no limitations as to
which processor that thread runs on.  And you're welcome to have as many
worker threads as you like so long as they don't try to update the UI.  So
it's entirely possible for a Windows Forms app to make use of multiple CPUs.

There's a Windows feature: thread affinity.  It's possible to tell Windows
to

No idea.

Here's some simple code that fires up two threads, and which does pointless
busy work that consumes 100% of the CPU on my dual-proc system (i.e. it is
using both CPUs completely) for a minute or two:

using System;
using System.Threading;

class app
{
 static void DoWork()
 {
   int i = 0;
   double total = 0;
   while (i < 1000000000)
   { ++i; total += 1.0 / i; }
   Console.WriteLine(total);
 }

 static void Main()
 {
   ThreadStart threadProc = new ThreadStart(DoWork);
   Thread t1 = new Thread(threadProc);
   Thread t2 = new Thread(threadProc);

   t1.Start();
   t2.Start();
   Console.WriteLine("Both threads running.");
   t1.Join();
   t2.Join();
   Console.WriteLine("Done");
 }
}

So I suggest you start by comparing that with your code to see how it
differs.

I compiled this with no special flags.  I didn't do anything special to the
environment.  I just ran the code on a Windows Server 2003 box with version
1.1 of the .NET Framework installed.

Are you running under a DEBUG or RELEASE build?  I have noticed (though not
consistantly) that my multi-threaded apps seem to have all their threads run
on one proc (on a quad proc machine).  when i make a RELEASE build, the
threads are always distributed across all procs.

I'm curious if there is something in the CLR that sets processor affinity
when under a debug build

Check for the following in machine.config file:

MaxConnection                   =    12 * #CPUs
MaxIOThreads                   =      100
maxWorkerThreads            =       100
MinFreeThreads                    =    88 * #CPUs
MinLocalRequestFreeThreads   = 76 * #CPUs

Regards,
Ajit