Note from jkriegshauser: This code sample does not work as expected on Multi-core processors (i.e. Core2 Duo/Quad, etc). The reason is because the newer multi-core processors define the HTT flag as "hardware multithreading" and the logical processor count will include the cores even though Core2 Duo processors do not have HTT. See the following Intel publications:

Intel Processor Identification and the CPUID Instruction: http://www.intel.com/assets/pdf/appnote/241618.pdf

Intel 64 and IA-32 Architectures Software Developer's Manual Vol 3A (section 7.10.2): http://www.intel.com/design/processor/manuals/253668.pdf

Also, the original MSDN article mentioned in this comment can be found here: http://msdn.microsoft.com/en-us/magazine/cc300701.aspx

The following code sample comes from "Juice Up Your App with the Power of Hyper-Threading", an MSDN article:

public void SetProcessAffinityToPhysicalCPUForHyperthreadOnly(int processid)
{
    int res;
    int hProcess;
    int ProcAffinityMask = 0, SysAffinityMask = 0;
    hProcess  = OpenProcess(PROCESS_ALL_ACCESS, 0, processid);
    res = GetProcessAffinityMask(
        hProcess, ref ProcAffinityMask, ref SysAffinityMask);
    if (SysAffinityMask == 3) // 1 proc, 2 logical CPUs
        res = SetProcessAffinityMask(hProcess, 1);
    else if (SysAffinityMask == 15)  //dual proc, 4 virtual CPUs
        res = SetProcessAffinityMask(hProcess, 3);
    res = CloseHandle(hProcess);
}

From the sample above, we see that the affinity mask is such that all physical processors come first in the mask, then the logical (Hyper-threaded) processors.

If your process is a heavy user of floating point instructions, setting the affinity mask to (number of processors/2) - 1 will make sure your threads will give preference for the physical processors which have FPU.

For a sample on how to detect if Hyper-Thread is on in C/C++, you could use this:

__inline BOOL hyperThreadingOn()
{
    DWORD       rEbx, rEdx;
    __asm {
         push   eax         // save registers used
         push   ebx
         push   ecx
         push   edx
         xor    eax,eax     // cpuid(1)
         add    al, 0x01
        _emit   0x0F
        _emit   0xA2
         mov    rEdx, edx   // Features Flags, bit 28 indicates if HTT (Hyper-Thread Technology) is
                            //  available, but not if it is on; if on, Count of logical processors > 1.
         mov    rEbx, ebx   // Bits 23-16: Count of logical processors.
                            //             Valid only if Hyper-Threading Technology flag is set.
         pop    edx         // restore registers used
         pop    ecx
         pop    ebx
         pop    eax
    }
    return (rEdx & (1<<28)) && (((rEbx & 0x00FF0000) >> 16) > 1);
}