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);
} 

The above information should not be used under any circumstances. There are 4 problems with the implementation itself, as well as a few problems with the concept as a whole.

Implementation problems:

1. With Hyper-Threading, each physical processor contains two logical processors. There is no distinction between a "physical" and "logical" processor. There are no special bits that correspond to a "physical" processor. There are only logical processors. It is not possible to "prefer a physical processor over a logical one" because that simply makes no sense. Each logical processor on the same physical chip shares the same FPU and provides access to the same resources. Even the title makes no sense... every logical processor is part of a real processor, you can't "prefer your real processors" because they are all your real processors. The example above fundamentally does not make sense. It will gain you nothing more than limiting a process to run on two completely arbitrary logical CPU's (which may or may not even be on the same physical CPU). It just doesn't work. Don't use it. This is the biggest problem.

2. Which CPUs correspond to which bits in the affinity mask is not guaranteed by the Win32 API at all, and should not be relied on (e.g. it is never correct to assume that a certain processor type comes first in the bit mask).

3. The example logic fails on machines without 2 or 4 logical CPU's (e.g., an 8-core Xeon server).

4. The Hyper-Threading detection "algorithm" is not "C or C++". It's MSVC-specific inline assembler. Another compiler, for example, GCC, would not recognize it as-is. And the _emit's are highly eyebrow-raising.

Conceptual problems:

1. Unless you have complete control over the system, and over the other software on the system, and over how the user is running your particular piece of software, you are far more likely to hurt performance by setting the affinity mask yourself. The above poster is incorrect in stating that a thread "prefers" a certain logical processor. In fact, this function forces the threads to run on those processors, no matter what. While the threads will be able to run on any of the CPU's in the process affinity mask, this is still inviting problems. For example, if another piece of software has also unwisely set it's affinity mask to the same logical CPU(s) as your software does, you're S.O.L. as you've bypassed the kernel's ability to make wise judgments about which CPU to run your application on, and forced your application to share busy CPU's with another process (e.g. a quad core system where two applications are forced to run on the same two cores... not too great). Also, what if you set the affinity mask, and the user runs your application twice? Again, bypassing the kernel's CPU scheduler forces your application to share busy CPU's with other processes while other CPU's sit idle.

2. Again, unless you have complete control over the system, making the assumption that the system you are on has a certain number of cores is never a good idea. At best, if you must set affinity masks, you can get information about which logical processors reside on the same physical processor using NUMA functions like GetNumaNodeProcessorMask and make judgments from there (rather than using unreliable hacks like assuming certain bits correspond to certain processors). At the very least, you can get the system affinity mask and count the bits if you want to count the number of logical processors and make decisions based on that. Ignoring the fact that the above technique fundamentally makes no sense (as mentioned above), even if it did make sense, it only "works" (I use the term loosely) in 2 very specific situations: HT CPU with 2 logical processors, or HT CPU with 4 logical processors. In all other cases, if HT is not present, or if there are not 2 or 4 logical processors, the affinity mask is not set. Inconsistent logic like that is never a recipe for optimization.

3. For the vast majority of applications, the kernel will always do a better job than you at making judgments about what logical CPU's to schedule a process and it's threads on. By not setting the affinity masks at all, you immediately make the best use of all available CPU's on any system, regardless of system configuration or other running applications. On the other hand, by setting the affinity masks, you lock yourself into very specific CPU configurations, and risk forcing yourself into far-less-than-optimal situations on machines with other processes running on them or with moderately complex layouts (e.g. Windows automatically schedules to logical CPU's on separate physical CPU's first in layouts where that makes sense, and in general always is assumed to schedule threads as optimally as possible given the system layout).

While there are certainly some valid reasons for setting the process affinity mask, the "technique" given above is fundamentally flawed in both concept and implementation, and should be disregarded.

For those of you interested in actually learning more about the concept of physical and logical processors, multi-threading in general, and a bit about Hyper-threading before attempting such "optimizations", here is a decent article about what goes on under the hood: http://arstechnica.com/articles/paedia/cpu/hyperthreading.ars

I also suggest actually reading the MSDN article linked to in the above post. It's a great article. Note how it correctly describes the difference between physical and logical processors. Also note that the kernel takes the system configuration into account and makes optimal decisions for you (e.g. by attempting to schedule processes on logical CPU's that reside on different physical CPU's first -- and doing that only in HT environments where it is appropriate).

JC

<DllImport("kernel32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> Public Shared Function SetProcessAffinityMask(ByVal handle As SafeProcessHandle, ByVal mask As IntPtr) As Boolean
End Function

[DllImport("kernel32.dll", CharSet=CharSet.Auto, SetLastError=true)]
public static extern bool SetProcessAffinityMask(SafeProcessHandle handle, IntPtr mask);