How to determine which core a program runs on?

[Tue_B_]

I have a single threaded program which I would like to run multiple times, and ideally I would like it to start up on any free core - though I realize that may not be possible. I know how to bind the program to a specific core for multi-threaded programs using  SETENVQQ and OpenMP, but that does not seem to work when I only use 1 thread, so how do you bind a single threaded program to a specific core/logicalproccessor?

[Nikitopoulos__Theodo]

You can check these articles:

http://www.howtogeek.com/howto/windows-vista/start-an-application-assigned-to-a-specific-cpu-in-windows-vista/
http://www.howtogeek.com/121775/how-to-force-windows-applications-to-use-a-specific-cpu/

I'm not sure if there is a windows api to change the affinity from within the program. Maybe someone else can help on this.

[JVanB]

Have you looked at SetProcessAffinityMask?

[Tue_B_]

Repeat Offender wrote:

Have you looked at SetProcessAffinityMask?

I have now, but I'm still not exactly sure whether this can do what I want. What I'm currently doing is using:

SETENVQQ("OMP_PLACES={0},{1},{2}")

But this does not get respected if I'm only using 1 thread, and so I'm not really sure how I would use SetProcessAffinityMask.

[JVanB · Accepted solution]

Now, hopefully you're not using OMP because I don't know whether that would play nice with code that asks the OS to reserve a specific CPU for its purposes. What I was thinking of was something like

program affinity
   use IFWIN
   implicit none
   integer, parameter :: dp = kind(1.0d0)
   integer(HANDLE) Process
   integer(DWORD_PTR) AffinityMask
   integer Core
   integer(DWORD) i
   real(dp) x, sum
   integer(BOOL) Retval
   integer(DWORD) iError

   Process = GetCurrentProcess()
   Core = 2 ! Set to run on core #2
   AffinityMask = ishft(1_DWORD_PTR, Core)
   Retval = SetProcessAffinityMask(Process,AffinityMask)
   if(Retval == 0) then
      iError = GetLastError()
      write(*,*) 'Failed with error code ',iError
      stop
   end if
   sum = 0
   do i = huge(i),1,-1
      x = i
      x = 1/x**2
      sum = sum+x
   end do
   write(*,*) huge(i),sum
end program affinity

Now, when I run the above and look in Resource Manager under the CPU tab, I see a spike in usage for CPU 2 and none for the others, so it looks like it is working.

EDIT: If you wanted the process to use CPUs 0, 1, and 2, you could have coded

   AffinityMask = iany(ishft(1_DWORD_PTR, [0,1,2]))

[Tue_B_]

Repeat Offender wrote:

Now, hopefully you're not using OMP because I don't know whether that would play nice with code that asks the OS to reserve a specific CPU for its purposes. What I was thinking of was something like

program affinity
   use IFWIN
   implicit none
   integer, parameter :: dp = kind(1.0d0)
   integer(HANDLE) Process
   integer(DWORD_PTR) AffinityMask
   integer Core
   integer(DWORD) i
   real(dp) x, sum
   integer(BOOL) Retval
   integer(DWORD) iError

   Process = GetCurrentProcess()
   Core = 2 ! Set to run on core #2
   AffinityMask = ishft(1_DWORD_PTR, Core)
   Retval = SetProcessAffinityMask(Process,AffinityMask)
   if(Retval == 0) then
      iError = GetLastError()
      write(*,*) 'Failed with error code ',iError
      stop
   end if
   sum = 0
   do i = huge(i),1,-1
      x = i
      x = 1/x**2
      sum = sum+x
   end do
   write(*,*) huge(i),sum
end program affinity

Now, when I run the above and look in Resource Manager under the CPU tab, I see a spike in usage for CPU 2 and none for the others, so it looks like it is working.

EDIT: If you wanted the process to use CPUs 0, 1, and 2, you could have coded

   AffinityMask = iany(ishft(1_DWORD_PTR, [0,1,2]))

Thank you, this did exactly what I wanted!

Is there any resource I could read about how it actually work, or what other commands I would be able to do with these kinds of things? I haven't been able to find anything about this, which is probably because I'm searching for this from a Fortran point of view.

[JVanB]

I don't know what to tell you further except to read the documentation provided in MSDN. For example, just search for

SetProcessAffinityMask msdn

And you will get all the information that I have. More general advice on using Google: try to state your query as an English sentence, including all or most of the keywords that you are really searching for, like you would be asking a person. Google thinks it's smarter than you and anthropomorphizing it in this way confirms its attitude and so gets you better service.