OpenMP on 80 cores Windows 2008 uses only 40 cores at most

OpenMP on 80 cores Windows 2008 uses only 40 cores at most

We have a 40 Core DL580, 4 sockets, Westmere Intel(R) Xeon(R) CPU E7- 4870  @ 2.40GHz, with Hyperthreading activated Windows Server 2008 Entreprise R2 sees 80 "logical processors". Windows splits these 80 "logical processors" into two groups of 40 "logical processors" (see http://msdn.microsoft.com/en-us/library/windows/desktop/dd405503(v=vs.85).aspx ). 

When testing our fortran 90 calculation kernel which uses OpenMP to parallelize, we see that on that machine Windows only schedules on up to 40 threads, not 80. On a machine with up to 64 "logical processors", we are able to use them all since only one group is created under Windows. When testing on the same machine with Linux, we are able to use 80 threads. 

Are there commands available from Intel Fortran or from OpenMP to allow the process to be scheduled over the two process groups to be able to access the full power of the windows server?

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I do not have such a system, but MSDN had an article about this and I do not have the link handy. MS chose to keep a somewhat compatible up to 64-bit bit mask structure for logical processors. An application, with no special coding, will function as it did on a lesser machine. To use more logical processors than 64, the newer Windows has a term (not sure now what they used), but functionally it is a logical processor set. With special coding, a process can manipulate multiple processor sets (groups). If you are using system level programming you will have to add code to use this feature. OpenMP should hide this from you... provided you have a version that supports the new API's. You may have to upgrade your compiler and OpenMP library.

Jim Dempsey

www.quickthreadprogramming.com

Here is the link: http://archive.msdn.microsoft.com/64plusLP

Jim Dempsey

www.quickthreadprogramming.com

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