I found that GHC runtime does not take advantage of processor groups under Windows. So it cannot use more than 64 logical processors (2x 18 core physical + HT). This is Reddit thread about: https://www.reddit.com/r/haskell/comments/3q1r0o/ghc_on_numa_72_core_2_processor_machine_cannot/ MSDN link https://msdn.microsoft.com/en-us/library/windows/desktop/dd405503(v=vs.85).aspx <details><summary>Trac metadata</summary> | Trac field | Value | | ---------------------- | -------------- | | Version | 7.10.2 | | Type | Bug | | TypeOfFailure | OtherFailure | | Priority | normal | | Resolution | Unresolved | | Component | Runtime System | | Test case | | | Differential revisions | | | BlockedBy | | | Related | | | Blocking | | | CC | simonmar | | Operating system | | | Architecture | | </details> <!-- {"blocked_by":[],"summary":"GHC on Windows could not use more than 64 logical processors","status":"New","operating_system":"","component":"Runtime System","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"7.10.2","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":["simonmar"],"type":"Bug","description":"I found that GHC runtime does not take advantage of processor groups under Windows. So it cannot use more than 64 logical processors (2x 18 core physical + HT).\r\n\r\nThis is Reddit thread about:\r\nhttps://www.reddit.com/r/haskell/comments/3q1r0o/ghc_on_numa_72_core_2_processor_machine_cannot/\r\n\r\nMSDN link\r\nhttps://msdn.microsoft.com/en-us/library/windows/desktop/dd405503(v=vs.85).aspx","type_of_failure":"OtherFailure","blocking":[]} -->