I_MPI_PIN_DOMAIN example

Hi,

   I am trying to pin 6 ranks on a dual-socket (12 cores per socket) node, with 4 OpenMP threads per MPI rank. I set I_MPI_PIN_DOMAIN=omp:compact, but I get this I_MPI_DEBUG output:

[0] MPI startup(): 0       14563    n2470      {0,1,12,13}

[0] MPI startup(): 1       14564    n2470      {2,3,14,15}

[0] MPI startup(): 2       14565    n2470      {4,5,16,17}

[0] MPI startup(): 3       14566    n2470      {6,7,18,19}

[0] MPI startup(): 4       14567    n2470      {8,9,20,21}

[0] MPI startup(): 5       14568    n2470      {10,11,22,23}

I would have expected

[0] MPI startup(): 0       14563    n2470      {0,1,2,3}
[0] MPI startup(): 1       14564    n2470      {4,5,6,7}
[0] MPI startup(): 2       14565    n2470      {8,9,10,11}
[0] MPI startup(): 3       14566    n2470      {12,13,14,15}
[0] MPI startup(): 4       14567    n2470      {16,17,18,19}
[0] MPI startup(): 5       14568    n2470      {20,21,22,23}

Intel Parallel Studio Cluster edition 2017 update 5. Am I setting something incorrectly, or is this a behavior to protect performance against myself?

Thanks!  - Chris