openmpi - assign two MPI processes per core

Question

How do I assign 2 MPI processes per core?

For example, if I do mpirun -np 4 ./application then it should use 2 physical cores to run 4 MPI processes (2 processes per core). I am using Open MPI 1.6. I did mpirun -np 4 -nc 2 ./application but wasn't able to run it.

It complains mpirun was unable to launch the specified application as it could not find an executable:

Answer 1

orterun (the Open MPI SPMD/MPMD launcher; mpirun/mpiexec are just symlinks to it) has some support for process binding but it is not flexible enough to allow you to bind two processes per core. You can try with -bycore -bind-to-core but it will err when all cores already have one process assigned to them.

But there is a workaround - you can use a rankfile where you explicitly specify which slot to bind each rank to. Here is an example: in order to run 4 processes on a dual-core CPU with 2 processes per core, you would do the following:

mpiexec -np 4 -H localhost -rf rankfile ./application

where rankfile is a text file with the following content:

rank 0=localhost slot=0:0
rank 1=localhost slot=0:0
rank 2=localhost slot=0:1
rank 3=localhost slot=0:1

This will place ranks 0 and 1 on core 0 of processor 0 and ranks 2 and 3 on core 1 of processor 0. Ugly but works:

$ mpiexec -np 4 -H localhost -rf rankfile -tag-output cat /proc/self/status | grep Cpus_allowed_list
[1,0]<stdout>:Cpus_allowed_list:     0
[1,1]<stdout>:Cpus_allowed_list:     0
[1,2]<stdout>:Cpus_allowed_list:     1
[1,3]<stdout>:Cpus_allowed_list:     1

Edit: From your other question is becomes clear that you are actually running on a hyperthreaded CPU. Then you would have to figure out the physical numbering of your logical processors (it's a bit confusing but physical numbering corresponds to the value of processor: as reported in /proc/cpuinfo). The easiest way to obtain it is to install the hwloc library. It provides the hwloc-ls tool that you can use like this:

$ hwloc-ls --of console
...
  NUMANode L#0 (P#0 48GB) + Socket L#0 + L3 L#0 (12MB)
    L2 L#0 (256KB) + L1 L#0 (32KB) + Core L#0
      PU L#0 (P#0)    <-- Physical ID 0
      PU L#1 (P#12)   <-- Physical ID 12
...

Physical IDs are listed after P# in the brackets. In your 8-core case the second hyperthread of the first core (core 0) would most likely have ID 8 and hence your rankfile would look something like:

rank 0=localhost slot=p0
rank 1=localhost slot=p8
rank 2=localhost slot=p1
rank 3=localhost slot=p9

(note the p prefix - don't omit it)

If you don't have hwloc or you cannot install it, then you would have to parse /proc/cpuinfo on your own. Hyperthreads would have the same values of physical id and core id but different processor and apicid. The physical ID is equal to the value of processor.