The MPICH programs use MPD daemon or SSH connection to spawn processes, no PBS support is needed. However, the PBS allocation is required to access compute nodes.
MPI process mapping may be controlled by PBS parameters.
The `mpiprocs` and `ompthreads` parameters allow for selection of number of running MPI processes per node as well as number of OpenMP threads per MPI process.
### One MPI Process Per Node
Follow this example to run one MPI process per node, 16 threads per process. Note that no options to `mpirun` are needed
In this example, we demonstrate recommended way to run an MPI application, using 1 MPI processes per node and 16 threads per socket, on 4 nodes.
### Two MPI Processes Per Node
Follow this example to run two MPI processes per node, 8 threads per process. Note the options to `mpirun` for `mvapich2`. No options are needed for `impi`.
In this example, we demonstrate recommended way to run an MPI application, using 2 MPI processes per node and 8 threads per socket, each process and its threads bound to a separate processor socket of the node, on 4 nodes
### 16 MPI Processes Per Node
Follow this example to run 16 MPI processes per node, 1 thread per process. Note the options to `mpirun` for `mvapich2`. No options are needed for `impi`.
In this example, we demonstrate recommended way to run an MPI application, using 16 MPI processes per node, single threaded. Each process is bound to separate processor core, on 4 nodes.
### OpenMP Thread Affinity
!!! note
Important! Bind every OpenMP thread to a core!
In the previous two examples with one or two MPI processes per node, the operating system might still migrate OpenMP threads between cores. You might want to avoid this by setting these environment variable for GCC OpenMP:
```console
$export GOMP_CPU_AFFINITY="0-15"
```
or this one for Intel OpenMP:
```console
$export KMP_AFFINITY=granularity=fine,compact,1,0
```
As of OpenMP 4.0 (supported by GCC 4.9 and later and Intel 14.0 and later) the following variables may be used for Intel or GCC:
```console
$export OMP_PROC_BIND=true
$export OMP_PLACES=cores
```
## MPICH2 Process Mapping and Binding
The `mpirun` allows for precise selection of how the MPI processes will be mapped to the computational nodes and how these processes will bind to particular processor sockets and cores.
### Machinefile
Process mapping may be controlled by specifying a machinefile input to the mpirun program. Altough all implementations of MPI provide means for process mapping and binding, following examples are valid for the `impi` and `mvapich2` only.
Example machinefile
```console
cn110.bullx
cn109.bullx
cn108.bullx
cn17.bullx
cn108.bullx
```
Use the machinefile to control process placement
```console
$mpirun -machinefile machinefile helloworld_mpi.x
Hello world! from rank 0 of 5 on host cn110
Hello world! from rank 1 of 5 on host cn109
Hello world! from rank 2 of 5 on host cn108
Hello world! from rank 3 of 5 on host cn17
Hello world! from rank 4 of 5 on host cn108
```
In this example, we see that ranks have been mapped on nodes according to the order in which nodes show in the machinefile
### Process Binding
The Intel MPI automatically binds each process and its threads to the corresponding portion of cores on the processor socket of the node, no options needed. The binding is primarily controlled by environment variables. Read more about mpi process binding on [Intel website][a]. The MPICH2 uses the `-bindto` option. Use `-bindto numa` or `-bindto core` to bind the process on single core or entire socket.
### Bindings Verification
In all cases, binding and threading may be verified by executing
```console
$mpirun -bindto numa numactl --show
$mpirun -bindto numa echo$OMP_NUM_THREADS
```
## Intel MPI on Xeon Phi
The [MPI section of Intel Xeon Phi chapter][1] provides details on how to run Intel MPI code on Xeon Phi architecture.
