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NEC Cluster Using MPI: Difference between revisions

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== OpenMPI example ==
=== OpenMPI ===


=== simple example ===
see [[Open MPI]]


To use OpenMPI with intel Compiler, create a .modulerc in your home
=== Intel MPI ===
with this contents:


#%Module1.0#
see [[Intel MPI]]
set version 1.0
module load compiler/intel/11.0
module load mpi/openmpi/1.3-intel-11.0


For compilationuse the mpi wrapper scripts like mpicc/mpic++/mpif90.
=== MVAPICH2 ===


The following example is for a pure MPI job, using 16 nodes (128 processes).
see [[MVAPICH2]]
For Illustration, this is done using an interactvie session (-I option)


First step: Batch submit to get the nodes
=== MPI I/O ===


  qsub -l nodes=16:nehalem:ppn=8,walltime=6:00:00 -I            # get the 16 nodes
see [[MPI-IO]]
 
In the session you will get after some time, the application is started with
mpirun -np 128 PathToYourApp
 
=== more complex examples ===
 
OpenMPI the resources in something called 'slots'.
By specifying 'ppn:X' to the batchsystem, the number of slots per node is specified.
So for a simple MPI job with 8 process per node (=1 process per core) ppn:8
is best choice, as in above example. Details can be specified on mpirun commandline.
PBS setup is adjusted for ppn:8, please do not use other values.
 
If you want to use less processes per node e.g. because you are restricted by memory per process,
or you have a hybrid parallel application using OpenMP and MPI,
MPI would always put the first 8 processes on the first node, second 8 on second and so on.
To avoid this, you can do
 
mpirun -np X -npernode 2 /path/to/app
 
This would start 2 processes per node. Like this, you can use a larger number of nodes
with a smaller number of processes, or you can e.g. start threads out of the processes.
 
If you want to pin your processes to a CPU (and enable NUMA memory affinity) use
 
 
mpirun -np X --mca mpi_paffinity_alone 1  /path/to/app
 
Warning: This will not behave as expected for hybrid multithreaded applications,
as the threads will be pinned to a single CPU as well! Use this only in case
of one process per core, no extra threads.
 
 
 
 
 
 
 
 
== Intel MPI example ==
 
As Nehalem system is a two socket system with local attached ccNUMA memory,
memory and process placmeent can be crucial.
 
Here is an example that shows a 16 node Job, using 1 process per socket and 4 threads
per socket and optimum NUMA placement of processes and memory.
 
Prerequiste: Use intel MPI and best intel compiler
To setup environment for this, use this .modulerc file in your home:
 
#%Module1.0#
set version 1.0
module load compiler/intel/11.0
module load mpi/impi/intel-11.0.074-impi-3.2.0.011
 
And compile your application using mpicc/mpif90.
 
First step: Batch submit to get the nodes
 
  qsub -l nodes=16:nehalem:ppn=8,walltime=6:00:00 -I          # get the 16 nodes
 
Second step: make a hostlist
 
  sort -u  $PBS_NODEFILE  > m
 
Third step: make a process ring to be used by MPI later
 
mpdboot  -n 16 -f m -r ssh 
 
Fourth step: start MPI application
 
mpiexec -perhost 2 -genv I_MPI_PIN 0  -np 32 ./wrapper.sh ./yourGloriousApp
 
With wrapper.sh looking like this
 
#!/bin/bash
export KMP_AFFINITY=verbose,scatter
export OMP_NUM_THREADS=4
if [ $(expr $PMI_RANK % 2) = 0  ]
then
        export GOMP_CPU_AFFINITY=0-3
        numactl --preferred=0 --cpunodebind=0 $@
else
        export GOMP_CPU_AFFINITY=4-7
        numactl --preferred=1 --cpunodebind=1 $@
fi
 
 
Result is an application running on 16 nodes, using 32 processes spawning
128 threads. One set of 4 therads is pinned to the one socket, the other set of 4 threads to the other socket.

Latest revision as of 14:44, 12 June 2013

OpenMPI

see Open MPI

Intel MPI

see Intel MPI

MVAPICH2

see MVAPICH2

MPI I/O

see MPI-IO