Parallel Execution
Parallel execution allows umx to use multiple cores, or distribute a model across machines. Single models can be processed using parallel resources, and multiple models can be run and results combined.
CRAN, umx uses all cores (actually, the result of a call to detectCores() - 1).
Set cores with umx
umx allows you to get and set the number of cores OpenMx will use with:
umx_set_cores() # Show many cores are currently requested
umx_set_cores(3) # Request use of 3 cores
Background
Modern CPUs contain multiple cores (processing units that share memory), and machines may contain more than one CPU.
Some chips also implement “virtual” cores which help allow the compiler to keep the hardware working while one process is data limited.
You can see total number of cores on your machine with detectCores() (on my 2016 iMac, the answer was 8 (4 real, with hyper-threading). In practice, only real cores advance model speed much, and 4 cores will complete a model in a little over 1/4 of the time compared to 1-core.
Under the hood, this is getting or setting an mxOption:
mxOption(NULL, "Number of Threads") # get current value
mxOption(NULL, "Number of Threads", n) # set value to n
Setting cores to 0 will use the most available
note: umx_set_cores is equivalent to:
mxOption(NULL, "Number of Threads", detectCores())
Now you don’t need to remember or type this long option string!
top tip: Use R’s tab-function completion (just type umx_set and tab to see the list of easy set and get helpers…):
top tip: If you’re a MacOS user, my TextMate OpenMx bundle contains snippets for many of umx’s features.
OpenMP on unix clusters
If you’re trying to use multiple cores on a managed cluster, you might need to request these in the control script that schedules your jobs.
Here’s an example for edinburgh’s cluster. The key is -pe OpenMP 12
#!/bin/sh
# This is a simple example of a batch script
# call me as
# qsub -P ppls_psychology -cwd -o ~/bin/makeOpenMx.out -e ~/bin/makeOpenMx.err ~/bin/makeOpenMx.sh
# Grid Engine options
#$ -cwd
# Set the maximum run time to 30 minutes, 0 seconds:
#$ -l h_rt=00:30:00
#$ -N parallel_example
#$ -pe OpenMP 12
# initialise environment module
. /etc/profile.d/modules.sh
# use Intel compilers (icc,ifort)
# Load R
module load R/3.2.2
module load gcc/4.8.3
module load intel
# set number of threads
export OMP_NUM_THREADS=$NSLOTS
echo OMP_NUM_THREADS=$OMP_NUM_THREADS
R CMD BATCH my.R my.out