Distributed computing in R with Rmpi

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Load modules and install Rmpi

Change to the new software stack and load required modules. Here we need MPI and R libraries.

$ env2lmod
$ module load gcc/6.3.0 openmpi/2.1.1 r/4.0.2
$ R
> install.packages("Rmpi")

Request an interactive session

Rmpi assigns one processor to be the master and other processors to be workers. Here we would like to use 5 processors on 2 nodes for computation. Therefore, we request 6 processors

 $ bsub -n 6 -R "span[ptile=3]" -Is bash
 Generic job.
 Job <155200980> is submitted to queue <normal.4h>.
 <<Waiting for dispatch ...>>
 <<Starting on eu-c7-105-05>>

Use Rmpi

Create an R script called test_rmpi.R

# Load Rmpi which calls mpi.initialize()
library(Rmpi)

# Spawn R-slaves to the host. nslaves = requested number of processors - 1
usize <- mpi_universe_size()
ns <- usize - 1
mpi.spawn.Rslaves(nslaves=ns)

# Set up a variable array
var = c(11.0, 22.0, 33.0, 44.0, 55.0)

# Root sends state variables and parameters to other ranks
mpi.bcast.data2slave(var, comm = 1, buffunit = 100)
# Get the rank number of that processor
mpi.bcast.cmd(id <- mpi.comm.rank())
# Check if each rank can use its own value
mpi.remote.exec(paste("The variable on rank ",id," is ", var[id]))

# Root orders other ranks to calculate
mpi.bcast.cmd(output <- var[id]*2)
# Root orders other ranks to gather the output
mpi.bcast.cmd(mpi.gather(output, 2, double(1)))

# Root gathers the output from other ranks
mpi.gather(double(1), 2, double(usize))

# Close down and quit
mpi.close.Rslaves(dellog = FALSE)
mpi.quit()

Run the script with mpirun

$ mpirun -np 1 Rscript test_rmpi.R
   5 slaves are spawned successfully. 0 failed.
master (rank 0, comm 1) of size 6 is running on: eu-c7-103-05 
slave1 (rank 1, comm 1) of size 6 is running on: eu-c7-103-05 
slave2 (rank 2, comm 1) of size 6 is running on: eu-c7-103-05 
slave3 (rank 3, comm 1) of size 6 is running on: eu-c7-103-11 
slave4 (rank 4, comm 1) of size 6 is running on: eu-c7-103-11 
slave5 (rank 5, comm 1) of size 6 is running on: eu-c7-103-11 
[1] "MPI is initialized."
$slave1
[1] "I am 1 of 6"

$slave2
[1] "I am 2 of 6"

$slave3
[1] "I am 3 of 6"

$slave4
[1] "I am 4 of 6"

$slave5
[1] "I am 5 of 6"

$slave1
[1] "The variable on rank  1  is  11"

$slave2
[1] "The variable on rank  2  is  22"

$slave3
[1] "The variable on rank  3  is  33"

$slave4
[1] "The variable on rank  4  is  44"

$slave5
[1] "The variable on rank  5  is  55"

[1]   0  22  44  66  88 110
[1] 1


Further reading

https://cran.r-project.org/web/packages/Rmpi/Rmpi.pdf

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