This tutorial describes how to compute the value of pi using the R statistical package on the OSG. For this example, we'll estimate the value of pi using a Monte Carlo method. We'll first run the program locally as a test. After that we'll create a submit file, submit it to OSG using OSG Connect, and then collate results when the jobs finish.


Some background is useful here. We define a square inscribed by a unit circle. We randomly sample points, and calculate the ratio of the points outside of the circle to the points inside for the first quadrant. This ratio approaches pi/4.

See also: http://mathfaculty.fullerton.edu/mathews/n2003/montecarlopimod.html

This method converges extremely slowly, which makes it great for a CPU-intensive exercise (but bad for a real estimation!).

Accessing R on the submit host

First we'll need to create a working directory, you can either run $ tutorial R or type the following:

$ mkdir tutorial-R
$ cd tutorial-R

First, we'll need to set up the system paths so we can access R correctly. This is done via OSG's [Distributed Environment Modules]. To access these modules and access R, enter:

$ module load r

Once we have the path set up, we can try to run R. Don't worry if you aren't an R expert, I'm not either.

$ R

R version 3.5.1 (2018-07-02) -- "Feather Spray"
Copyright (C) 2018 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

  Natural language support but running in an English locale

R is a collaborative project with many contributors.
Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for an HTML browser interface to help.
Type 'q()' to quit R.


Great! R works. You can quit out with "q()".

> q()
Save workspace image? [y/n/c]: n

Running R code

Now that we can run R, let's try using the pi estimation code. Create the file mcpi.R:

montecarloPi <- function(trials) {
  count = 0
  for(i in 1:trials) {
    if((runif(1,0,1)^2 + runif(1,0,1)^2)<1) {
      count = count + 1


R normally runs as an interactive shell, but it is easy to run in batch mode too.

$ Rscript --no-save mcpi.R
[1] 3.14

This should take few seconds to run. Now edit the file so the line montecarloPi(1000) reads montecarloPi(10000000). Increasing the trials ten times (10000000) it will take little over a minute to run, but the estimation still isn't very good. Fortunately, this problem is pleasingly parallel since we're just sampling random points. So what do we need to do to run R on the campus grid?

Building the HTCondor job

The first thing we're going to need to do is create a wrapper for our R environment, based on the setup we did in previous sections. Create the file R-wrapper.sh:



if [ $# -ne $EXPECTED_ARGS ]; then
  echo "Usage: R-wrapper.sh file.R"
  exit 1
  module load r
  Rscript $1

Notice here that we're using Rscript (equivalent to R --slave). It accepts the script as command line argument, it makes R much less verbose, and it's easier to parse the output later. If you run it at the command line, you should get similar output as above. This lets the wrapper launch R on any generic worker node under HTCondor.

$ ./R-wrapper.sh mcpi.R
[1] 3.142524

Now that we've created a wrapper, let's build a HTCondor submit file around it. We'll call this one R.submit:

Executable = R-wrapper.sh
arguments = mcpi.R
transfer_input_files = mcpi.R

log = log/mcpi.log.$(Cluster).$(Process)
error = log/mcpi.err.$(Cluster).$(Process)
output = log/mcpi.out.$(Cluster).$(Process)

requirements = HAS_MODULES == True && OSGVO_OS_STRING == "RHEL 7" && Arch == "X86_64"
queue 100

Notice the requirements line? You'll need to put HAS_MODULES == True any time you need software from the modules. There's also one small gotcha here – make sure the "log" directory used in the submit file exists before you submit! Else HTCondor will fail because it has nowhere to write the logs.

$ mkdir log

Submit and analyze

Finally, submit the job to OSG Connect!

$ condor_submit R.submit
Submitting job(s)............................
100 job(s) submitted to cluster 14027.
$ condor_q user

-- Submitter: login01.osgconnect.net : <> : login01.osgconnect.net
14027.0   user           8/25 22:51   0+00:00:43 R  0   0.0  R-wrapper.sh mcpi.
14027.1   user           8/25 22:51   0+00:00:43 R  0   0.0  R-wrapper.sh mcpi.
14027.2   user           8/25 22:51   0+00:00:31 R  0   0.0  R-wrapper.sh mcpi.
14027.4   user           8/25 22:51   0+00:00:41 R  0   0.0  R-wrapper.sh mcpi.
14027.5   user           8/25 22:51   0+00:00:41 R  0   0.0  R-wrapper.sh mcpi.

You can follow the status of your job cluster with the watch condor_q command, which shows condor_q output that refreshes each 2 seconds. Press control-C to stop watching.

Since our jobs just output their results to standard out, we can do the final analysis from the log files. Let's see what one looks like:

$ cat log/mcpi.out.14027.1
[1] 3.141246

After job completion we have 100 Monte Carlo estimates of the value of pi. Taking an average across them all should give us a closer approximation.

We'll use a bit of awk magic to do the averaging:

$ grep "[1]" log/mcpi.out.* | awk '{sum += $2} END { print "Average =", sum/NR}'
Average = 3.14151

That's pretty close! With even more sample sets — that is, more Queue jobs in the cluster — we can statistically come even closer.

What to do next?

The R.submit file may have included a few lines that you are unfamiliar with. For example, $(Cluster) and $(Process) are variables that will be replaced with the job's cluster and process id. This is useful when you have many jobs submitted in the same file. Each output and error file will be in a separate directory.

Also, did you notice the transfer_input_files line? This tells HTCondor what files to transfer with the job to the worker node. You don't have to tell it to transfer the executable, HTCondor is smart enough to know that the job will need that. But any extra files, such as our MonteCarlo R file, will need to be explicitly listed to be transferred with the job. You can use transfer_input_files for input data to the job, as shown in Transferring data with HTCondor.

Getting Help

For assistance or questions, please email the OSG User Support team at support@opensciencegrid.org or visit the help desk and community forums.


This page was updated on Feb 25, 2020 at 18:00 from tutorials/tutorial-R/README.md.