[R] Could concurrent R sessions mix up variables?

Phil Spector spector at stat.berkeley.edu
Fri Dec 10 19:58:01 CET 2010

Anthony -
    I would advise you to use the multicore or snowfall packages
to utilize multiple CPUs.  As an example using multicore:

> library(multicore)
> sim = function(mu)max(replicate(100000,max(rnorm(100,mu))))
> library(multicore)
> unlist(mclapply(c(1,5,10,20),sim))
[1]  6.569332 10.268091 15.335847 25.291502

Using snowfall:

> library(snowfall)
> sim = function(mu)max(replicate(100000,max(rnorm(100,mu))))
> sfInit(cpus=4,type='SOCK',parallel=TRUE)
> sfSapply(c(1,5,10,20),sim)
[1]  6.200161 10.307807 15.271581 25.055950

Hope this helps.

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu

On Fri, 10 Dec 2010, Anthony Damico wrote:

> Hi, I'm working in R 2.11.1 x64 on Windows x86_64-pc-mingw32.
> I'm experiencing a strange problem in R that I'm not even sure how to
> begin to fix.
> I've got a huge (forty-pages printed) simulation written in R that I'd
> like to run multiple times.  When I open up R and run it on its own,
> it works fine.  At the beginning of the program, there's a variable X
> that I set to 1, 5, 10, 20, depending on how sensitive I want the
> simulation to be to a certain parameter.  When I just run one instance
> of R, the X variable stays the same throughout the program.
> I have a quad-core machine, so I'd like to take advantage of all four
> processors.
> If I open up four sessions and set X to 1, 5, 10, and 20 in those
> different sessions, then run all four simulations all the way through
> (about eighteen hours of processing time) at the same time, the
> variable X ends up being 20 at the end of all four sessions.  It's as
> if R mixed up the variable setting between the four concurrent
> sessions.  I can't figure out why else my variable X would ever get
> changed to 20 in the three simulations that I set it to 1, 5, and 10,
> respeectively (it doesn't get updated anywhere during the simulation).
> When I have all four of these simulations running concurrently, I am
> absolutely maxing out my computer.  All four processors are at 100%,
> and my Windows Task Manager says I'm using almost 100% of my 16 GB of
> RAM.  Is it possible that intense resource use would cause a variable
> conflict like this?  I have no idea where to start troubleshooting
> this error, so any advice would be appreciated.
> Thanks!
> Anthony Damico
> Kaiser Family Foundation
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