for /l %i in (1,1,1) do @start /b rscript -e "system.time(for(i in
1:1e4){fisher.test(matrix(c(200,300,400,500),nrow=2))})"

   user  system elapsed
  54.50    0.04   54.63

for /l %i in (1,1,2) do @start /b rscript -e "system.time(for(i in
1:1e4){fisher.test(matrix(c(200,300,400,500),nrow=2))})"

   user  system elapsed
  55.64    0.00   55.77
   user  system elapsed
  55.92    0.02   56.08

Test of multiple concurrent R processes on Windows XP on a dual-core machine(R version 2.9.2)

##</code><code>##

$ for i in $(seq 1 1); do Rscript -e 'system.time(for(i in 1:1e4){fisher.test(matrix(c(200,300,400,500),nrow=2))})' & done

   user  system elapsed 
 46.694   0.088  47.189 

$ for i in $(seq 1 2); do Rscript -e 'system.time(for(i in 1:1e4){fisher.test(matrix(c(200,300,400,500),nrow=2))})' & done

   user  system elapsed 
 48.007   0.060  48.188 
   user  system elapsed 
 47.838   0.072  49.487 

##</code><code>##

$ tail Rtest1.txt 

0.05
0.05
0.05
0.05
0.05
0.05
0.13
0.08
0.10
0.15

$ wc -l Rtest1.txt 

91135 Rtest1.txt

$ cp Rtest1.txt Rtest2.txt 

$ cat Rtest1.R

library(splines)
library(survival)
library(NADA)
outcome=scan("Rtest1.txt")
cenoutcome=rep(FALSE, length(outcome))
cenoutcome[outcome==min(outcome)]=TRUE
pyros=cenros(outcome,cenoutcome)
mean(pyros)
proc.time()

# Rtest2.R is the same, but it reads Rtest2.txt

$ Rscript Rtest1.R 2> /dev/null &

[1] 0.03195313
   user  system elapsed 
 22.201   4.096  26.293 


$ Rscript Rtest1.R 2> /dev/null & Rscript Rtest2.R 2> dev/null &

[1] 0.03195313
   user  system elapsed 
 40.358   6.328  46.839 
[1] 0.03195313
   user  system elapsed 
 39.706   6.044  48.128