[Rd] mcmapply Core Usage

[Dario Strbenac]

Hello,

I have a minimal example that shows a problem I'm having with parallel processing.

library(parallel)
mcmapply(function(x, y)
{
  print("Running")
  Sys.sleep(10)
}, as.list(1:10), as.list(10:1), mc.cores = 16, SIMPLIFY = FALSE)

I see "Running" printed once every ten seconds. I read the documentation for mcmapply, but I don't understand why it wouldn't print all ten output statements almost at once, then wait for about ten seconds, then return to the prompt. Is it documented in another location ?

My problem is that I have a section of code that takes a long time, which I have replaced with Sys.sleep in the above example, but when I look at the output of top, I only see 1 CPU being used. The computer has 16 processors.

> sessionInfo()
R version 3.1.0 (2014-04-10)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_AU.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=en_AU.UTF-8        LC_COLLATE=en_AU.UTF-8    
 [5] LC_MONETARY=en_AU.UTF-8    LC_MESSAGES=en_AU.UTF-8   
 [7] LC_PAPER=en_AU.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_AU.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] parallel  stats     graphics  grDevices utils     datasets  methods  
[8] base     

loaded via a namespace (and not attached):
[1] tools_3.1.0

--------------------------------------
Dario Strbenac
PhD Student
University of Sydney
Camperdown NSW 2050
Australia