[R-sig-hpc] How to check if %dopar% really run parallel?

[Stephen Weston]

There is a mistake.  Rather than:

    times(10000) %dopar% fun

you should write:

    times(10000) %dopar% fun()

On my machine, "fun" executes in about 0.4 seconds, so executing
it 10,000 times should take over an hour to execute.  Your error turned
a real program into a toy program.  The error also resulted in more
communication, since now the function itself is being returned by the
workers.

When I ran your benchmark on my machine with 100, rather than 10,000
tasks, I got the following results:

   user  system elapsed
 43.573   0.191  43.823
   user  system elapsed
  0.093   0.007  24.890

That's not so bad.

- Steve


On Tue, May 4, 2010 at 12:22 AM, Mario Valle <mvalle at cscs.ch> wrote:
> Is there any way to check that %dopar% really runs parallel?
> The following code (on a dual core laptop running windows+R 2.11.0pat and on
> Linux+R2.11.0) runs %dopar% more slowly than the same %do% code.
> BTW, if you see any obvious mistake in the code...
> Thanks!
>                mario
>
>
> library(doSNOW)
> library(foreach)
>
> fun <- function() for(q in 1:1000000) sqrt(3)
>
> system.time(times(10000) %do% fun, gcFirst = TRUE)
> #   user  system elapsed
> #   5.74    0.01    6.24
>
> cl <- makeCluster(2, type = "SOCK")
> registerDoSNOW(cl)
>
> system.time(times(10000) %dopar% fun, gcFirst = TRUE)
> #   user  system elapsed
> #   7.89    0.19    9.01
>
> stopCluster(cl)
>
> --
> Ing. Mario Valle
> Data Analysis and Visualization Group            |
> http://www.cscs.ch/~mvalle
> Swiss National Supercomputing Centre (CSCS)      | Tel:  +41 (91) 610.82.60
> v. Cantonale Galleria 2, 6928 Manno, Switzerland | Fax:  +41 (91) 610.82.82
>
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