[R] parallel SNOW slower than single core?

[Prof Brian Ripley]

On 02/08/2012 19:13, Jie wrote:
> Dear All,
>
> I am learning parallel in R and start with the package "snow". I did a test
> about running time and the parallel version is much slower than the regulat
> code. My laptop is X200s with dual core intel L9400 cpu.

The OS matters far more.  Overheads on Windows are much higher than on 
Linux for example.

> Should I make more clusters than 2? Or how to improve the performance?

Not use such small tasks.  This is discussed in the 'parallel' vignette 
(and we do suggest you start with 'parallel' these days).

> # install.packages("snow")
> library(snow)
> cl <- makeCluster(2)
> t1 <- proc.time()
> a <- c()
> for (i in 1:1000)
> {
> a[i] <- sum(parSapply(cl, 1:15, get("+"), 2))
> }
> proc.time()-t1
> t2 <- proc.time()

See ?system.time

> a <- c()
> for (i in 1:1000)
> {
> a[i] <- sum(sapply(1:15, "+", 2))
> }
> proc.time()-t2



-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595