[R] parallel SNOW slower than single core?

[R. Michael Weylandt]

I'm not surprised -- you're doing a fairly trivial calculation so the
overhead of setting up the parallelization is likely more than the
benefit from halving the computation time. Do it on a larger
calculation and I imagine the performance will be better -- I might
just be pulling this out of thin air, but I think I remember reading
slides from Luke Tierney saying one had to be adding vectors of size
>= 30k or so for parallelization to have a meaningful benefit.
Obviously sizes will be lower for operations more expensive than
addition.

Michael

On Thu, Aug 2, 2012 at 1:13 PM, Jie <jimmycloud at gmail.com> 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.
> Should I make more clusters than 2? Or how to improve the performance?
>
> # 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()
> a <- c()
> for (i in 1:1000)
> {
> a[i] <- sum(sapply(1:15, "+", 2))
> }
> proc.time()-t2
>
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