[R] foreach takes foreever?

[R. Michael Weylandt]

You're probably being killed by the overhead of parallelization which
is, in this case, far more than actual computation time. I've not dug
through foreach() in a while, but I think this winds up spawning many
many subprocesses which isn't cheap in Windows.

MW

On Mon, Jan 21, 2013 at 3:59 PM, Andre Zege <azege at yahoo.com> wrote:
> I started to look at ways to improve times of certain very parallel tasks and thought that foreach should be a valid candidate to do the job.
> So, i opened foreach tutorial by Steve Weston and started timing examples from it. First example from tutorial is
>
>
>>system.time(for(i in 1:100000) sqrt(i))
>
>    user  system elapsed
>    0.06    0.00    0.06
>> system.time(foreach(i=1:100000) %do% sqrt(i))
>    user  system elapsed
>  102.37    0.21  103.38
>
> Hmm, 1700 time slower?
>
> second example is
>> system.time(x <- exp(1:1000000))
>    user  system elapsed
>    0.34    0.03    0.42
>>system.time(x <- foreach(i=1:1000000, .combine='c') %do% exp(i))
>
>
> I stopped it at 958 seconds, didn't have enough patience -- it basically seems that foreach  slows down this one down naive  by more than 2000 times. I must be  doing something very wrong. Am i supposed to set some environment variables before it works properly? I am running 64bit R on win7 dual core 2.27GHZ CPUs and 4GB memory laptop.
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