Question on foreach package
The example that you're trying is tiny. The overhead in most of the parallel programming packages in R is such that you don't get a speed improvement for problems that take less than a few seconds, let alone 0.05 seconds. Actually, since the parallel version isn't slower, I wonder if you registered a parallel backend. Did you, and if so, which one? - Steve
On Mon, Jul 18, 2011 at 3:25 PM, Megh Dal <megh700004 at yahoo.com> wrote:
As per the documentation of foreach package, if I use "%dopar%" then computation happens parallaly and on the contrary for %do%", it happens?sequentially. Here, I tried both "%dopar%" and "%do%" for one of the examples given in the help page of ?foreach:
a <- matrix(1:1600, 40, 40) b <- t(a) system.time(foreach(b=iter(b, by='col'), .combine=cbind) %dopar% ? (a %*% b))
? ?user ?system elapsed ? ?0.04 ? ?0.00 ? ?0.05
a <- matrix(1:1600, 40, 40) b <- t(a) system.time(foreach(b=iter(b, by='col'), .combine=cbind) %do% ? (a %*% b))
? ?user ?system elapsed ? ?0.05 ? ?0.00 ? ?0.05 However?surprisingly, I did not see any improvement in the computation time. I am using windows vista with dual core CPU (I think it is dual core as when I open Task manager -> Performance, I see there are 2 windows for CPU Usage History......... I am correct that it is dual core, right?) Therefore as it is dual core, shouldn't the computation time with "%dopar%" will be half of "%do%"? Am I missing something? Your help will be highly appreciated. Thanks
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