[R] Two R sessions on multicore computer seem to inhibit each other ?

[Ulrike Grömping]

Dear R-helpers,

I am puzzled by the following observation: 
On my home dual core Windows desktop computer, I am used to running two R
sessions in parallel. These do very well in using the full CPU of the
computer (half each) and don't seem to slow each other down.

Today I have started some large computation effort in one of our university
labs, and I intended to go for two R sessions per computer as well. These
computers are all identical and have Intel Core i5, 3.3 GHz processors with
four cores, and they are used for my computations and nothing else at the
moment. When I run one R session (32 bit) on them, the Windows task manager
shows CPU usage at about 25%. If I add another R session that should also
use as much CPU as it can get, the windows task manager showed overall CPU
usage of the two sessions below 25%. 
This made me very suspicious, and I decided to use separate computers for
all tasks (glad I did) and started a small investigation into what is going
on here: one extra computer runs two R sessions that do the exact same
calculations done on two other computers. I am surprised to see the enormous
speed advantage of doing calculations on separate computers: a calculation
that took 15 minutes on a computer that ran a single R session took 156
minutes in an R session on a computer with two R sessions running; the other
example: 8 minutes to 71 minutes. 

Does anyone have any idea why running two R sessions simultaneously slows
everything down so much? 

Thanks and regards,
Ulrike

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