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

[Jim Holtman]

what would be nice is to understand the amount of cpu time vs. the elapsed time consumed.  you might be paging if you don't have sufficient real memory.  I would venture a guess that the cpu time is very similar and the differnce is elapsed time due to some contention like memory.

Sent from my iPad

On Jul 12, 2012, at 13:54, Ulrike Grömping <groemping at bht-berlin.de> wrote:

> 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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