[R] bam (mgcv) not using the specified number of cores

[Andrew Crane-Droesch]

I am getting strange behavior when trying to fit models to large 
datasets using bam.  I am working on a 4-core machine, but I think that 
there may be 2 physical cores that the computer uses as 4 cores in some 
sense that I don't understand.

When I run the bam using makeCluster(3), the model runs on one core.  
But when I run it on makeCluster(2), top shoes me that three of my cores 
are taken up to full capacity, and my computer slows down or crashes.

How can I get it to truly run on 2 cores?

I'm on a thinkpad X230, running ubuntu.

Thanks,
Andrew