[Gllug] Why do jobs migrate between cores?

[John Winters]

I'm running a couple of CPU intensive jobs on a dual quad-core Xeon 
system (so 8 cores in total).  In the fullness of time (once the 
algorithm has been tested) the jobs will run multi-threaded and so I 
hope will make full use of all the cores, but I'm intrigued about what's 
happening at present.

With 2 jobs running they sit quite happily on two different cores, one 
in each processor and they each use about 75% user mode and 25% system 
mode.  The system is otherwise pretty much idle and yet every now and 
again one of the jobs will migrate to a different core (as seen using 
top).  Two questions:

1) Is it by design that the kernel uses one core from each physical 
processor?
2) Why would a job move from one core to another when there's sod all 
else going on?

John
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