Using hyperthreading on 2 Proc Xeon cluster nodes

[Patrick Geoffray]

Hi Craig,

Craig Tierney wrote:
> Has anyone seen this behavior?  Is anyone running a cluster

Yes, exactely. Nothing is done in the Linux scheduler to care about 
allocating the 2 heavy processes on 2 virtual processors mapped on 2 
real processors. So it may be possible that 2 HPL processes run 
effectively on the same CPU.

Worse, the scheduler assume that there is 4 independant CPUs,
so processes may be reassigned to different virtual processors
at each quantum, depending of the context. In short, processes
can be balanced over the 2 real CPUs at one time, and then they
can share one, and so on.

HyperThreading is worth only when there is much more threads/processes 
than processors, blocking often on IO for example (webserver, etc.)

BTW, the 2 virtual processors share the same FPU, so not interesting for 
HPC.

Patrick

----------------------------------------------------------
|   Patrick Geoffray, Ph.D.      patrick at myri.com
|   Myricom, Inc.                http://www.myri.com
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