[Beowulf] about hyperthreading in HPC

Mark Hahn hahn at physics.mcmaster.ca
Mon Aug 22 09:37:31 PDT 2005

> I have a cluster with 6 nodes Intel Xeon processors with Hyperthreading
> technology. My question is about if this technology has advantages on HPC
> clustering, or deppends of the application only?.

depeneds entirely on the application.  hyperthreading is nothing more than
hardware-assisted time-slicing.  namely, the CPU pretends that it's two 
CPUs, though in reality there is still a single pipeline.  one thread 
runs until it hits something like a cache miss or slow instruction, then 
the other thread runs for a while.  this can lead to moderately improved
throughput, but never to faster single-thread performance.  and often
the threads will conflict (demanding the same resource, such as competing 
fro cache).  when that happens, you can see _lower_ throughput...

> Then i found an article here:
> http://www.daemonology.net/hyperthreading-considered-harmful/ that talks about
> a security problem on servers with this technology. I wannt to know your
> opinions about this.

it's ridiculously mis-focused, and not all that novel.  mis-focused because
anyoen who can actually run malicious code on your cluster will probably 
find easier ways to own you.  not all that novel because there is a very long
history of this sort of timing-based information leaks; no one is interested
except perhaps governments.  and they have easier ways to go about it.

it would have been a lot more interesting during the cold war ;)

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