[Beowulf] Opinions of Hyper-threading?

Bill Broadley bill at cse.ucdavis.edu
Wed Feb 13 14:00:15 EST 2008


If you don't want handwaving, I'd just test it.  There are jobs that do work 
with HT, and those that don't.

 From the tests I've done it's not particularly reliable.  So the performance 
you get depends on what else the CPU is doing.  So if you have jobs A and B
on a single CPU with 2 HT threads the performance of A and B vary depending on 
the phase of the moon.  So if jobs A and B use 16 CPUs and have to make 
progress in lock step (common in parallel jobs) you get the worst case of
16 CPUs, which is VERY likely to be less than turning HT off.

I have seen occasional improvements in throughput of 5-10% or so.

So without testing I'd vote turn it off.

The best benchmark is your code.

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