[Beowulf] Opinions of Hyper-threading?

Vincent Diepeveen diep at xs4all.nl
Mon Feb 25 10:16:38 PST 2008

Let's suppose you've inherited 3Ghz dual Xeon nodes and that the  
power costs get paid anyway.

Then the choice then is between:

without hyperthreading you've got
    2 cores @ 3Ghz

with hyperthreading you've got if you're lucky:
   2 cores @ 3Ghz which can split itself to
   4 cores @ 1.6Ghz

If you'd run 2 processes at each node, then there is 4 cores {A.1,A. 
So from scheduling points seen there is a number of possibilities.

We can compress those possibilities.

{A.1,A.2}     2 x 1.6ghz
{A.1,B.1}    2  x 3Ghz
{A.1,B.2}   2 x  3Ghz

So odds is roughly 33% that you end up getting dicked as your total  
throughput is
in 33% of the cases 3.2Ghz instead of 6.0Ghz

Seymour Crays principle comes to mind.

Now there seems to exist software on planet earth that just needs a  
lot of throughput.

Like the LL/LLR type software, provided that the FFT size isn't too big.
You schedule 4 processes and it wins 5% in throughput compared to 2  
Not the predicted 20% nor 30%, but 5%.

Heep Heep Huray, Seymour Crays principle refuted.

So for software that just needs throughput and where you run that  
might be faster
under specific circumstance.

That's however very risky.

Therefore most likely, you want to turn off hyperthreading in hardware.


p.s. it's nice if someone else pays your power bill, isn't it?

On Feb 13, 2008, at 6:58 PM, Jon Forrest wrote:

> I inherited a cluster containing a bunch
> of Xeon-based compute nodes. The compute
> nodes were configured with hyper-threading
> turned on. I'm wondering what you HPC cluster
> people think of hyper-threading. I haven't
> heard much about it recently since most
> modern processors are true multi-core.
> The main thing I'd like to know is whether
> hyper-threading can do any harm when cpu
> bound jobs are run.
> Cordially,
> -- 
> Jon Forrest
> Research Computing Support
> College of Chemistry
> 173 Tan Hall
> University of California Berkeley
> Berkeley, CA
> 94720-1460
> 510-643-1032
> jlforrest at berkeley.edu
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