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.
2,B.1,B.2}
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
Xeons.
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.
Vincent
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
[EMAIL PROTECTED]
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