I'll very soon give a try to using Hyperthreading with our app,
and keep you posted about the improvements, if any.
Our current cluster is made out of 4-core dual-socket Nehalem nodes.
Cheers, Gilbert.
Le 7 janv. 11 à 16:17, Tim Prince a écrit :
On 1/7/2011 6:49 AM, Jeff Squyres wrote:
My understanding is that hyperthreading can only be activated/
deactivated at boot time -- once the core resources are allocated
to hyperthreads, they can't be changed while running.
Whether disabling the hyperthreads or simply telling Linux not to
schedule on them makes a difference performance-wise remains to be
seen. I've never had the time to do a little benchmarking to
quantify the difference. If someone could rustle up a few cycles
(get it?) to test out what the real-world performance difference is
between disabling hyperthreading in the BIOS vs. telling Linux to
ignore the hyperthreads, that would be awesome. I'd love to see
such results.
My personal guess is that the difference is in the noise. But
that's a guess.
Applications which depend on availability of full size instruction
lookaside buffer would be candidates for better performance with
hyperthreads completely disabled. Many HPC applications don't
stress ITLB, but some do.
Most of the important resources are allocated dynamically between
threads, but the ITLB is an exception.
We reported results of an investigation on Intel Nehalem 4-core
hyperthreading where geometric mean performance of standard
benchmarks for certain commercial applications was 2% better with
hyperthreading disabled at boot time, compared with best 1 rank per
core scheduling with hyperthreading enabled. Needless to say, the
report wasn't popular with marketing. I haven't seen an equivalent
investigation for the 6-core CPUs, where various strange performance
effects have been noted, so, as Jeff said, the hyperthreading effect
could be "in the noise."
--
Tim Prince
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