[Beowulf] Hyperthreading and 'OS jitter'

John Hearns hearnsj at googlemail.com
Sat Jul 22 03:11:11 PDT 2017

Several times in the past I have jokingly asked if there shoudl eb another
lower powered CPU core ina  system to run OS tasks (hello Intel - are you
Also int he past there was advice to get best possible throughpur on AMD
Bulldozer CPUs to run only on every second core (as they share FPUs).
When I managed a large NUMA system we used cpusets, and the OS ran in a
smal l'boot cpuset' which was physically near the OS disks and IO cards.

I had a thought about hyperthreading though. A few months ago we did a
quick study with Blener rendering, and got 30% more througput with HT
switched on. Also someone who I am workign with now would liek to assess
the effect on their codes of HT on/HT off.
I kow that HT has nromally not had any advantages with HPC type codes - as
the core should be 100% flat out.

I am thinking though - what woud be the effect of enabling HT, and usign a
cgroup to constrain user codes to run on all the odd-numbered CPU cores,
with the OS tasks on the even numbered ones?
I would hope this would be at least performance neutral? Your thoughts
please! Also thoughts on candidate benchmark programs to test this idea.

John Hearns........
 ....... John Hearns
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