[Tfug] Dual cores and hyperthreading

[Shawn Nock]

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John Gruenenfelder wrote:
> Is is possible to take advantage of this as well?  My understanding is that
> hyperthreading is 2 virtual CPUs in a single core and so it should be separate
> from the hardware which enables two physical cores thus making it possible to
> use both at the same time.

Yes, but...
Last I checked (this was for FreeBSD) the story was this:

- -SMP kernel
- -Hyperthreading enabled in BIOS

and now the ``but'' part:

- -HyperThreading slowed down performance measurably during high-load
operations (Typical server work-flow; I don't know if there were
workstation simulations... you'll have to do some research.)

- -HyperThreading opens a severe memory sharing vulnerability, where One
process running on the proc (or core) could snoop/modify the other
processes stack and heap. (Could have been fixed... but when I checked
this was a hardware issue that couldn't be easily fixed din software.
The FreeBSD SMP folks mentioned that this vulnerability should have
similar implications in other OS's and that the safe course of action
was to disable HT).

These two issues lead me to disable HT in my production machines, but
they are servers, YMMV on your lappie (In theory, no processes you
didn't initiate and a distinctly non-server work-flow).

Cheers and good luck,

Shawn
- --
Shawn Nock (OpenPGP: 0xB64200E1)
Broadcast Engineer; KUAT Communications Group
University of Arizona
nock 'at ' arizona 'dot' edu
- --
"Despair leads to boredom, electronic games, computer hacking, poetry,
and other bad habits."  -Edward Abbey
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