no hyperthreading in FreeBSD 9?

b. f. bf1783 at
Fri Jan 13 18:33:55 UTC 2012

> > Hi,
> >
> > I just upgraded from 8-STABLE to 9-STABLE on my dual Xeon (nocona). Now I have in my boot messages:
> > ...
> > root: /etc/rc.d/sysctl: WARNING: sysctl machdep.hlt_logical_cpus does not exist.
> > root: /etc/rc.d/sysctl: WARNING: sysctl machdep.hyperthreading_allowed does not exist.
> > ...
> >
> > So isn't hyperthreading not available anymore in FreeBSD 9?

I'm not sure what you mean by this double negative.  If you mean "Is
hyperthreading still available on FreeBSD?", the answer is yes.  If
you mean "Can hyperthreading still be disabled on FreeBSD?" the answer
is still yes-- only some problematic and redundant means of disabling
it that were present in earlier versions of FreeBSD have been removed.
 (The primary commit is:

.)  You can still use machdep.hyperthreading_allowed, but it is now
only a (loader) tunable, and not also a sysctl, so you can only set it
at boot time (via loader.conf(5), or by using "set ..." on the
loader(8) command line), and you cannot change it on the fly after the
system is up and running.

> Seems to imply HT is enabled by default and new sysctls are used to take
> logical CPUs offline.

Yes -- although that has been the case for a while, and the OIDs are
not all sysctls (some are (loader) tunables or device hints (cf.
device.hints(5)), and they are not new.


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