The machdep.hyperthreading_allowed & ULE weirdness in 7.1

[Maxim Sobolev]

Robert Watson wrote:
> In the mean time, it sounds like the sysctl does need to be 
> reimplemented or removed, but one question is how far to take it -- 
> caches are shared to varying degrees at varying levels of the topology.  
> However, I believe the recommendation has generally moved to disabling 
> hyperthreading using the BIOS, as that uses the vendor's notion of 
> hyperthreading.  The idea of changing the setting at run-time is 
> currently untenable because we don't have the OS infrastructure to take 
> CPUs out of service, although growing it would be useful in order to 
> support virtual machine dynamic CPU reconfiguration.

Well, as far as I know, what SCHED_4BSD does is simply stopping 
scheduling threads to the logical core(s). One doesn't need 
infrastructure to take CPU off-line for doing the same in SCHED_ULE.

Unfortunately access to BIOS is not always an option and also some 
BIOSes don't even provide a feature to turn HTT off.

-Maxim