Status of hyperthreading in FreeBSD

michael michael.copeland at
Sat Dec 20 20:53:10 UTC 2008

Brett Glass wrote:
> "Netbooks" based on Intel's "Atom" microprocessor are turning into big 
> hits this Christmas season. The Atom, a super-low-power x86 processor, 
> is an "in-order" machine, which means that except for a few special 
> cases it can spend a lot of time waiting for data to arrive when it 
> encounters a cache miss. So, hyperthreading may make sense on this 
> kind of processor as compared to one with out-of-order execution.
> Which raises a question: What's the status of FreeBSD's support for 
> hyperthreading? As far as I know, after it was revealed that some 
> processes on a machine with hyperthreading could "spy" on others, and 
> also that hyperthreading didn't always improve performance on high end 
> processors, the feature was turned off by default. But on single-user 
> machines, or on servers where the CPU was likely to be shared by two 
> processes that were both privileged anyway, it might make sense to 
> re-enable it. But has this feature of the scheduler been maintained 
> well enough for this to be a good idea? If not, would it worth looking 
> into updating it so that FreeBSD runs well on the Atom?
> --Brett Glass
as far as i know, just enabling smp will allow ht to function. also, i 
don't know if intel changed ht in the new atom processor, they could have.
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