Status of hyperthreading in FreeBSD

Brett Glass brett at
Sat Dec 20 20:39:29 UTC 2008

"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

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