Status of hyperthreading in FreeBSD

Ivan Voras ivoras at
Sun Dec 21 22:32:56 UTC 2008

Brett Glass wrote:

> 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

Yes, but that is a hardware problem which is independent of the
operating system (it's present in all of them).

> also that hyperthreading didn't always improve performance on high end
> processors, the feature was turned off by default. But on single-user

Yes, especially on the early Pentium 4 CPUs. This is also OS-independent.

> 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?

It's as good as it can be on recent ULE2 scheduler. ULE2 has support for
HTT but there's not much that can be done at the scheduler level as the
Atom is single-socket, single-core CPU.

Atom's HTT is actually pretty good - I saw up to 25% more performance
simply by using multithreading in 7zip's compression benchmark (on
WinXP, though). Of course, OTOH it uses about that much more transistors
on the CPU die so it's not exactly free performance.

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