"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?
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