How is hyperthreading handled on amd64?
wmoran at collaborativefusion.com
Mon Mar 13 15:41:58 UTC 2006
On Mon, 13 Mar 2006 15:16:15 +0000
Pete French <petefrench at ticketswitch.com> wrote:
> > I don't see this behaviour on i386 - my desktop, for example uses only
> > CPU 0 if the sysctl is 0, and switches between 0 and 1 when the sysctl
> > is 1.
> Uh, a typo on my part - substitute *machine* for *kernel*. Sorry!
> A machine with one physical processor works fine - can enable and disable
> hyperthreading as per the manual. A machine with two physical processors
> always gets hyperthreading enabled though. All of these machines are
> running 6.0 currently - though I will verify that the bug is still there
> on 6.1 when I get a moment to do an update on them.
Again, not what I'm seeing here. It appears as if the machine behaves
identically no matter what machdep.hyperthreading_allowed is set to.
The machine I'm testing on only has a single physical processor.
I've been running ubench multiple times to test this, and I see processes
on CPUs 1 and 0 no matter what I set the above sysctl to. I also don't
see any difference in ubench's performance numbers.
Collaborative Fusion Inc.
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