RFC: Simplfying hyperthreading distinctions
Rui Paulo
rpaulo at me.com
Fri Mar 6 20:59:23 UTC 2015
> On 6 Mar 2015, at 12:44, John Baldwin <jhb at FreeBSD.org> wrote:
>
> Currently we go out of our way a bit to distinguish Pentium4-era
> hyperthreading from more recent ("modern") hyperthreading. I suspect that
> this distinction probably results in confusion more than anything else.
> Intel's documentation does not make near as broad a distinction as far as I
> can tell. Both types of SMT are called hyperthreading in the SDM for example.
> However, we have the astonishing behavior that
> 'machdep.hyperthreading_allowed' only affects "old" hyperthreads, but not
> "new" ones. We also try to be overly cute in our dmesg output by using HTT
> for "old" hyperthreading, and SMT for "new" hyperthreading.
Yes, this is annoying.
> I propose the
> following changes to simplify things a bit:
>
> 1) Call both "old" and "new" hyperthreading HTT in dmesg.
Yes.
> 2) Change machdep.hyperthreading_allowed to apply to both new and old HTT.
> However, doing this means a POLA violation in that we would now disable
> modern HTT by default. Balanced against re-enabling "old" HTT by default
> on an increasingly-shrinking pool of old hardware, I think the better
> approach here would be to also change the default to allow HTT.
I think that's ok given 3).
> 3) Possibly add a different knob (or change the behavior of
> machdep.hyperthreading_allowed) to still bring up hyperthreads, but leave
> them out of the default cpuset (set 1). This would allow those threads
> to be re-enabled dynamically at runtime by adjusting the mask on set 1.
> The original htt settings back when 'hyperthreading_allowed' was
> introduced actually permitted this via by adjusting 'machdep.hlt_cpus' at
> runtime.
Sounds good.
Thanks,
--
Rui Paulo
More information about the freebsd-arch
mailing list