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Date:      Fri, 06 Mar 2015 18:41:48 -0500
From:      John Baldwin <jhb@freebsd.org>
To:        Adrian Chadd <adrian@freebsd.org>
Cc:        Andriy Gapon <avg@freebsd.org>, "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org>
Subject:   Re: RFC: Simplfying hyperthreading distinctions
Message-ID:  <2092193.qt8NhEKglv@ralph.baldwin.cx>
In-Reply-To: <CAJ-VmoknoYw-jkihmutN6qB=Piy4O73bzV50ijDaEaNvEncGpA@mail.gmail.com>
References:  <1640664.8z9mx3EOQs@ralph.baldwin.cx> <CAJ-VmoknoYw-jkihmutN6qB=Piy4O73bzV50ijDaEaNvEncGpA@mail.gmail.com>

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On Friday, March 06, 2015 01:37:04 PM Adrian Chadd wrote:
> Hi!
> 
> 1) I'd rather we leave them as SMT/HTT as they're slightly different
> things. Who knows if intel will re-introduce this stuff in their more
> embedded CPU line at a future time, or add another threading type in
> the future. Being told about the distinction is nice.
> 2) I'd rather we had it more clearly defind - machdep.htt_allowed /
> machdep.smt_allowed . Again, I'd rather have the distinction in case
> Intel decide again to make their embedded things use old-style
> threading. (The intel edison/galilleo boards use P1 style cores that
> are low power, I can imagine a world where they reuse HTT for that.)

I don't think Netburst is coming back.  Even the Atom stuff is based on the 
PIII/Core line, not Netburst (and Atom CPUs that support HTT use the newer-
style HTT, not Netburst).  In the SDM it mostly seems to be Netburst vs 
everything else where the distinction is made (and it's not always made).

We are now in the odd situation where we refer to a small (and shrinking) set 
of CPUs that support HTT as HTT and we refer to a much larger (and growing) 
set of CPUs that support HTT as something else.  This means that if a random 
user wants to see if FreeBSD supports HTT they won't see that in dmesg on a 
modern CPU without having some sort of magic decoder ring.

I also think the set of folks who actually care about the slight differences 
in HTT is quite small and not worth the cost of looking as if we don't support 
HTT on the majority of processors that support it.

Note that BIOS manufacturers have gotten away with labeling the two things the 
same without people bringing out pitchforks, so I think having FreeBSD be 
consistent with that (and other OS's) is less confusing to users, not more.

-- 
John Baldwin



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