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Date: Thu, 28 Jul 2011 10:49:54 +0100 (BST)
From: Robert Watson <rwatson@FreeBSD.org>
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To: Andriy Gapon <avg@FreeBSD.org>
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Cc: FreeBSD Hackers <freebsd-hackers@FreeBSD.org>
Subject: Re: HTT vs SMT in x86 SMP topology reporting
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On Tue, 26 Jul 2011, Andriy Gapon wrote:

> Can anybody explain to me why our _x86_ SMP topology discovery and reporting 
> code sometimes reports "HTT" and sometimes "SMT"? As in FreeBSD/SMP: %d 
> package(s) x %d core(s) x %d HTT threads vs FreeBSD/SMP: %d package(s) x %d 
> core(s) x %d SMT threads
>
> As I understand, and quoting Wikipedia (I know, I know), SMT stands for 
> simultaneous multithreading and is a generic term for a particular kind of 
> hardware multithreading: 
> http://en.wikipedia.org/wiki/Simultaneous_multithreading
>
> The only known (to me) implementation of SMT for x86 is Intel's 
> Hyper-Threading Technology aka HTT aka HT Technology aka hyperthreading: 
> http://en.wikipedia.org/wiki/Hyper-threading 
> http://software.intel.com/en-us/articles/intel-hyper-threading-technology-your-questions-answered/?wapkw=%28Intel+Hyper-Threading+Technology%29

Several MIPS platforms we run on support SMT.  Typically this means a set of 
"weaker" threads sharing a single core, usually context switching as a result 
of memory access stalls in other threads, and perhaps sharing particularly 
expensive CPU features, such as a TLB.  They sometimes come with 
high-performance message-passing facilities between threads, or even between 
cores, to supplement shared memory and IPIs.

It may be that HTT is, among other things, a trademark of Intel.

Robert