Date: Thu, 28 Jul 2011 10:49:54 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Andriy Gapon <avg@FreeBSD.org> Cc: FreeBSD Hackers <freebsd-hackers@FreeBSD.org> Subject: Re: HTT vs SMT in x86 SMP topology reporting Message-ID: <alpine.BSF.2.00.1107281047050.30580@fledge.watson.org> In-Reply-To: <4E2ED546.2080401@FreeBSD.org> References: <4E2ED546.2080401@FreeBSD.org>
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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
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