Date: Wed, 22 May 2002 12:19:38 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Alfred Perlstein <bright@mu.org> Cc: "Dorr H. Clark" <dclark@applmath.scu.edu>, freebsd-smp@FreeBSD.ORG Subject: Re: hyperthreading: myth or legend? (was Re: hyperthreading? (was Re: question)) Message-ID: <3CEBEF4A.7394402C@mindspring.com> References: <20020514222840.GB1585@elvis.mu.org> <Pine.GHP.4.21.0205220940410.28331-100000@hpux38.dc.engr.scu.edu> <20020522172759.GV54960@elvis.mu.org> <3CEBE6FD.626DC5DD@mindspring.com> <20020522185102.GY54960@elvis.mu.org>
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Alfred Perlstein wrote: > Terry, it boots and probes as a 4way box with 2 cpus. I'm sure > the listed optimizations you provided would be of some assistance > however I HAVE seen it boot and probe as a 4way. Well, then it really comes down to what "support" means. It's like saying a sound card is supported when all you can do with it is make it make an error "beep" equivalent to that of a PC speaker: yes, there's presentation; no, there's not utilization. I expect that, unless you are running threaded code, and have a uniform load (i.e. you have few contending processes), then a FreeBSD box with an SMT processor will gain about an 18% speedup for the SMT -- if that. Most FreeBSD code is not threaded, and you lose all the locality that you'd otherwise have, were that not the case, and were the scheduler careful. The same contention problems that prevent reasonably scaling to more than 4 CPUs will be magnified in the SMT system. So from one perspective: Q: "Does FreeBSD support Hyperthreading?" A: "That's a stupid question. Yes, it supports detecting resources which are listed in the MP Table in the BIOS." And from another: Q: "Does FreeBSD support Hyperthreading?" A: "Not really. You will gain little benefit from spending the extra money, compared to significantly better benefits from a traditional SMP system." I think saying that Hyperthreading is supported in FreeBSD (or Linux, or even Windows, for that matter) is really being disingenous. Red Hat Linux 7.1 has significantly better support for Hyperthreading than FreeBSD or Windows. It's still disingenuous for them to claim that it's really supported, but they are unlikely to do the necessary compiler work any time soon, so it's probably as supported as it's going to be by anyone, for quite some time. I know for a fact that the CPU affinity in Linux degrtades not very gracefully under real world loads involving non-uniform and contending processes. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-smp" in the body of the message
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