Date: Tue, 23 May 2006 11:48:10 +1000 From: Mikhail Goriachev <mikhailg@webanoide.org> To: Jon Falconer <jfalconer@puc.edu> Cc: questions@freebsd.org Subject: Re: machdep.hlt_logical_cpus being ignored Message-ID: <447269DA.6040108@webanoide.org> In-Reply-To: <Pine.BSI.4.05L.10605221717090.1211-100000@ecf2.puc.edu> References: <Pine.BSI.4.05L.10605221717090.1211-100000@ecf2.puc.edu>
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Jon Falconer wrote: > Greetings All, > > I have installed FreeBSD 6.1 Release on a Dell 2850. I is has two Xeon > CPUs. Seeing the comment in the SMP man page about hyperthreading being a > problem in some situations, I added "machdep.hlt_logical_cpus=1" in > /boot/loader.conf file. Upon rebooting, the system still finds four CPUs > and starts all of them. I would have expected it to start only the two > physical CPUs. Am I missunderstanding what machdep.hlt_logical_cpus is > supposed to do? Should I even be concerned about the hyperthreading > problem. This system will be an email server for a small college campus > with about 2000 active accounts. > > Also, the man page for SMP say that the machdep.hlt_cpus can be used to > halt a specific CPU. I have not set this value, but sysctl shows it has a > value of 10, even before I set the machdep.hlt_logical_cpus to 1. Why > would this be the default value? I would think this would mean that one of > the system CPUs is halted. But dmesg output claims it launched four CPUs. > > Can someone fill me in on what this all means? Hi, This issue has been addressed in: http://security.freebsd.org/advisories/FreeBSD-SA-05:09.htt.asc You could also read this: http://www.daemonology.net/hyperthreading-considered-harmful/ I hope this helps. Cheers, Mikhail. -- Mikhail Goriachev Webanoide Telephone: +61 (0)3 62252501 Mobile Phone: +61 (0)4 38255158 E-Mail: mikhailg@webanoide.org Web: http://www.webanoide.org PGP Key ID: 0x4E148A3B PGP Key Fingerprint: D96B 7C14 79A5 8824 B99D 9562 F50E 2F5D 4E14 8A3B
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