Date: Fri, 27 Jan 2006 11:32:01 -0800 From: Brooks Davis <brooks@one-eyed-alien.net> To: Pete French <petefrench@ticketswitch.com> Cc: freebsd-stable@freebsd.org Subject: Re: How do I turn off hyperthreading on 6.0 ? Message-ID: <20060127193201.GB20549@odin.ac.hmc.edu> In-Reply-To: <E1F2ZCY-000Dvx-Ey@dilbert.firstcallgroup.co.uk> References: <200601271019.59088.fcash@ocis.net> <E1F2ZCY-000Dvx-Ey@dilbert.firstcallgroup.co.uk>
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--98e8jtXdkpgskNou Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Jan 27, 2006 at 07:24:06PM +0000, Pete French wrote: > > Disable it in the BIOS. The kernel can't use what isn't advertised as= =20 > > available. :) >=20 > true - but surely this is a bug in FreeBSD ? >=20 > man smp says: >=20 > "Since using logical CPUs can cause performance penalties under certain > loads, the logical CPUs can be disabled by setting the > machdep.hlt_logical_cpus sysctl to one." >=20 > I have: >=20 > websvr04# sysctl machdep.hlt_logical_cpus > machdep.hlt_logical_cpus: 1 >=20 > but I am still seeing 4 CPU's as I have two physical processors, each with > two logical ones onboard. >=20 > I find it very hard to believe theres a bug in something this basic thoug= h, > as it's specificly mentioned in the relase notes as being there to disable > hyperthreading... You misunderstand the point of this sysctl. All machdep.hlt_logical_cpus does is prevent anything from scheduling anything on the extra logical CPUs. Since nothing is scheduled on them, they don't contend with each other and the performance issues are mitigated. They are still there and there's nothing the OS can do about that. -- Brooks --=20 Any statement of the form "X is the one, true Y" is FALSE. PGP fingerprint 655D 519C 26A7 82E7 2529 9BF0 5D8E 8BE9 F238 1AD4 --98e8jtXdkpgskNou Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iD8DBQFD2nUvXY6L6fI4GtQRAr1MAJ0ftEN/UVQIER8PvAalrpxRyeF4rQCgkNPR +aJJGCfbXcj+ojwCVaQkETU= =kGMf -----END PGP SIGNATURE----- --98e8jtXdkpgskNou--
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