From owner-freebsd-smp@FreeBSD.ORG  Sun Apr 25 16:34:34 2010
Return-Path: <owner-freebsd-smp@FreeBSD.ORG>
Delivered-To: freebsd-smp@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id EE3DD1065674
	for <freebsd-smp@freebsd.org>; Sun, 25 Apr 2010 16:34:34 +0000 (UTC)
	(envelope-from bounces@nabble.com)
Received: from kuber.nabble.com (kuber.nabble.com [216.139.236.158])
	by mx1.freebsd.org (Postfix) with ESMTP id 93CA98FC17
	for <freebsd-smp@freebsd.org>; Sun, 25 Apr 2010 16:34:34 +0000 (UTC)
Received: from isper.nabble.com ([192.168.236.156])
	by kuber.nabble.com with esmtp (Exim 4.63)
	(envelope-from <bounces@nabble.com>) id 1O64X6-00028j-Q9
	for freebsd-smp@freebsd.org; Sun, 25 Apr 2010 09:18:12 -0700
Message-ID: <28357071.post@talk.nabble.com>
Date: Sun, 25 Apr 2010 09:18:12 -0700 (PDT)
From: Hubert Tournier <hubert@frbsd.org>
To: freebsd-smp@freebsd.org
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Nabble-From: hubert@frbsd.org
Subject: Disabling HyperThreading?
X-BeenThere: freebsd-smp@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: FreeBSD SMP implementation group <freebsd-smp.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-smp>,
	<mailto:freebsd-smp-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-smp>
List-Post: <mailto:freebsd-smp@freebsd.org>
List-Help: <mailto:freebsd-smp-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-smp>,
	<mailto:freebsd-smp-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sun, 25 Apr 2010 16:34:35 -0000


Hello (anybody there?),

Is there a way to disable "Hyper-Threading Technology" in modern FreeBSD
systems?

At the time of FreeBSD-SA-05:09.htt, it was suggested to use the following
workaround:

setting the "machdep.hlt_logical_cpus" tunable:
# echo "machdep.hlt_logical_cpus=1" >> /boot/loader.conf
Use of this workaround is not recommended on "dual-core" systems, as this
workaround will also disable one of the processor cores.

But this OID no longer seems to exist and I do not want to disable other
processor cores.

My use case is a VirtualBox dedicated server where I believe using HTT might
be problematic because:
1/ running two busy virtual machines on the same physical core could
severely impact their performance
2/ the virtualizer might expose the real CPU to the virtual machines, which
might in turn attempt to use HTT for themselves

Best regards,

Hubert

For example on a bi-Xeon X5650 FreeBSD 8.0p2 system, the relevant sysctl
OIDs are:


kern.ccpu: 0
  <cpu count="24" mask="0xffffff">0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23</cpu>
    <cpu count="12" mask="0xfff">0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11</cpu>
      <cpu count="2" mask="0x3">0, 1</cpu>
      <cpu count="2" mask="0xc">2, 3</cpu>
      <cpu count="2" mask="0x30">4, 5</cpu>
      <cpu count="2" mask="0xc0">6, 7</cpu>
      <cpu count="2" mask="0x300">8, 9</cpu>
      <cpu count="2" mask="0xc00">10, 11</cpu>
    <cpu count="12" mask="0xfff000">12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
22, 23</cpu>
      <cpu count="2" mask="0x3000">12, 13</cpu>
      <cpu count="2" mask="0xc000">14, 15</cpu>
      <cpu count="2" mask="0x30000">16, 17</cpu>
      <cpu count="2" mask="0xc0000">18, 19</cpu>
      <cpu count="2" mask="0x300000">20, 21</cpu>
      <cpu count="2" mask="0xc00000">22, 23</cpu>
kern.smp.cpus: 24
debug.kdb.stop_cpus: 1
hw.ncpu: 24
machdep.hlt_cpus: 0

Perhaps by playing with the mask?
-- 
View this message in context: http://old.nabble.com/Disabling-HyperThreading--tp28357071p28357071.html
Sent from the freebsd-smp mailing list archive at Nabble.com.