From owner-freebsd-hackers@FreeBSD.ORG  Sat Oct  4 14:56:33 2003
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 1F5E516A4B3
	for <freebsd-hackers@freebsd.org>;
	Sat,  4 Oct 2003 14:56:33 -0700 (PDT)
Received: from salmon.maths.tcd.ie (salmon.maths.tcd.ie [134.226.81.11])
	by mx1.FreeBSD.org (Postfix) with SMTP id B86DF43FBF
	for <freebsd-hackers@freebsd.org>;
	Sat,  4 Oct 2003 14:56:31 -0700 (PDT)
	(envelope-from iedowse@maths.tcd.ie)
Received: from walton.maths.tcd.ie by salmon.maths.tcd.ie with SMTP
          id <aa22200@salmon>; 4 Oct 2003 22:56:31 +0100 (BST)
To: Kris Kennaway <kris@obsecurity.org>
In-Reply-To: Your message of "Sat, 04 Oct 2003 13:04:35 PDT."
             <20031004200435.GA60432@rot13.obsecurity.org> 
Date: Sat, 04 Oct 2003 22:56:27 +0100
From: Ian Dowse <iedowse@maths.tcd.ie>
Message-ID: <200310042256.aa22200@salmon.maths.tcd.ie>
cc: Richard Coleman <richardcoleman@mindspring.com>
cc: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
cc: freebsd-hackers@freebsd.org
Subject: Re: Hyperthreading slowdown 
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sat, 04 Oct 2003 21:56:33 -0000

In message <20031004200435.GA60432@rot13.obsecurity.org>, Kris Kennaway writes:
>Yes, that's because (as discussed in the archives) the kernel treats
>it like an extra, completely decoupled physical CPU and schedules
>processes on it without further consideration.  This is presumably the
>cause of the slowdown, because it's only efficient to use the virtual
>CPU under certain workload patterns.  HTT is not magic performance
>beans.

Try also setting the sysctl variable "machdep.cpu_idle_hlt" to 1, as
it doesn't help to have the idle logical CPUs spinning.

Ian