From owner-freebsd-hackers@FreeBSD.ORG  Sat Oct  4 19:55:17 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 A587716A4B3
	for <freebsd-hackers@freebsd.org>;
	Sat,  4 Oct 2003 19:55:17 -0700 (PDT)
Received: from artax.karlin.mff.cuni.cz (artax.karlin.mff.cuni.cz
	[195.113.31.125])
	by mx1.FreeBSD.org (Postfix) with ESMTP id E76B543FE5
	for <freebsd-hackers@freebsd.org>;
	Sat,  4 Oct 2003 19:55:16 -0700 (PDT)
	(envelope-from mikulas@artax.karlin.mff.cuni.cz)
Received: by artax.karlin.mff.cuni.cz (Postfix, from userid 17421)
	id 075F33FAA; Sun,  5 Oct 2003 04:55:14 +0200 (CEST)
Received: from localhost (localhost [127.0.0.1])
	by artax.karlin.mff.cuni.cz (Postfix) with ESMTP
	id EEEFB2FBD9; Sun,  5 Oct 2003 04:55:14 +0200 (CEST)
Date: Sun, 5 Oct 2003 04:55:14 +0200 (CEST)
From: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
To: Ian Dowse <iedowse@maths.tcd.ie>
In-Reply-To: <200310042256.aa22200@salmon.maths.tcd.ie>
Message-ID: <Pine.LNX.4.58.0310050452140.7806@artax.karlin.mff.cuni.cz>
References: <200310042256.aa22200@salmon.maths.tcd.ie>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
cc: Richard Coleman <richardcoleman@mindspring.com>
cc: freebsd-hackers@freebsd.org
cc: Kris Kennaway <kris@obsecurity.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: Sun, 05 Oct 2003 02:55:17 -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.

I did and it solved the problem (-j1 is as fast on hyperthreading kernel
as on singlethreading kernel). It should be default setting on
hyperthreading system (or at least there should be comment around option
HTT that people must set it), because otherwise performance is really
terrible --- the idle thread is spinning, taking half of the CPU.

Mikulas