From owner-freebsd-performance@FreeBSD.ORG Tue Oct 25 15:12:10 2005 Return-Path: X-Original-To: freebsd-performance@freebsd.org Delivered-To: freebsd-performance@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id DC9E416A420 for ; Tue, 25 Oct 2005 15:12:10 +0000 (GMT) (envelope-from rwatson@FreeBSD.org) Received: from cyrus.watson.org (cyrus.watson.org [209.31.154.42]) by mx1.FreeBSD.org (Postfix) with ESMTP id 785F043D53 for ; Tue, 25 Oct 2005 15:12:10 +0000 (GMT) (envelope-from rwatson@FreeBSD.org) Received: from fledge.watson.org (fledge.watson.org [209.31.154.41]) by cyrus.watson.org (Postfix) with ESMTP id 04D4346B06; Tue, 25 Oct 2005 11:12:10 -0400 (EDT) Date: Tue, 25 Oct 2005 16:12:09 +0100 (BST) From: Robert Watson X-X-Sender: robert@fledge.watson.org To: Lukas Razik In-Reply-To: <378216560@web.de> Message-ID: <20051025160620.O31152@fledge.watson.org> References: <378216560@web.de> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: freebsd-performance@freebsd.org Subject: Re: Performance issue with 5.4-RELEASE-p8 but not with 5.4-RELEASE X-BeenThere: freebsd-performance@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Performance/tuning List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 25 Oct 2005 15:12:11 -0000 On Tue, 25 Oct 2005, Lukas Razik wrote: >> >> http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/i386/i386/mp_machdep.c?only_with_tag=RELENG_5_4 >> >> "Add a knob for disabling/enabling HTT, >> "machdep.hyperthreading_allowed". >> Default off due to information disclosure on multi-user systems." >> >> Does turning HTT back on fix the regression? > > Yes, it does! :-) Many thanks!!! Nice to know this important detail... It would be quite interesting to know if you see the same problem in 6.0 or not. One of the things that is turned on in 6.0 by default is kernel preemption -- it was off in the 5.x series for stability reasons, as it depends on a fair amount of locking refinement, etc. One of the impacts of having four logical processors instead of two is that when interrupts come in, a logical processor is more likely to be free, so the ithread may start running earlier on 5.x if HTT is on, as it will get scheduled to another CPU. In 6.x, it can simply preempt any currently running kernel code, meaning it runs faster. So in 5.x, with a somewhat busy kernel, you see quite a bit lower interrupt processing latency with more processors, whereas in 6.x, it is consistently a lot lower because the ithread can run much more quickly regardless of the number of processors. (This isn't 100% true, but it's fairly true). Robert N M Watson