From owner-freebsd-hackers@FreeBSD.ORG Sat Oct 4 14:54:47 2003 Return-Path: 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 A953F16A4B3 for ; Sat, 4 Oct 2003 14:54:47 -0700 (PDT) Received: from stork.mail.pas.earthlink.net (stork.mail.pas.earthlink.net [207.217.120.188]) by mx1.FreeBSD.org (Postfix) with ESMTP id BE8BD43FE3 for ; Sat, 4 Oct 2003 14:54:46 -0700 (PDT) (envelope-from richardcoleman@mindspring.com) Received: from c-24-98-233-138.atl.client2.attbi.com ([24.98.233.138] helo=mindspring.com) by stork.mail.pas.earthlink.net with asmtp (Exim 3.33 #1) id 1A5uMP-0004hM-00; Sat, 04 Oct 2003 14:54:45 -0700 Message-ID: <3F7F41A5.7020202@mindspring.com> Date: Sat, 04 Oct 2003 17:54:45 -0400 From: Richard Coleman Organization: Critical Magic, Inc. User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.4) Gecko/20030624 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Kris Kennaway References: <20031004190251.GA60026@rot13.obsecurity.org> <3F7F1D63.2010703@mindspring.com> <20031004200435.GA60432@rot13.obsecurity.org> In-Reply-To: <20031004200435.GA60432@rot13.obsecurity.org> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-ELNK-Trace: 1ee258965991efcb0865379cdb43356e5e89bb4777695beb702e37df12b9c9ef0c5f199c920c3aab780ffe4a04a8ad24350badd9bab72f9c350badd9bab72f9c cc: freebsd-hackers@freebsd.org cc: Mikulas Patocka Subject: Re: Hyperthreading slowdown X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list Reply-To: richardcoleman@mindspring.com List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 04 Oct 2003 21:54:47 -0000 Kris Kennaway wrote: > On Sat, Oct 04, 2003 at 03:20:03PM -0400, Richard Coleman wrote: > >>Kris Kennaway wrote: >> >>>On Sat, Oct 04, 2003 at 04:39:03PM +0200, Mikulas Patocka wrote: >>> >>>>I installed FreeBSD 4.9RC1 on P4 3GHz with hyperthreading and I see >>>>drastic slowdown when kernel with hyperthreading is booted. For example >>>>program compilation took this time: >>>> >>>>hyperthreading kernel, make -j 1 --- 1:09 >>>>hyperthreading kernel, make -j 2 --- 0:42 >>>>singlethreading kernel, make -j 1 --- 0:45 >>>>singlethreading kernel, make -j 2 --- 0:41 >>>> >>>>Compilation does very few system calls so when I compile with only one >>>>process (-j 1), it should be as fast as with singlethreading kernel. Do >>>>you have any idea why is it so slow? >>> >>>Do you realise that hyperthreading != a secret extra CPU in your system? >>> >>>Kris >> >>I didn't see anywhere in the message where he implied that. To me, the >>interesting thing is that there is such a larger difference between the >>compile time for -j1 and -j2 when using hyperthreading as compared to >>the difference between -j1 and -j2 for a single threaded kernel. It's >>over a 50% slowdown. > > > 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. > > Kris Sigh. No one is claiming HTT is magic performance beans. The 50% slowdown I'm talking about is between -j1 and -j2 BOTH ARE WHICH ARE USING HTT. It's just an interesting observation. That's all. Richard Coleman