From owner-freebsd-questions@FreeBSD.ORG Sat Dec 20 20:39:29 2008 Return-Path: Delivered-To: questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id C034D1065673 for ; Sat, 20 Dec 2008 20:39:29 +0000 (UTC) (envelope-from brett@lariat.net) Received: from lariat.net (lariat.net [66.119.58.2]) by mx1.freebsd.org (Postfix) with ESMTP id 6AD1C8FC12 for ; Sat, 20 Dec 2008 20:39:29 +0000 (UTC) (envelope-from brett@lariat.net) Received: from anne-o1dpaayth1.lariat.org (IDENT:ppp1000.lariat.net@lariat.net [66.119.58.2]) by lariat.net (8.9.3/8.9.3) with ESMTP id NAA10290 for ; Sat, 20 Dec 2008 13:39:26 -0700 (MST) Message-Id: <200812202039.NAA10290@lariat.net> X-Mailer: QUALCOMM Windows Eudora Version 7.1.0.9 Date: Sat, 20 Dec 2008 13:39:23 -0700 To: questions@freebsd.org From: Brett Glass Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed Cc: Subject: Status of hyperthreading in FreeBSD X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 20 Dec 2008 20:39:29 -0000 "Netbooks" based on Intel's "Atom" microprocessor are turning into big hits this Christmas season. The Atom, a super-low-power x86 processor, is an "in-order" machine, which means that except for a few special cases it can spend a lot of time waiting for data to arrive when it encounters a cache miss. So, hyperthreading may make sense on this kind of processor as compared to one with out-of-order execution. Which raises a question: What's the status of FreeBSD's support for hyperthreading? As far as I know, after it was revealed that some processes on a machine with hyperthreading could "spy" on others, and also that hyperthreading didn't always improve performance on high end processors, the feature was turned off by default. But on single-user machines, or on servers where the CPU was likely to be shared by two processes that were both privileged anyway, it might make sense to re-enable it. But has this feature of the scheduler been maintained well enough for this to be a good idea? If not, would it worth looking into updating it so that FreeBSD runs well on the Atom? --Brett Glass