From owner-freebsd-current@FreeBSD.ORG Tue Nov 9 19:17:08 2010 Return-Path: Delivered-To: current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 7C8AF1065679; Tue, 9 Nov 2010 19:17:08 +0000 (UTC) (envelope-from julian@freebsd.org) Received: from out-0.mx.aerioconnect.net (outx.internet-mail-service.net [216.240.47.247]) by mx1.freebsd.org (Postfix) with ESMTP id 5ABD68FC25; Tue, 9 Nov 2010 19:17:08 +0000 (UTC) Received: from idiom.com (postfix@mx0.idiom.com [216.240.32.160]) by out-0.mx.aerioconnect.net (8.13.8/8.13.8) with ESMTP id oA9JH5hV019398; Tue, 9 Nov 2010 11:17:05 -0800 X-Client-Authorized: MaGic Cook1e X-Client-Authorized: MaGic Cook1e X-Client-Authorized: MaGic Cook1e X-Client-Authorized: MaGic Cook1e X-Client-Authorized: MaGic Cook1e Received: from julian-mac.elischer.org (h-67-100-89-137.snfccasy.static.covad.net [67.100.89.137]) by idiom.com (Postfix) with ESMTP id BA8282D6015; Tue, 9 Nov 2010 11:17:04 -0800 (PST) Message-ID: <4CD99E34.10002@freebsd.org> Date: Tue, 09 Nov 2010 11:17:08 -0800 From: Julian Elischer User-Agent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X 10.4; en-US; rv:1.9.2.12) Gecko/20101027 Thunderbird/3.1.6 MIME-Version: 1.0 To: Alan Cox References: <4CD97A9A.8000007@freebsd.org> <20101109170453.942D95B89@mail.bitblocks.com> <4CD982E9.70500@freebsd.org> <4CD98BE7.7030208@rice.edu> In-Reply-To: <4CD98BE7.7030208@rice.edu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.67 on 216.240.47.51 Cc: alc@freebsd.org, sbruno@freebsd.org, FreeBSD Current Subject: Re: limits to memory on amd64 X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 09 Nov 2010 19:17:08 -0000 On 11/9/10 9:59 AM, Alan Cox wrote: > Julian Elischer wrote: >> On 11/9/10 9:04 AM, Bakul Shah wrote: >>> On Tue, 09 Nov 2010 08:45:14 PST Julian >>> Elischer wrote: >>>> During the discussion at MeetBSD the question came up as to what >>>> the real >>>> limiting factors were with regard to how much RAM a system could >>>> have. >>>> it was put to us that the limit was currently around 512 GB, >>>> though no-one >>>> at teh discussion knew what the mechanism of the limitation was or >>>> what might ligh beyond it. >>>> >>>> Could anyone who knows, pipe upt and let use know what the >>>> factors are, >>>> and if the current limit is overcome, what the next one after >>>> that will be? >>> You mean beyond architectural limits? >> >> no, though of course they are relevant. >> I was thinking more of details like limits to the KVM space or >> any limitations there may be on the size of the direct-map region, >> or maybe some limit on some data structure size in the kernel. >> Since I don 't know the details, this is exactly the question.. >> what IS the limit? > > The changes to support more than 512GB RAM should be > straightforward. Off the top of my head, it will require some > constant definitions in vmparam.h to change, and the allocation of > some additional PDP-level page table pages in create_pagetables(). > In contrast, the changes to break the original 2GB KVM barrier > involved touching a number of different places in the kernel. Alan, since some people (e.g. Sean Bruno) were hitting this, do you think you could provide a patch to allow people to test this? Sean in Particular seemed keen to try go to 1TB ram in a machine he had access to. Julian > > Alan > > > >