From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [RFC] new totalmem= boot parameter Date: Tue, 20 Jul 2010 17:55:42 -0400 Message-ID: <20100720215542.GA17791@phenom.dumpdata.com> References: <682e4bcf-71e3-4b49-a25b-79404ae470bb@default> <20100720212616.GB15056@ca-server1.us.oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20100720212616.GB15056@ca-server1.us.oracle.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Sarina Canelake Cc: "xen-devel@lists.xensource.com" , Keir Fraser List-Id: xen-devel@lists.xenproject.org On Tue, Jul 20, 2010 at 02:26:16PM -0700, Sarina Canelake wrote: > On Mon, Jul 19, 2010 at 07:11:19PM +0100, Keir Fraser wrote: > > On 19/07/2010 18:56, "Sarina Canelake" wrote: > > > > > We have a need for ensuring the total RAM available to [Xen / the kernel] at > > > boot is X MB because there are situations in which you wish to limit the > > > amount of RAM available to a box. The existing mem= option doesn't work > > > because it limits the maximum physical address, NOT the amount of available > > > RAM. Many, if not all, systems contain a substantial memory hole below 4 Gb, > > > typically a 0.5 or 1 Gb hole from 3-4 Gb. Thus, on a system with 6 Gb of RAM, > > > requesting mem=4096M will yield a box with maximum physical address in the 4 > > > Gb neighborhood but perhaps only 3 or 3.5 actual gigs of RAM available. > > > > It doesn't sound *very* useful. But then neither is mem= really. We can add > > something like this if you really need it. So what's the motivation? > > > > I found it useful while I was testing various core dumping capabilities. > Using a boot-time argument to limit memory eliminates the need for pulling > out DIMMs (which I couldn't do anyways, as the machines I was working > on are remote). However mem= didn't suffice for this purpose > beyond 3 Gb since, as I mentioned, it limits the physical address > rather than the amount of RAM, which is what I thought it was > supposed to do. Hence the implementation of totalmem=, which made my > 16Gb+ boxes capable of imitating various, specific smaller configurations. > > Alternatively, if mem= isn't used very frequently, perhaps it wouldn't I use it for testing combinations where memory below the 4GB mark (for PCI devices) makes Dom0/DomU work. This helps to figure out what went wrong. And that means I actually need RAM (and the PCI hole) to be below the 32-bit mark.