From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758996Ab0GIAeU (ORCPT ); Thu, 8 Jul 2010 20:34:20 -0400 Received: from one.firstfloor.org ([213.235.205.2]:59813 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758939Ab0GIAeO (ORCPT ); Thu, 8 Jul 2010 20:34:14 -0400 Date: Fri, 9 Jul 2010 02:34:09 +0200 From: Andi Kleen To: Jeremy Fitzhardinge Cc: Andi Kleen , Daniel Kiper , xen-devel@lists.xensource.com, linux-kernel@vger.kernel.org Subject: Re: [Xen-devel] Re: GSoC 2010 - Migration from memory ballooning to memory hotplug in Xen Message-ID: <20100709003409.GC15950@basil.fritz.box> References: <20100708194553.GA30124@router-fw-old.local.net-space.pl> <871vbdr4ey.fsf@basil.nowhere.org> <4C36648F.6020006@goop.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4C36648F.6020006@goop.org> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Yes. Another approach would be to fiddle with the E820 maps early at > boot to add more RAM, but then early_reserve it and hand it over to the > control of the balloon driver. But it does mean you need to statically > come up with the max ever at boot time. You need to do that too for memory hotadd -- you need predeclared hotadd regions. Linux mainly needs it to know in which node to put the memory. Other OS use it for other things too. > > The only advantage of using memory hotadd is that the mem_map doesn't > > need to be pre-allocated, but that's only a few percent of the memory. > > > > So it would only help if you want to add gigantic amounts of memory > > to a VM (like >20-30x of what it already has). > > > > That's not wildly unreasonable on the face of it; consider a domain > which starts at 1GB but could go up to 32GB as demand requires. But The programs which need 32GB will probably not even start in 1GB :) > > One trap is also that memory hotadd is a frequent source of regressions, > > so you'll likely run into existing bugs. > > That could be painful, but I expect the main reason for regressions is > that the code is fairly underused. Adding new users should help. Yes, and we fixed a lot of the bugs, but still a lot of them were tricky and frankly new ones might be too difficult for a SoC. -Andi -- ak@linux.intel.com -- Speaking for myself only.