From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50206) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1adFaW-00063S-Nc for qemu-devel@nongnu.org; Tue, 08 Mar 2016 06:14:05 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1adFaT-0000Ab-2o for qemu-devel@nongnu.org; Tue, 08 Mar 2016 06:14:04 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51655) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1adFaS-0000AX-TR for qemu-devel@nongnu.org; Tue, 08 Mar 2016 06:14:00 -0500 Date: Tue, 8 Mar 2016 16:43:43 +0530 From: Amit Shah Message-ID: <20160308111343.GM15443@grmbl.mre> References: <1457001868-15949-1-git-send-email-liang.z.li@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1457001868-15949-1-git-send-email-liang.z.li@intel.com> Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Liang Li Cc: ehabkost@redhat.com, kvm@vger.kernel.org, mst@redhat.com, quintela@redhat.com, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, linux-mm@kvack.org, pbonzini@redhat.com, akpm@linux-foundation.org, virtualization@lists.linux-foundation.org, dgilbert@redhat.com, rth@twiddle.net On (Thu) 03 Mar 2016 [18:44:24], Liang Li wrote: > The current QEMU live migration implementation mark the all the > guest's RAM pages as dirtied in the ram bulk stage, all these pages > will be processed and that takes quit a lot of CPU cycles. > > From guest's point of view, it doesn't care about the content in free > pages. We can make use of this fact and skip processing the free > pages in the ram bulk stage, it can save a lot CPU cycles and reduce > the network traffic significantly while speed up the live migration > process obviously. > > This patch set is the QEMU side implementation. > > The virtio-balloon is extended so that QEMU can get the free pages > information from the guest through virtio. > > After getting the free pages information (a bitmap), QEMU can use it > to filter out the guest's free pages in the ram bulk stage. This make > the live migration process much more efficient. > > This RFC version doesn't take the post-copy and RDMA into > consideration, maybe both of them can benefit from this PV solution > by with some extra modifications. I like the idea, just have to prove (review) and test it a lot to ensure we don't end up skipping pages that matter. However, there are a couple of points: In my opinion, the information that's exchanged between the guest and the host should be exchanged over a virtio-serial channel rather than virtio-balloon. First, there's nothing related to the balloon here. It just happens to be memory info. Second, I would never enable balloon in a guest that I want to be performance-sensitive. So even if you add this as part of balloon, you'll find no one is using this solution. Secondly, I suggest virtio-serial, because it's meant exactly to exchange free-flowing information between a host and a guest, and you don't need to extend any part of the protocol for it (hence no changes necessary to the spec). You can see how spice, vnc, etc., use virtio-serial to exchange data. Amit