From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60966) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aj6va-0005ZQ-KZ for qemu-devel@nongnu.org; Thu, 24 Mar 2016 11:12:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aj6vU-0000zc-RU for qemu-devel@nongnu.org; Thu, 24 Mar 2016 11:12:02 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45757) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aj6vU-0000zY-Bx for qemu-devel@nongnu.org; Thu, 24 Mar 2016 11:11:56 -0400 Date: Thu, 24 Mar 2016 17:11:49 +0200 From: "Michael S. Tsirkin" Message-ID: <20160324165530-mutt-send-email-mst@redhat.com> References: <1458632629-4649-1-git-send-email-liang.z.li@intel.com> <20160322190530.GI2216@work-vm> <20160324012424.GB14956@linux-gk3p> <20160324090004.GA2230@work-vm> <20160324102354.GB2230@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping free pages List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Li, Liang Z" Cc: "rkagan@virtuozzo.com" , "linux-kernel@vger.kenel.org" , "ehabkost@redhat.com" , "kvm@vger.kernel.org" , "quintela@redhat.com" , "simhan@hpe.com" , "Dr. David Alan Gilbert" , "qemu-devel@nongnu.org" , "jitendra.kolhe@hpe.com" , "mohan_parthasarathy@hpe.com" , "amit.shah@redhat.com" , "pbonzini@redhat.com" , Wei Yang , "rth@twiddle.net" On Thu, Mar 24, 2016 at 02:50:56PM +0000, Li, Liang Z wrote: > > > > > >> Given the typical speed of networks; it wouldn't do too much > > > > > >> harm to start sending assuming all pages are dirty and then > > > > > >> when the guest finally gets around to finishing the bitmap then > > > > > >> update, so it's asynchronous - and then if the guest never > > > > > >> responds we don't really > > > > care. > > > > > > > > > > > >Indeed, thanks! > > > > > > > > > > > > > > > > This is interesting. By doing so, the threshold I mentioned in > > > > > another mail is not necessary, since we can do it in parallel. > > > > > > > > Actually I just realised it's a little more complex; we can't sync > > > > the dirty bitmap again from the guest until after we've received the > > guests 'free' > > > > bitmap; that's because we wouldn't know if a 'dirty' page reflected > > > > that a page declared as 'free' had now been reused - so there is > > > > still an ordering there. > > > > > > > > Dave > > > > > > Not very complex, we can implement like this: > > > > > > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2. Clear > > > all the bits in ram_list. dirty_memory[DIRTY_MEMORY_MIGRATION] > > > 3. Send the get_free_page_bitmap request 4. Start to send pages to > > > destination and check if the free_page_bitmap is ready > > > if (is_ready) { > > > filter out the free pages from migration_bitmap_rcu->bmap; > > > migration_bitmap_sync(); > > > } > > > continue until live migration complete. > > > > > > > > > Is that right? > > > > The order I'm trying to understand is something like: > > > > a) Send the get_free_page_bitmap request > > b) Start sending pages > > c) Reach the end of memory > > [ is_ready is false - guest hasn't made free map yet ] > > d) normal migration_bitmap_sync() at end of first pass > > e) Carry on sending dirty pages > > f) is_ready is true > > f.1) filter out free pages? > > f.2) migration_bitmap_sync() > > > > It's f.1 I'm worried about. If the guest started generating the free bitmap > > before (d), then a page marked as 'free' in f.1 might have become dirty > > before (d) and so (f.2) doesn't set the dirty again, and so we can't filter out > > pages in f.1. > > > > As you described, the order is incorrect. > > Liang So to make it safe, what is required is to make sure no free list us outstanding before calling migration_bitmap_sync. If one is outstanding, filter out pages before calling migration_bitmap_sync. Of course, if we just do it like we normally do with migration, then by the time we call migration_bitmap_sync dirty bitmap is completely empty, so there won't be anything to filter out. One way to address this is call migration_bitmap_sync in the IO handler, while VCPU is stopped, then make sure to filter out pages before the next migration_bitmap_sync. Another is to start filtering out pages upon IO handler, but make sure to flush the queue before calling migration_bitmap_sync. > > Dave > > > > > > > > Liang > > > > > > > > > > > > > > >Liang > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK