From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f171.google.com (mail-pf0-f171.google.com [209.85.192.171]) by kanga.kvack.org (Postfix) with ESMTP id D92436B0260 for ; Tue, 15 Mar 2016 21:20:43 -0400 (EDT) Received: by mail-pf0-f171.google.com with SMTP id x3so51765199pfb.1 for ; Tue, 15 Mar 2016 18:20:43 -0700 (PDT) Received: from mga09.intel.com (mga09.intel.com. [134.134.136.24]) by mx.google.com with ESMTP id 79si1371242pfm.61.2016.03.15.18.20.42 for ; Tue, 15 Mar 2016 18:20:42 -0700 (PDT) From: "Li, Liang Z" Subject: RE: [RFC qemu 0/4] A PV solution for live migration optimization Date: Wed, 16 Mar 2016 01:20:39 +0000 Message-ID: References: <1457001868-15949-1-git-send-email-liang.z.li@intel.com> <20160308111343.GM15443@grmbl.mre> <20160310075728.GB4678@grmbl.mre> <20160310111844.GB2276@work-vm> <20160314170334.GK2234@work-vm> <20160315121613-mutt-send-email-mst@redhat.com> <20160315195515.GL11728@work-vm> In-Reply-To: <20160315195515.GL11728@work-vm> Content-Language: en-US Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: "Dr. David Alan Gilbert" Cc: "Michael S. Tsirkin" , Amit Shah , "quintela@redhat.com" , "qemu-devel@nongnu.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "pbonzini@redhat.com" , "rth@twiddle.net" , "ehabkost@redhat.com" , "linux-mm@kvack.org" , "virtualization@lists.linux-foundation.org" , "kvm@vger.kernel.org" , "mohan_parthasarathy@hpe.com" , "jitendra.kolhe@hpe.com" , "simhan@hpe.com" > > > > > > I'm just catching back up on this thread; so without > > > > > > reference to any particular previous mail in the thread. > > > > > > > > > > > > 1) How many of the free pages do we tell the host about? > > > > > > Your main change is telling the host about all the > > > > > > free pages. > > > > > > > > > > Yes, all the guest's free pages. > > > > > > > > > > > If we tell the host about all the free pages, then we migh= t > > > > > > end up needing to allocate more pages and update the host > > > > > > with pages we now want to use; that would have to wait for= the > > > > > > host to acknowledge that use of these pages, since if we d= on't > > > > > > wait for it then it might have skipped migrating a page we > > > > > > just started using (I don't understand how your series sol= ves that). > > > > > > So the guest probably needs to keep some free pages - how > many? > > > > > > > > > > Actually, there is no need to care about whether the free pages > > > > > will be > > > used by the host. > > > > > We only care about some of the free pages we get reused by the > > > > > guest, > > > right? > > > > > > > > > > The dirty page logging can be used to solve this, starting the > > > > > dirty page logging before getting the free pages informant from g= uest. > > > > > Even some of the free pages are modified by the guest during the > > > > > process of getting the free pages information, these modified > > > > > pages will > > > be traced by the dirty page logging mechanism. So in the following > > > migration_bitmap_sync() function. > > > > > The pages in the free pages bitmap, but latter was modified, > > > > > will be reset to dirty. We won't omit any dirtied pages. > > > > > > > > > > So, guest doesn't need to keep any free pages. > > > > > > > > OK, yes, that works; so we do: > > > > * enable dirty logging > > > > * ask guest for free pages > > > > * initialise the migration bitmap as everything-free > > > > * then later we do the normal sync-dirty bitmap stuff and it all = just > works. > > > > > > > > That's nice and simple. > > > > > > This works once, sure. But there's an issue is that you have to > > > defer migration until you get the free page list, and this only > > > works once. So you end up with heuristics about how long to wait. > > > > > > Instead I propose: > > > > > > - mark all pages dirty as we do now. > > > > > > - at start of migration, start tracking dirty > > > pages in kvm, and tell guest to start tracking free pages > > > > > > we can now introduce any kind of delay, for example wait for ack > > > from guest, or do whatever else, or even just start migrating pages > > > > > > - repeatedly: > > > - get list of free pages from guest > > > - clear them in migration bitmap > > > - get dirty list from kvm > > > > > > - at end of migration, stop tracking writes in kvm, > > > and tell guest to stop tracking free pages > > > > I had thought of filtering out the free pages in each migration bitmap > synchronization. > > The advantage is we can skip process as many free pages as possible. No= t > just once. > > The disadvantage is that we should change the current memory > > management code to track the free pages, instead of traversing the free > page list to construct the free pages bitmap, to reduce the overhead to g= et > the free pages bitmap. > > I am not sure the if the Kernel people would like it. > > > > If keeping the traversing mechanism, because of the overhead, maybe it'= s > not worth to filter out the free pages repeatedly. >=20 > Well, Michael's idea of not waiting for the dirty bitmap to be filled doe= s make > that idea of constnatly using the free-bitmap better. >=20 No wait is a good idea. Actually, we could shorten the waiting time by pre allocating the free page= s bit map and update it when guest allocating/freeing pages. it requires to modify th= e mm=20 related code. I don't know whether the kernel people like this. > In that case, is it easier if something (guest/host?) allocates some memo= ry in > the guests physical RAM space and just points the host to it, rather than > having an explicit 'send'. >=20 Good idea too. Liang > Dave -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org