linux-kernel - RE: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization

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Date:	Fri, 4 Mar 2016 09:12:12 +0000
From:	"Li, Liang Z" <liang.z.li@...el.com>
To:	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	Roman Kagan <rkagan@...tuozzo.com>,
	"ehabkost@...hat.com" <ehabkost@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mst@...hat.com" <mst@...hat.com>,
	"quintela@...hat.com" <quintela@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"amit.shah@...hat.com" <amit.shah@...hat.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"rth@...ddle.net" <rth@...ddle.net>
Subject: RE: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration
 optimization

> * Roman Kagan (rkagan@...tuozzo.com) wrote:
> > On Fri, Mar 04, 2016 at 08:23:09AM +0000, Li, Liang Z wrote:
> > > > On Thu, Mar 03, 2016 at 05:46:15PM +0000, Dr. David Alan Gilbert wrote:
> > > > > * Liang Li (liang.z.li@...el.com) 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.
> > > > >
> > > > > Hi,
> > > > >   An interesting solution; I know a few different people have
> > > > > been looking at how to speed up ballooned VM migration.
> > > > >
> > > > >   I wonder if it would be possible to avoid the kernel changes
> > > > > by parsing /proc/self/pagemap - if that can be used to detect
> > > > > unmapped/zero mapped pages in the guest ram, would it achieve
> > > > > the
> > > > same result?
> > > >
> > > > Yes I was about to suggest the same thing: it's simple and makes
> > > > use of the existing infrastructure.  And you wouldn't need to care
> > > > if the pages were unmapped by ballooning or anything else
> > > > (alternative balloon implementations, not yet touched by the
> > > > guest, etc.).  Besides, you wouldn't need to synchronize with the guest.
> > > >
> > > > Roman.
> > >
> > > The unmapped/zero mapped pages can be detected by parsing
> > > /proc/self/pagemap, but the free pages can't be detected by this.
> > > Imaging an application allocates a large amount of memory , after
> > > using, it frees the memory, then live migration happens. All these free
> pages will be process and sent to the destination, it's not optimal.
> >
> > First, the likelihood of such a situation is marginal, there's no
> > point optimizing for it specifically.
> >
> > And second, even if that happens, you inflate the balloon right before
> > the migration and the free memory will get umapped very quickly, so
> > this case is covered nicely by the same technique that works for more
> > realistic cases, too.
> 
> Although I wonder which is cheaper; that would be fairly expensive for the
> guest wouldn't it? And you'd somehow have to kick the guest before
> migration to do the ballooning - and how long would you wait for it to finish?

About 5 seconds for an 8G guest, balloon to 1G. Get the free pages bitmap take about 20ms
for an 8G idle guest.

Liang

> 
> Dave
> 
> >
> > Roman.
> --
> Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK