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

[Li, Liang Z]

> On Thu, Mar 03, 2016 at 05:46:15PM +0000, Dr. David Alan Gilbert wrote:
> > * Liang Li (address@hidden) 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.

Liang