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

On Thu, Mar 03, 2016 at 05:46:15PM +0000, Dr. David Alan Gilbert wrote:

* Liang Li (liang.z.li@intel.com) wrote:

quoted

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.