Re: [Qemu-devel] [RFC kernel 0/2]A PV solution for KVM live migration optimization

On 3/8/2016 4:44 PM, Amit Shah wrote:

On (Fri) 04 Mar 2016 [15:02:47], Jitendra Kolhe wrote:

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

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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.

Ooh, different solutions for the same purpose, and both based on the balloon.

We were also tying to address similar problem, without actually needing to modify
the guest driver. Please find patch details under mail with subject.
migration: skip sending ram pages released by virtio-balloon driver

The scope of this patch series seems to be wider: don't send free
pages to a dest at all, vs. don't send pages that are ballooned out.

		Amit

Hi,

Thanks for your response. The scope of this patch series doesn’t seem to take care 
of ballooned out pages. To balloon out a guest ram page the guest balloon driver does 
a alloc_page() and then return the guest pfn to Qemu, so ballooned out pages will not 
be seen as free ram pages by the guest.
Thus we will still end up scanning (for zero page) for ballooned out pages during 
migration. It would be ideal if we could have both solutions.

Thanks,
- Jitendra
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