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

From: Roman Kagan
Date: Wed Mar 09 2016 - 12:05:06 EST

On Wed, Mar 09, 2016 at 05:41:39PM +0200, Michael S. Tsirkin wrote:
> On Wed, Mar 09, 2016 at 05:28:54PM +0300, Roman Kagan wrote:
> > For (1) I've been trying to make a point that skipping clean pages is
> > much more likely to result in noticable benefit than free pages only.
> I guess when you say clean you mean zero?

No I meant clean, i.e. those that could be evicted from RAM without
causing I/O.

> Yea. In fact, one can zero out any number of pages
> quickly by putting them in balloon and immediately
> taking them out.
> Access will fault a zero page in, then COW kicks in.

I must be missing something obvious, but how is that different from
inflating and then immediately deflating the balloon?

> We could have a new zero VQ (or some other option)
> to pass these pages guest to host, but this only
> works well if page size matches the host page size.

I'm afraid I don't yet understand what kind of pages that would be and
how they are different from ballooned pages.

I still tend to think that ballooning is a sensible solution to the
problem at hand; it's just the granularity that makes things slow and
stands in the way.