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

On Fri, Mar 04, 2016 at 09:08:44AM +0000, Li, Liang Z wrote:

quoted

On Fri, Mar 04, 2016 at 01:52:53AM +0000, Li, Liang Z wrote:

quoted

quoted

  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?

quoted

Only detect the unmapped/zero mapped pages is not enough. Consider

the

quoted

situation like case 2, it can't achieve the same result.

Your case 2 doesn't exist in the real world.  If people could stop their main
memory consumer in the guest prior to migration they wouldn't need live
migration at all.

The case 2 is just a simplified scenario, not a real case.
As long as the guest's memory usage does not keep increasing, or not always run out,
it can be covered by the case 2.

The memory usage will keep increasing due to ever growing caches, etc,
so you'll be left with very little free memory fairly soon.

quoted

I tend to think you can safely assume there's no free memory in the guest, so
there's little point optimizing for it.

If this is true, we should not inflate the balloon either.

We certainly should if there's "available" memory, i.e. not free but
cheap to reclaim.

quoted

OTOH it makes perfect sense optimizing for the unmapped memory that's
made up, in particular, by the ballon, and consider inflating the balloon right
before migration unless you already maintain it at the optimal size for other
reasons (like e.g. a global resource manager optimizing the VM density).

Yes, I believe the current balloon works and it's simple. Do you take the performance impact for consideration?
For and 8G guest, it takes about 5s to  inflating the balloon. But it only takes 20ms to  traverse the free_list and
construct the free pages bitmap.

I don't have any feeling of how important the difference is.  And if the
limiting factor for balloon inflation speed is the granularity of
communication it may be worth optimizing that, because quick balloon
reaction may be important in certain resource management scenarios.

By inflating the balloon, all the guest's pages are still be processed (zero page checking).

Not sure what you mean.  If you describe the current state of affairs
that's exactly the suggested optimization point: skip unmapped pages.

The only advantage of ' inflating the balloon before live migration' is simple, nothing more.

That's a big advantage.  Another one is that it does something useful in
real-world scenarios.

Roman.