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

[Li, Liang Z]

> > > > > Yes, we really can teach qemu to skip these pages and it's not hard.
> > > > > The problem is the poor performance, this PV solution
> > > >
> > > > Balloon is always PV. And do not call patches solutions please.
> > > >
> > > > > is aimed to make it more
> > > > > efficient and reduce the performance impact on guest.
> > > >
> > > > We need to get a bit beyond this.  You are making multiple
> > > > changes, it seems to make sense to split it all up, and analyse
> > > > each change separately.
> > >
> > > Couldn't agree more.
> > >
> > > There are three stages in this optimization:
> > >
> > > 1) choosing which pages to skip
> > >
> > > 2) communicating them from guest to host
> > >
> > > 3) skip transferring uninteresting pages to the remote side on
> > > migration
> > >
> > > For (3) there seems to be a low-hanging fruit to amend
> > > migration/ram.c:iz_zero_range() to consult /proc/self/pagemap.  This
> > > would work for guest RAM that hasn't been touched yet or which has
> > > been ballooned out.
> > >
> > > 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 am considering to drop the pagecache before getting the free pages.
> >
> > > As for (2), we do seem to have a problem with the existing balloon:
> > > according to your measurements it's very slow; besides, I guess it
> > > plays badly
> >
> > I didn't say communicating is slow. Even this is very slow, my
> > solution use bitmap instead of PFNs, there is fewer data traffic, so it's
> faster than the existing balloon which use PFNs.
> 
> By how much?
> 

Haven't measured yet. 
To identify a page, 1 bit is needed if using bitmap, 4 Bytes(32bit) is needed 
if using PFN, 

For a guest with 8GB RAM,  the corresponding free page bitmap size is 256KB.
And the corresponding total PFNs size is 8192KB. Assuming the inflating size
is 7GB, the total PFNs size is 7168KB.

Maybe this is not the point.

Liang

> > > with transparent huge pages (as both the guest and the host work
> > > with one 4k page at a time).  This is a problem for other use cases
> > > of balloon (e.g. as a facility for resource management); tackling
> > > that appears a more natural application for optimization efforts.
> > >
> > > Thanks,
> > > Roman.