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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 10, 2016 at 01:41:16AM +0000, Li, Liang Z wrote:
> > > > > > 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.

Yes but this is not how balloon works, instead, it will reuse a single
4K page multiple times. We can also trade off more memory for speed
if we want to, it's completely up to guest.

> 
> 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.