Re: [Qemu-devel] RFC migration of zero pages

[Avi Kivity]

On Jan 31, 2013 11:14 PM, "Michael S. Tsirkin" <address@hidden> wrote:
>
> On Thu, Jan 31, 2013 at 05:20:02PM +0200, Avi Kivity wrote:
> > On Thu, Jan 31, 2013 at 4:42 PM, Gleb Natapov <address@hidden> wrote:
> > > On Thu, Jan 31, 2013 at 04:36:25PM +0200, Avi Kivity wrote:
> > >> On Thu, Jan 31, 2013 at 4:30 PM, Gleb Natapov <address@hidden> wrote:
> > >> >> >
> > >> >> > Avi/Michael do you remember why mincore can't be used to check if a guest
> > >> >> page is unmapped?
> > >> >>
> > >> >> A page may be not in core, but also nonzero (for example swap).
> > >> > Yes, mincore() should not be used as zero page check, but it can be used
> > >> > as an indicator that page can be dealt with in dead stage of migration
> > >> > since it is likely zero and will not take much time to send.
> > >>
> > >> Or it's swapped, in which case we know nothing about it.
> > >>
> > >> > It is
> > > Of course, that is why I said "likely' :)
> > >
> > >> > possible to call madvise(MADV_WILLNEED) on them meanwhile to pre-load
> > >> > swap without faulting on each page individually.
> > >>
> > >> During migration you're faulting like mad anyway.
> > > That's guest faulting on dirty bit logging, as far as I understand Orit
> > > says that in addition to that she sees a lot of faults generated by
> > > migration reading unmapped guest memory. She wants to eliminate at least
> > > those.
> >
> > For that, we once talked about extending mincore() to return info
> > whether a page is known zero (anonymous memory without a backing page,
> > or backing page == zero page).
>
> Shoudn't be hard to expose in /proc, no?

Unfortunately.

>
> > But I doubt it's worth it, yes we're
> > faulting a lot, but it's still a lot cheaper than actually sending a
> > page, so we're still ahead of a non-idle guest.
>
> It's not just fault, reading in the page is bad for the cache.
>

There's just one zero page.