On Thu, Jan 31, 2013 at 05:20:02PM +0200, Avi Kivity wrote: > On Thu, Jan 31, 2013 at 4:42 PM, Gleb Natapov <g...@redhat.com> 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 <g...@redhat.com> 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? > 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. -- MST
