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

[Gleb Natapov]

On Thu, Jan 31, 2013 at 03:59:23PM +0200, Avi Kivity wrote:
> On Jan 31, 2013 12:29 PM, "Orit Wasserman" <address@hidden> wrote:
> >
> > On 01/31/2013 11:48 AM, Gleb Natapov wrote:
> > > On Thu, Jan 31, 2013 at 09:47:24AM +0200, Orit Wasserman wrote:
> > >> On 01/31/2013 08:57 AM, Peter Lieven wrote:
> > >>> Hi,
> > >>>
> > >>> I just came across an idea and would like to have feedback if it
> makes sence or not.
> > >>>
> > >>> If a VM is started without preallocated memory all memory that has
> not been written to
> > >>> reads as zeros, right?
> > >> Hi,
> > >> No the memory will be unmapped (we allocate on demand).
> > >>> If a VM with a lot of unwritten memory is migrated or if the memory
> contains a lot
> > >>> of zeroed out memory (e.g. Windows or Linux guest with page
> sanitization) all this memory
> > >>> is allocated on the target during live migration. Especially with KSM
> this leads
> > >>> to the problem that this memory is allocated and might be not
> available completely as
> > >>> merging of the pages will happen async.
> > >>>
> > >>> Wouldn't it make sense to not send zero pages in the first round
> where the complete
> > >>> ram is sent (if it is detectable that we are in this stage)?
> > >> We send one byte per zero page at the moment (see is_dup_page) we can
> further optimizing it
> > >> by not sending it.
> > >> I have to point out that this is a very idle guest and we need to work
> on a loaded guest
> > >> which is the more hard problem in migration.
> > >>
> > >> Also I notice that the bottle neck in migrating unmapped pages is the
> detection of those pages
> > >> because we map the pages in order to check them, for a large guest
> this is very expensive as mapping a page
> > >> results in a page fault in the host.
> > >> So what will be very helpful is actually locating those pages without
> mapping them
> > >> which looks very complicated.
> > >>
> > > What is wrong with mincore()?
> >
> > 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. It is
possible to call madvise(MADV_WILLNEED) on them meanwhile to pre-load
swap without faulting on each page individually.

--
                        Gleb.