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

[Avi Kivity]

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).