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

[Gleb Natapov]

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()?

--
                        Gleb.