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

[Orit Wasserman]

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.

Regards,
Orit
> 
> Peter
> 
>