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

[Orit Wasserman]

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?

> 
> --
>                       Gleb.
>