Re: [Qemu-devel] Re: [Qemu-commits] [COMMIT 3086844] Instead of writing a zero page, madvise it away

[Anthony Liguori <aliguori@us.ibm.com>]

Paul Brook wrote:
>>>>> We have the same issue on the migration source node.  I don't see a
>>>>> simple way to solve it, though.
>>>>>           
>>>> I don't follow.  In this case, the issue is:
>>>>
>>>> 1) Start a guest with 1024, balloon down to 128MB.  RSS size is now
>>>> ~128MB
>>>> 2) Live migrate to a different node
>>>> 3) RSS on different node jumps to ~1GB
>>>>         
>>> 3.5) RSS on source node jumps to ~1GB, since reading the page
>>> instantiates the pte
>>>       
>> Surely we can do better here...
>>     
>
> Huh, I'm surprised we have to. I was expecting linux to populate the region 
> with CoW mappings of a single zero page, but apparently not. I guess reading a 
> zero page without writing to it is a fairly rare case, and it's usually better 
> to instantiate a writable page on the first access.
>   

I did too. If you mmap(/dev/zero) you'll get that behavior but not with 
a normal allocation.

Regards,

Anthony Liguori

> Paul
>   


-- 
Regards,

Anthony Liguori