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

[Anthony Liguori]

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