Re: [PATCH 4 of 4] xenpaging: initial libxl support

[Olaf Hering <olaf@aepfle.de>]

On Tue, Nov 22, Stefano Stabellini wrote:

> On Mon, 21 Nov 2011, Olaf Hering wrote:
> > On Mon, Nov 21, Stefano Stabellini wrote:
> > 
> > > what if tot_memkb is bigger than target_memkb? Or even bigger than
> > > max_memkb?
> > 
> > tot_memkb is unrelated to target_memkb, also somewhat unrelated to
> > max_memkb.
> 
> At build time ballooning is not active yet and target_memkb represents
> the amount of memory available to the VM plus the videoram (see
> libxl__build_hvm).
> As a consequence I think that tot_memkb cannot be higher than
> target_memkb - videoram_memkb (that is build_start in the diagram). 

It can because with xenpaging the target_memkb turns from real memory
into virtual memory, and tot_memkb is the new amount of real memory.

The actual checking wether the tot_memkb/target_memkb/max_memkb are sane
can be either done when they are changed with xl mem-XY like its done
now. Or we add new code to do such checking already during config
parsing.

> So, what is going to happen if tot_memkb is higher than target_memkb -
> videoram_memkb?

Nothing happens, since xenpaging is the only consumer of that variable
(via xenstore). See below.

> Also, what is going to happen if it is lower?

If its lower, xenpaging will page-out some pages, adds them back if the
guest happens to access them and page-out some other pages. The guest
still has access to all memory it thinks it has (target_memkb).

> > xenpaging will look at tot_memkb value (at "memory/target-tot_pages" to
> > be precise) and try to reach that number of domain->tot_pages. If the
> > tot_memkb number is larger than max_memkb nothing will happen.
> 
> How is it going to reach the tot_pages target? Where is it going to take
> the memory from? Is it going to automatically page out memory from other
> VMs?

xenpaging does not add new memory. If it has no pages to page-in and
tot_pages is still higher, it will do nothing.

> > Right now there is not much checking anyway, memory=1024 maxmem=1 in the
> > config is accepted in my testing.
>  
> That is a correct configuration: it means that the domain has 1024MB of
> RAM but it cannot allocate any more (maximum allocation limit being 1MB).
> maxmem doesn't influence the current memory of the VM, only future
> allocations.

It causes stall in the host, perhaps due to an interger overflow (I have
not analyzed it yet).

Olaf