Re: [PATCH v8 1/2] hypervisor: XENMEM_claim_pages (subop of existing) hypercall

[David Vrabel <david.vrabel@citrix.com>]

On 28/11/12 15:50, Dan Magenheimer wrote:
> This is patch 1of2 of an eighth cut of the patch of the proposed
> XENMEM_claim_pages hypercall/subop, taking into account review
> feedback from Jan and Keir and IanC and Matthew Daley, plus some
> fixes found via runtime debugging (using printk and privcmd only).
> 
[...]
>
> Proposed:
> - call claim for mem=N amount of memory
> - if claim succeeds:
>     call populate_physmap repeatedly to achieve mem=N memory (failsafe)
>   else
>     report -ENOMEM up the stack
> - claim is held until mem=N is achieved or the domain dies or
>    the toolstack changes it to 0
> - memory is held until domain dies or the toolstack decreases it

There is no mechanism for per-NUMA node claim.  Isn't this needed?

More fundamentally, doesn't this approach result in a worse user
experience?  It's guaranteeing that a new VM can be started but at the
expense of existing VMs on that node.

When making a VM placement decision, the toolstack needs to consider the
future memory requirements of the new and existing VMs on the host and
not just the current (or more correctly, the recently) memory.

It seems more useful to me to have the toolstack (for example) to track
historical memory usage of a VM to allow it to make better predictions
about memory usage.  With a better prediction, the number of failed VM
creates due to memory shortage will be minimized.  Then, combined with
reducing the cost of a VM create by optimizing the allocator, the cost
of occasionally failing a create will be minimal.

For example, Sally starts her CAD application at 9am, tripling her
desktop VM instances memory usage.  If at 0858, the toolstack claimed a
most of the remaining memory for a new VM, then Sally's VM is going to
grind to a halt as it swaps to death.

If the toolstack could predict that that desktop instances memory usage
was about to spike (because it had historical data showing his), it
could have selected a different host and Sally's VM would perform as
expected.

David