Re: Pointed questions re Xen memory overcommit

[Olaf Hering <olaf@aepfle.de>]

On Fri, Feb 24, Dan Magenheimer wrote:

> With the Xen memory overcommit (Satori and xenpaging) work getting
> closer to the real world (and indeed one gating factor for a major
> Xen release), I wonder if it is time to ask some very pointed
> questions:

A few comments from me below. I just learned about Satori now, its not
clear to me how it is related to memory overcommit.

To me memory overcommit means swapping, which is what xenpaging does:
turn the whole guest gfn range into some sort of virtual memory,
transparent to the guest.

> 1) How is the capability and implementation similar or different
> from VMware's?  And specifically I'm asking for hard information
> relating to:
> 
> http://lwn.net/Articles/309155/ 
> http://lwn.net/Articles/330589/ 

KSM looks more like page sharing which also Xen can do to some degree.
I'm not familiar with the sharing code in Xen or Linux Kernel.

> 2) Assuming no legal issues, how is Xen memory overcommit different
> or better than VMware's, which is known to have lots of issues
> in the real world, such that few customers (outside of a handful
> of domains such as VDI) enable it?  Or is this effort largely to
> remove an item from the VMware sales team's differentiation list?
> And a comparison vs Hyper-V and KVM would be interesting also.

I have no idea what VMware provides to fill the memory overcommit
checkbox.

The Win8 preview I tested recently offers some sort of memory handling
for guests. So far I have not looked into that feature.

Since KVM guests are ordinary processes AFAIK they are most likely
victims of the Linux kernel process swapping (unless KVM mlocks the
gfns?). So KVM gets memory overcommit in the way xenpaging does it for
free.

> 3) Is there new evidence that a host-based-policy-driven memory
> balancer works sufficiently well on one system, or for
> multiple hosts, or for a data center?  It would be nice for
> all Xen developers/vendors to understand the intended customer
> (e.g. is it the desktop user running a handful of VMs running
> known workloads?)

xenpaging is the red emergency knob to free some host memory without
caring about the actual memory constraints within the paged guests.

Olaf