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[Xen-devel] Re: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") f

Dan Magenheimer wrote:
CMM2's focus is on increasing the number of VM's that
can run on top of the hypervisor.  To do this, it
depends on hints provided by Linux to surreptitiously
steal memory away from Linux.  The stolen memory still
"belongs" to Linux and if Linux goes to use it but the
hypervisor has already given it to another Linux, the
hypervisor must jump through hoops to give it back.

It depends on how you define "jump through hoops".

If it guesses wrong and overcommits too aggressively,
the hypervisor must swap some memory to a "hypervisor
swap disk" (which btw has some policy challenges).
IMHO this is more of a "mainframe" model.

No, not at all. A guest marks a page as being "volatile", which tells the hypervisor it never needs to swap that page. It can discard it whenever it likes.

If the guest later tries to access that page, it will get a special "discard fault". For a lot of types of memory, the discard fault handler can then restore that page transparently to the code that generated the discard fault.

AFAICT, ephemeral tmem has the exact same characteristics as volatile CMM2 pages. The difference is that tmem introduces an API to explicitly manage this memory behind a copy interface whereas CMM2 uses hinting and a special fault handler to allow any piece of memory to be marked in this way.

In other words, CMM2, despite its name, is more of a
"subservient" memory management system (Linux is
subservient to the hypervisor) and tmem is more
collaborative (Linux and the hypervisor share the
responsibilities and the benefits/costs).

I don't really agree with your analysis of CMM2. We can map CMM2 operations directly to ephemeral tmem interfaces so tmem is a subset of CMM2, no?

What's appealing to me about CMM2 is that it doesn't change the guest semantically but rather just gives the VMM more information about how the VMM is using it's memory. This suggests that it allows greater flexibility in the long term to the VMM and more importantly, provides an easier implementation across a wide range of guests.


Anthony Liguori

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