[Xen-devel] Intended functioning of Xenpaging

[Sahil Suneja <sahilsuneja@xxxxxxxxx>]

Hi,

This query is with regards to how xenpaging is envisioned to work in future.

In the current state (Xen 4.2..1_04-1.1.2 / Dom0 OpenSUSE linux
3.7.9-1.4-xen), I am able to run xenpaging pretty nicely following the
steps mentioned in xen/docs/misc/xenpaging.txt.

Now, from what I understand, once the 'target-tot_pages' value is set
for a guest, xenpaging tires to keep the guest's memory at that value
throughout, even when the guest memory demands increase. That is, to
page in the guest's swapped out pages, other pages of the same guest
have to paged out, keeping the overall guest memory usage at around
the 'target-tot_pages'  value. And even if there happens to be some
free memory available at the host and the guest is under some memory
pressure, no extra memory gets allocated to the guest even when
max_pages > tot_pages for the guest.

Is my understanding correct? Atleast that's what I understood from the
code and tested with my experiments- essentially different versions of
allocating memory inside the (hvm) guest upto its original configured
limit.

But is this really how xenpaging is supposed to work? Shouldn't the
memory reclaimed from a guest be returned to it when free memory
becomes available at the host? Or maybe a xenpaging reclaimed memory
pool be maintained from which memory can be allocated to the
participating guests depending upon certain policies?

When I map the guest's memory in Dom0, however, I do see all of the
guest's pages being brought into RAM together at once.

Thanks,
Sahil Suneja
Ph.D. Student
University of Toronto

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