[Xen-devel] Re: RFC: Doing a superpage zero-sweep on decrease_reservation

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Wed, 2011-03-09 at 14:23 +0000, Tim Deegan wrote:
> Do you think ballooned-out pages are likely to be in 2MB chunks of
> zeroed memory?

Sorry, didn't answer this.  I think that for a large guest, the initial
balloon is very likely that for the first mfn freed in a superpage, that
mfn will turn out to have been in a 2MB chunks of zeroed memory, all of
which will eventually passed back to Xen anyway.  (e.g., imagine a VM
with 32GiB static_max but a 16GiB target -- of the 16GiB of individual
4k pages passed back by the balloon driver, the vast majority of them
will be in a page which (1) is zeroed already and (2) will end up being
passed back entirely anyway.)

One could imagine the PV drivers allocating the full balloon size, then
going through and sorting the pages by pfn, consolidating the
superpages, and passing them back as one big chunk -- but that would be
a big waste of time compared to just having Xen optimistically reclaim
the whole superpage.

 -George



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