I did something similar. Filled the balloon with 15GB for a 16GB idle
guest, by
using bitmap, the madvise count was reduced to 605. when using the
PFNs, the madvise count
was 3932160. It means there are quite a lot consecutive bits in the
bitmap.
I didn't test for a guest with heavy memory workload.
Would it then even make sense to go one step further and report {pfn,
length} combinations?
So simply send over an array of {pfn, length}?
Li's current patches do that. Well, maybe not pfn/length, but they do
take a pfn and page-order, which fits perfectly with the kernel's
concept of high-order pages.
So we can send length in powers of two. Still, I don't see any benefit
over a simple pfn/len schema. But I'll have a more detailed look at the
implementation first, maybe that will enlighten me :)
And it makes sense if you think about:
a) hugetlb backing: The host may only be able to free huge pages (we
might want to communicate that to the guest later, that's another
story). Still we would have to send bitmaps full of 4k frames (512 bits
for 2mb frames). Of course, we could add a way to communicate that we
are using a different bitmap-granularity.
Yeah, please read the patches. If they're not clear, then the
descriptions need work, but this is done already.
I missed the page_shift, thanks for the hint.
--
David
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