Re: [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

On 12/07/2016 07:42 AM, David Hildenbrand wrote:

Am 07.12.2016 um 14:35 schrieb Li, Liang Z:

quoted

quoted

Am 30.11.2016 um 09:43 schrieb Liang Li:

quoted

This patch set contains two parts of changes to the virtio-balloon.

One is the change for speeding up the inflating & deflating process,
the main idea of this optimization is to use bitmap to send the page
information to host instead of the PFNs, to reduce the overhead of
virtio data transmission, address translation and madvise(). This can
help to improve the performance by about 85%.

Do you have some statistics/some rough feeling how many consecutive
bits are
usually set in the bitmaps? Is it really just purely random or is
there some
granularity that is usually consecutive?

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.

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.