Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER

From: Wei Wang
Date: Fri Mar 10 2017 - 06:36:16 EST

On 03/09/2017 10:14 PM, Matthew Wilcox wrote:
On Fri, Mar 03, 2017 at 01:40:28PM +0800, Wei Wang wrote:
From: Liang Li <>
1) allocating pages (6.5%)
2) sending PFNs to host (68.3%)
3) address translation (6.1%)
4) madvise (19%)

This patch optimizes step 2) by transfering pages to the host in
chunks. A chunk consists of guest physically continuous pages, and
it is offered to the host via a base PFN (i.e. the start PFN of
those physically continuous pages) and the size (i.e. the total
number of the pages). A normal chunk is formated as below:
| Base (52 bit) | Size (12 bit)|
For large size chunks, an extended chunk format is used:
| Base (64 bit) |
| Size (64 bit) |
What's the advantage to extended chunks? IOW, why is the added complexity
of having two chunk formats worth it? You already reduced the overhead by
a factor of 4096 with normal chunks ... how often are extended chunks used
and how much more efficient are they than having several normal chunks?

Right, chunk_ext may be rarely used, thanks. I will remove chunk_ext if there is no objection from others.