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

[Wei Wang <wei.w.wang@intel.com>]

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 <liang.z.li@intel.com>
> > 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.

Best,
Wei