Re: [Qemu-devel] [PATCH v1] migration: skip sending ram pages released by virtio-balloon driver.

[Jitendra Kolhe]

On 3/10/2016 3:19 PM, Roman Kagan wrote:
> On Fri, Mar 04, 2016 at 02:32:47PM +0530, Jitendra Kolhe wrote:
> > Even though the pages which are returned to the host by virtio-balloon
> > driver are zero pages, the migration algorithm will still end up
> > scanning the entire page ram_find_and_save_block() -> ram_save_page/
> > ram_save_compressed_page -> save_zero_page() -> is_zero_range().  We
> > also end-up sending some control information over network for these
> > page during migration. This adds to total migration time.
>
> I wonder if it is the scanning for zeros or sending the whiteout which
> affects the total migration time more.  If it is the former (as I would
> expect) then a rather local change to is_zero_range() to make use of the
> mapping information before scanning would get you all the speedups
> without protocol changes, interfering with postcopy etc.
>
> Roman.

Localizing the solution to zero page scan check is a good idea. I too
agree that most of the time is send in scanning for zero page in which
case we should be able to localize solution to is_zero_range().
However in case of ballooned out pages (which can be seen as a subset
of guest zero pages) we also spend a very small portion of total
migration time in sending the control information, which can be also
avoided.
From my tests for 16GB idle guest of which 12GB was ballooned out, the
zero page scan time for 12GB ballooned out pages was ~1789 ms and
save_page_header + qemu_put_byte(f, 0); for same 12GB ballooned out
pages was ~556 ms. Total migration time was ~8000 ms
    if (is_zero_range(p, TARGET_PAGE_SIZE)) {
        acct_info.dup_pages++;
        *bytes_transferred += save_page_header(f, block,
                                               offset |
RAM_SAVE_FLAG_COMPRESS);
        qemu_put_byte(f, 0);
        *bytes_transferred += 1;
        pages = 1;
    }
Would moving the solution to save_zero_page() be good enough?

Thanks,
- Jitendra