Re: [Qemu-devel] [RFC][PATCH 8/9] migration: do not sent zero_pages in bulk stage

[Eric Blake <eblake@redhat.com>]

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On 03/12/2013 09:54 AM, Peter Lieven wrote:
> if the dup_page is a zero_page do not send it at all.
> the memory at the destination reads as zero anyway.

It might help to comment that this is only true for the bulk migration
stage.  If the guest is actively zeroing pages, then later stages might
encounter an all-zero page that was not all zeros in the bulk migration,
so we must transmit the page in that case.

> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  arch_init.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 04c82e4..abd54a7 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -447,9 +447,11 @@ static int ram_save_block(QEMUFile *f, bool
> last_stage)
>              bytes_sent = -1;
>              if (ram_bulk_stage && is_dup_page(p)) {
>                  acct_info.dup_pages++;
> -                bytes_sent = save_block_hdr(f, block, offset, cont,
> +                if (p[0]) {

Your code has it right, because of the ram_bulk_stage check guarding
this if body, but I did a double take when reading just your commit message.

> +                    bytes_sent = save_block_hdr(f, block, offset, cont,
>                                              RAM_SAVE_FLAG_COMPRESS);
> -                qemu_put_byte(f, *p);
> +                    qemu_put_byte(f, *p);
> +                }
>                  bytes_sent += 1;
>              } else if (!ram_bulk_stage && migrate_use_xbzrle()) {
>                  current_addr = block->offset + offset;

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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