Re: [Qemu-devel] [PATCHv4 9/9] migration: use XBZRLE only after bulk stage

[Orit Wasserman <owasserm@redhat.com>]

On 03/22/2013 02:46 PM, Peter Lieven wrote:
> at the beginning of migration all pages are marked dirty and
> in the first round a bulk migration of all pages is performed.
> 
> currently all these pages are copied to the page cache regardless
> of whether they are frequently updated or not. this doesn't make sense
> since most of these pages are never transferred again.
> 
> this patch changes the XBZRLE transfer to only be used after
> the bulk stage has been completed. that means a page is added
> to the page cache the second time it is transferred and XBZRLE
> can benefit from the third time of transfer.
> 
> since the page cache is likely smaller than the number of pages
> it's also likely that in the second round the page is missing in the
> cache due to collisions in the bulk phase.
> 
> on the other hand a lot of unnecessary mallocs, memdups and frees
> are saved.
> 
> the following results have been taken earlier while executing
> the test program from docs/xbzrle.txt. (+) with the patch and (-)
> without. (thanks to Eric Blake for reformatting and comments)
> 
> + total time: 22185 milliseconds
> - total time: 22410 milliseconds
> 
> Shaved 0.3 seconds, better than 1%!
> 
> + downtime: 29 milliseconds
> - downtime: 21 milliseconds
> 
> Not sure why downtime seemed worse, but probably not the end of the world.
> 
> + transferred ram: 706034 kbytes
> - transferred ram: 721318 kbytes
> 
> Fewer bytes sent - good.
> 
> + remaining ram: 0 kbytes
> - remaining ram: 0 kbytes
> + total ram: 1057216 kbytes
> - total ram: 1057216 kbytes
> + duplicate: 108556 pages
> - duplicate: 105553 pages
> + normal: 175146 pages
> - normal: 179589 pages
> + normal bytes: 700584 kbytes
> - normal bytes: 718356 kbytes
> 
> Fewer normal bytes...
> 
> + cache size: 67108864 bytes
> - cache size: 67108864 bytes
> + xbzrle transferred: 3127 kbytes
> - xbzrle transferred: 630 kbytes
> 
> ...and more compressed pages sent - good.
> 
> + xbzrle pages: 117811 pages
> - xbzrle pages: 21527 pages
> + xbzrle cache miss: 18750
> - xbzrle cache miss: 179589
> 
> And very good improvement on the cache miss rate.
> 
> + xbzrle overflow : 0
> - xbzrle overflow : 0
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> ---
>  arch_init.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index b2b932a..86f7e28 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -464,7 +464,7 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>                      acct_info.skipped_pages++;
>                      bytes_sent = 0;
>                  }
> -            } else if (migrate_use_xbzrle()) {
> +            } else if (!ram_bulk_stage && migrate_use_xbzrle()) {
>                  current_addr = block->offset + offset;
>                  bytes_sent = save_xbzrle_page(f, p, current_addr, block,
>                                                offset, cont, last_stage);
> 
Reviewed-by: Orit Wasserman <owasserm@redhat.com>