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  • Peter Lieven's avatar
    migration: use XBZRLE only after bulk stage · 5cc11c46
    Peter Lieven authored
    
    
    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: default avatarPeter Lieven <pl@kamp.de>
    Reviewed-by: default avatarEric Blake <eblake@redhat.com>
    Reviewed-by: default avatarOrit Wasserman <owasserm@redhat.com>
    Signed-off-by: default avatarJuan Quintela <quintela@redhat.com>
    5cc11c46