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    • jamal's avatar
      net_sched: fix dequeuer fairness · d5b8aa1d
      jamal authored
      Results on dummy device can be seen in my netconf 2011
      slides. These results are for a 10Gige IXGBE intel
      nic - on another i5 machine, very similar specs to
      the one used in the netconf2011 results.
      It turns out - this is a hell lot worse than dummy
      and so this patch is even more beneficial for 10G.
      
      Test setup:
      ----------
      
      System under test sending packets out.
      Additional box connected directly dropping packets.
      Installed prio qdisc on the eth device and default
      netdev default length of 1000 used as is.
      The 3 prio bands each were set to 100 (didnt factor in
      the results).
      
      5 packet runs were made and the middle 3 picked.
      
      results
      -------
      
      The "cpu" column indicates the which cpu the sample
      was taken on,
      The "Pkt runx" carries the number of packets a cpu
      dequeued when forced to be in the "dequeuer" role.
      The "avg" for each run is the number of times each
      cpu should be a "dequeuer" if the system was fair.
      
      3.0-rc4      (plain)
      cpu         Pkt run1        Pkt run2        Pkt run3
      ================================================
      cpu0        21853354        21598183        22199900
      cpu1          431058          473476          393159
      cpu2          481975          477529          458466
      cpu3        23261406        23412299        22894315
      avg         11506948        11490372        11486460
      
      3.0-rc4 with patch and default weight 64
      cpu 	     Pkt run1        Pkt run2        Pkt run3
      ================================================
      cpu0        13205312        13109359        13132333
      cpu1        10189914        10159127        10122270
      cpu2        10213871        10124367        10168722
      cpu3        13165760        13164767        13096705
      avg         11693714        11639405        11630008
      
      As you can see the system is still not perfect but
      is a lot better than what it was before...
      
      At the moment we use the old backlog weight, weight_p
      which is 64 packets. It seems to be reasonably fine
      with that value.
      The system could be made more fair if we reduce the
      weight_p (as per my presentation), but we are going
      to affect the shared backlog weight. Unless deemed
      necessary, I think the default value is fine. If not
      we could add yet another knob.
      Signed-off-by: default avatarJamal Hadi Salim <jhs@mojatatu.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      d5b8aa1d