git: ifq/staging: Initial implementation of IFQ packet staging mechanism

[Sepherosa Ziehau <sephe@xxxxxxxxxxxxxxxxxxxxxxx>]

commit 28cc0c295c957f68a6e8afbac62d0e50d56ccb25
Author: Sepherosa Ziehau <sephe@dragonflybsd.org>
Date:   Sun Dec 23 20:31:32 2012 +0800

    ifq/staging: Initial implementation of IFQ packet staging mechanism
    
    The packets enqueued into IFQ are staged to a certain amount before the
    ifnet's if_start is called.  In this way, the driver could avoid writing
    to hardware registers upon every packet, instead, hardware registers
    could be written when certain amount of packets are put onto hardware
    TX ring.  The measurement on several modern NICs (emx(4), igb(4), bnx(4),
    bge(4), jme(4)) shows that the hardware registers writing aggregation
    could save ~20% CPU time when 18bytes UDP datagrams are transmitted at
    1.48Mpps.
    
    IFQ packets staging is performed for direct ifnet's if_start calling,
    i.e. ifq_try_ifstart()
    
    IFQ packets staging will be stopped upon any of the following conditions:
    - If the count of packets enqueued on the current CPU is great than or
      equal to ifq_stage_cntmax.
    - If the total length of packets enqueued on the current CPU is great
      than or equal to the hardware's MTU - max_protohdr.  max_protohdr is
      cut from the hardware's MTU mainly bacause a full TCP segment's size
      is usually less than hardware's MTU.
    - if_start interlock (if_snd.altq_started) is not released.
    - The if_start_rollup(), which is registered as low priority netisr
      rollup function, is called; probably because no more work is pending
      for netisr.
    
    Currently IFQ packet staging is only performed in netisr threads.
    
    Inspired-by: Luigi Rizzo's netmap paper
        (http://info.iet.unipi.it/~luigi/netmap/)
    Also-Suggested-by: dillon@

Summary of changes:
 sys/net/altq/if_altq.h |   13 +++
 sys/net/if.c           |  195 +++++++++++++++++++++++++++++++++++++----------
 2 files changed, 166 insertions(+), 42 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/28cc0c295c957f68a6e8afbac62d0e50d56ccb25


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