cvs commit: src/sys/dev/em if_em.c if_em.h

[Scott Long]

scottl      2006-01-11 00:30:25 UTC

  FreeBSD src repository

  Modified files:
    sys/dev/em           if_em.c if_em.h 
  Log:
  Significant performance improvements for the if_em driver:
  
  - Only update the rx ring consumer pointer after running through the rx loop,
    not with each iteration through the loop.
  - If possible, use a fast interupt handler instead of an ithread handler.  Use
    the interrupt handler to check and squelch the interrupt, then schedule a
    taskqueue to do the actual work.  This has three benefits:
    - Eliminates the 'interrupt aliasing' problem found in many chipsets by
      allowing the driver to mask the interrupt in the NIC instead of the
      OS masking the interrupt in the APIC.
    - Allows the driver to control the amount of work done in the interrupt
      handler.  This results in what I call 'adaptive polling', where you get
      the latency benefits of a quick response to interrupts with the
      interrupt mitigation and work partitioning of polling.  Polling is still
      an option in the driver, but I consider it orthogonal to this work.
    - Don't hold the driver lock in the RX handler.  The handler and all data
      associated is effectively serialized already.  This eliminates the cost of
      dropping and reaquiring the lock for every receieved packet.  The result
      is much lower contention for the driver lock, resulting in lower CPU usage
      and lower latency for interactive workloads.
  
  The amount of work done in the taskqueue is controlled by the sysctl
  dev.em.N.rx_processing_limit
  
  and tunable
  hw.em.rx_process_limit
  
  Setting these to -1 effectively removes the limit.
  
  The fast interrupt and taskqueue can be disabled by defining NO_EM_FASTINTR.
  This work has been shown to increase fast-forwarding from ~570 kpps to
  ~750 kpps (note that the same NIC hardware seems unable to transmit more than
  800 kpps, so this increase appears to be limited almost solely by the
  hardware).  Gains have been shown in other workloads, ranging from better
  performance to elimination of over-saturation livelocks.
  
  Thanks to Andre Opperman for his time and resources from his network
  performance project in performing much of the testing.  Thanks to Gleb
  Smirnoff and Danny Braniss for their help in testing also.
  
  Revision  Changes    Path
  1.98      +193 -18   src/sys/dev/em/if_em.c
  1.41      +10 -1     src/sys/dev/em/if_em.h