cvs commit: src/sys/dev/bge if_bge.c if_bgereg.h

[Bruce Evans]

bde         2007-02-15 10:33:49 UTC

  FreeBSD src repository

  Modified files:        (Branch: RELENG_4)
    sys/dev/bge          if_bge.c if_bgereg.h 
  Log:
  MFC (if_bge.c 1.108, etc., less some style bugs: eliminate one PCI
  read per call to bge_start()).
  
  In packet blasting tests using ttcp with tiny udp packets on an A64-3200
  with a 64-bit 5701 on a 32-bit 33MHz PCI bus, this gives a speedup
  from 347 kpps to 623 kpps.  sendto() has a lot of software overheads,
  but even with these the single PCI write per call to bge_start() almost
  doubled the per-packet time.  This is partly because the software
  overheads are so large that the CPU can't keep up with a Gbps NIC that
  can actually get anywhere near Gbps speed for tiny packets (347 kppps
  for tiny packets is only about 21% of wire speed).  When the CPU can't
  keep up, it gets further behind because it ends up calling bge_start()
  at least once for every packet, so any overheads in bge_start() are
  not amortized across multiple packets.  Thus the PCI read had an
  especially high overhead.
  
  For larger packets, the speedup is closer to the 1.8% claimed in rev.
  1.108.
  
  Rev.1.108 claims to eliminate a PCI write but actually eliminates a
  PCI read.  The write of the tx product index is not so costly as its
  read, and cannot be eliminated completely.  It could be coalesced in
  some cases.
  
  Revision  Changes    Path
  1.3.2.42  +8 -4      src/sys/dev/bge/if_bge.c
  1.1.2.22  +1 -0      src/sys/dev/bge/if_bgereg.h