em interface slow down on 8.0R

[Hiroki Sato]

Hi,

 I noticed that network connection of one of my boxes got
 significantly slow just after upgrading it to 8.0R.  The box has an
 em0 (82547EI) and worked fine with 7.2R.

 The symptoms are:

 - A ping to a host on the same LAN takes 990ms RTT, it reduces
   gradually to around 1ms, and then it returns to around 1s.  The
   rate was about 2ms/ping.

 - The response is quite slow, but no packet loss and network services
   on the box seem to work fine as far as I can check.  There does not
   seem interrupt storm according to "vmstat -i".  No error message
   such as "watchdog timeout" appears.

 Any ideas to narrow down the cause?  It maybe a linkup problem with a
 specific model of hub like full-duplex/half-duplex mismatch, but the
 link is "1000baseT <full-duplex>" and setting it manually did not
 solve it.  I think it is certain that upgrading to 8.0R triggered it,
 at least.

 Another box with an em interface works fine after upgrading to 8.0R.
 It has a different chip (82573E).

 Details of the em interface and vmstat -i are the following:

 em0 at pci0:1:1:0: class=0x020000 card=0x302c8086 chip=0x10198086 rev=0x00 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Gigabit Ethernet Controller (LOM) (82547EI)'
    class      = network
    subclass   = ethernet

 Adapter hardware address = 0xc42e1424
 em0: CTRL = 0x183c0241 RCTL = 0x8002
 em0: Packet buffer = Tx=10k Rx=30k
 em0: Flow control watermarks high = 28672 low = 27172
 em0: tx_int_delay = 66, tx_abs_int_delay = 66
 em0: rx_int_delay = 0, rx_abs_int_delay = 66
 em0: fifo workaround = 0, fifo_reset_count = 0
 em0: hw tdh = 49, hw tdt = 49
 em0: hw rdh = 238, hw rdt = 187
 em0: Num Tx descriptors avail = 250
 em0: Tx Descriptors not avail1 = 0
 em0: Tx Descriptors not avail2 = 0
 em0: Std mbuf failed = 0
 em0: Std mbuf cluster failed = 0
 em0: Driver dropped packets = 0
 em0: Driver tx dma failure in encap = 0

 dev.em.0.%desc: Intel(R) PRO/1000 Network Connection 6.9.14
 dev.em.0.%driver: em
 dev.em.0.%location: slot=1 function=0 handle=\_SB_.PCI0.P0P2.TANA
 dev.em.0.%pnpinfo: vendor=0x8086 device=0x1019 subvendor=0x8086 subdevice=0x302c class=0x020000
 dev.em.0.%parent: pci1
 dev.em.0.debug: -1
 dev.em.0.stats: -1
 dev.em.0.rx_int_delay: 0
 dev.em.0.tx_int_delay: 66
 dev.em.0.rx_abs_int_delay: 66
 dev.em.0.tx_abs_int_delay: 66
 dev.em.0.rx_processing_limit: 100
 dev.em.0.wake: 0

 % vmstat -i
 interrupt                          total       rate
 irq4: uart0                         3585          3
 irq14: ata0                         1811          1
 irq15: ata1                          112          0
 irq16: uhci0 uhci3                    15          0
 irq18: em0 uhci2+                  92457         99
 irq19: uhci1                           1          0
 irq23: ehci0                           2          0
 cpu0: timer                      1849981       1997
 cpu1: timer                      1849961       1997
 Total                            3797925       4101

-- Hiroki
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