RELENG_8 em(4) -- input errors ("Missed Packets")

[Jeremy Chadwick]

Something I came across today on a RELENG_8 (8.1-PRERELEASE, amd64,
built Jun 8th) system we have:

$ netstat -i -n -d -I em1
Name    Mtu Network       Address              Ipkts Ierrs Idrop    Opkts Oerrs  Coll Drop
em1    1500 <Link#2>      xx:xx:xx:xx:xx:xx 16117371    17     0 11011087     0     0    0
em1    1500 xxxxxxxxx/xx  xxxxxxxxxxx       16108452     -     - 11024972     -     -    -

The input errors are what concerned me.  I poked dev.em.1.stats and got
the following:

em1: Excessive collisions = 0
em1: Sequence errors = 0
em1: Defer count = 0
em1: Missed Packets = 17
em1: Receive No Buffers = 0
em1: Receive Length Errors = 0
em1: Receive errors = 0
em1: Crc errors = 0
em1: Alignment errors = 0
em1: Collision/Carrier extension errors = 0
em1: watchdog timeouts = 0
em1: XON Rcvd = 0
em1: XON Xmtd = 0
em1: XOFF Rcvd = 0
em1: XOFF Xmtd = 0
em1: Good Packets Rcvd = 16117349
em1: Good Packets Xmtd = 11046837
em1: TSO Contexts Xmtd = 17609
em1: TSO Contexts Failed = 0

What exactly does "Missed Packets" mean here?  How is a packet "missed"?
Said port on our switch doesn't any sign of problems:

hp2510g# show interfaces 2

 Status and Counters - Port Counters for port 2

  Name  : port2
  Link Status     : Up
  Totals (Since boot or last clear) :
   Bytes Rx        : 3,548,949,136      Bytes Tx        : 2,613,086,119
   Unicast Rx      : 182,088,023        Unicast Tx      : 255,981,685
   Bcast/Mcast Rx  : 14,674             Bcast/Mcast Tx  : 81,852
  Errors (Since boot or last clear) :
   FCS Rx          : 0                  Drops Rx        : 0
   Alignment Rx    : 0                  Collisions Tx   : 0
   Runts Rx        : 0                  Late Colln Tx   : 0
   Giants Rx       : 0                  Excessive Colln : 0
   Total Rx Errors : 0                  Deferred Tx     : 0
  Rates (5 minute weighted average) :
   Total Rx  (bps) : 1457984            Total Tx  (bps) : 1494568
   Unicast Rx (Pkts/sec) : 0            Unicast Tx (Pkts/sec) : 0
   B/Mcast Rx (Pkts/sec) : 0            B/Mcast Tx (Pkts/sec) : 0
   Utilization Rx  : 00.04 %            Utilization Tx  : 00.04 %

Relevant userland and kernel stuff:

$ ifconfig em1
em1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=219b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,WOL_MAGIC>
        ether xx:xx:xx:xx:xx:xx
        inet xxxxxxxxxxx netmask xxxxxxxxxx broadcast xxxxxxxxxxx
        media: Ethernet autoselect (1000baseT <full-duplex>)
        status: active

$ netstat -m
2162/1678/3840 mbufs in use (current/cache/total)
2048/1030/3078/25600 mbuf clusters in use (current/cache/total/max)
2048/896 mbuf+clusters out of packet secondary zone in use (current/cache)
0/104/104/12800 4k (page size) jumbo clusters in use (current/cache/total/max)
0/0/0/6400 9k jumbo clusters in use (current/cache/total/max)
0/0/0/3200 16k jumbo clusters in use (current/cache/total/max)
4636K/2895K/7532K bytes allocated to network (current/cache/total)
0/0/0 requests for mbufs denied (mbufs/clusters/mbuf+clusters)
0/0/0 requests for jumbo clusters denied (4k/9k/16k)
0/0/0 sfbufs in use (current/peak/max)
0 requests for sfbufs denied
0 requests for sfbufs delayed
0 requests for I/O initiated by sendfile
0 calls to protocol drain routines

$ vmstat -i
interrupt                          total       rate
irq4: uart0                          868          0
irq6: fdc0                             1          0
irq23: uhci3 ehci1+                  332          0
cpu0: timer                   1818467717       2000
irq256: em0                       375532          0
irq257: em1                      5004095          5
irq258: ahci0                    5879898          6
cpu1: timer                   1818466783       2000
cpu3: timer                   1818466831       2000
cpu2: timer                   1818466828       2000
Total                         7285128885       8012

$ dmesg | grep em1
em1: <Intel(R) PRO/1000 Network Connection 7.0.5> port 0x3000-0x301f mem 0xd0300000-0xd031ffff irq 17 at device 0.0 on pci15
em1: Using MSI interrupt
em1: [FILTER]

$ pciconf -lvc 
em1 at pci0:15:0:0:        class=0x020000 card=0x109a15d9 chip=0x109a8086 rev=0x00 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Intel PRO/1000 PL Network Adaptor (82573L)'
    class      = network
    subclass   = ethernet
    cap 01[c8] = powerspec 2  supports D0 D3  current D0
    cap 05[d0] = MSI supports 1 message, 64 bit enabled with 1 message
    cap 10[e0] = PCI-Express 1 endpoint max data 128(256) link x1(x1)

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |