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    SubjectRe: 2.6.17-rc6-rt1
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    Date
    On Fri, 2006-06-09 at 14:20 +0200, Mike Galbraith wrote:
    > >
    > > I'm starting to believe that it's network related. Pinging my box from
    > > a remote host gives a ~.3 ms round trip whereas pinging localhost gives
    > > ~500ms. Something real weird is going on here.
    >
    > Wow. I don't see anything near 500ms, but I am dropping packets, and
    > recvmsg, when it isn't saying -EAGAIN, is indeed taking way too long.
    >
    > 14:04:02.062550 sendmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, msg_iov(1)=[{"\10\0$@\217\36\0\2\262c\211D\35\364\0\0\10\t\n\v\f\r\16"..., 64}], msg_controllen=0, msg_flags=0}, 0) = 64 <0.000048>
    > 14:04:02.062731 setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={10, 0}}, NULL) = 0 <0.000016>
    > 14:04:02.062826 recvmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(19889), sin_addr=inet_addr("127.0.0.1")}, msg_iov(1)=[{"E\0\0T\177]\0\0@\1\375I\177\0\0\1\177\0\0\1\0\0,@\217\36"..., 192}], msg_controllen=20, {cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */, ...}, msg_flags=0}, 0) = 84 <0.037660>
    >

    Even weirder:

    $ping localhost
    PING localhost.localdomain (127.0.0.1) 56(84) bytes of data.
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=64 time=0.157 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=2 ttl=64 time=107 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=3 ttl=64 time=94.7 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=4 ttl=64 time=741 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=5 ttl=64 time=179 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=6 ttl=64 time=732 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=7 ttl=64 time=728 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=8 ttl=64 time=725 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=9 ttl=64 time=178 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=10 ttl=64 time=96.0 ms
    64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=11 ttl=64 time=342 ms
    ...

    $strace -tt -T ping localhost
    14:48:07.764888 sendmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0),
    sin_addr=inet_addr("127.0.0.1")}, msg_iov(1)=[{"\10\0:\221\341\v\0\3\7n\211DU\25
    2\v\0\10\t\n\v\f\r\16\17"..., 64}], msg_controllen=0, msg_flags=0}, MSG_CONFIRM)
    = 64 <0.000078>
    14:48:07.765930 recvmsg(3, {msg_name(16)={sa_family=AF_INET, sin_port=htons(0),
    sin_addr=inet_addr("127.0.0.1")}, msg_iov(1)=[{"E\0\0T\\\241\0\0@\1 \6\177\0\0\1
    \177\0\0\1\0\0B\221\341"..., 192}], msg_controllen=20, {cmsg_len=20, cmsg_level=
    SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */, ...}, msg_flags=0}, 0) = 84 <0.000402>
    14:48:07.767598 write(1, "64 bytes from localhost.localdom"..., 8064 bytes from
    localhost.localdomain (127.0.0.1): icmp_seq=3 ttl=64 time=1.86 ms
    ) = 80 <0.000307>

    All the round trips under strace are in the 2ms range.

    How on earth can it be that stracing the ping gives better
    performance???

    Sébastien.







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