What Causes TCP Send Queue to Grow ("netstat" -an output)?

Discussion in 'Linux Networking' started by John Davis, Aug 13, 2003.

  1. John Davis

    John Davis Guest


    I keep seeing the Send Queue Column growing from "netstat -an", can
    someone tell me what causes this to happen? I believe in RH Linux that
    the buffer is at 128K, but I'm not sure. We have been seeing some
    problems lately with performance, but am not sure how to pinpoint

    The column with the 53576 number seems very problematic...

    kernel - 2.4.20 on RH Linux 7.3


    Active Internet connections (servers and established)
    tcp 0 53576

    John Davis, Aug 13, 2003
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  2. What we have hear is a failure to communicate. It means that either is not accepting data right now, or there is some problem
    between the two (your nic, the other nic, or network/routing between
    them). Recv-Q and Send-Q should normally be near zero.

    If these are dummy IPs and actually connects over the internet, it is not
    uncommon to have occasional router interruptions or looping (traceroute
    the other IP when that happens to see if that gives a clue).
    David Efflandt, Aug 14, 2003
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  3. The buffer may be 128k, but linux assumes half the buffer is used for
    kernal structures so you only have 64k of space.

    Terry Sanders, Aug 14, 2003
  4. John Davis

    John Davis Guest

    How would I go about increasing the buffer size then? Does that mean
    it's 128K buffer in total for sending/receiving? I may need to
    increase these as we have been having some issues in the office.
    John Davis, Aug 14, 2003
  5. You can use setsockopt() to change the size of the send/receive buffers
    within an application and sysctl to change default system settings for
    all applications. See the man page socket(7) which gives details
    on what values to change.
    Terry Sanders, Aug 14, 2003
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