TCP congestion stats + performance tuning parameters

Discussion in 'Linux Networking' started by vishal..., Dec 9, 2005.

  1. vishal...

    vishal... Guest


    I am doing some performance thruput measurements of our TCP stack by
    using Linux 2.6.11 as a directly connected test client.

    There seem to be some problem with congestion window leading to poor
    thruput numbers.

    Is there a way I can see (or infer) congestion window on linux side. I
    see that the
    receive window (advertised in the TCP pkt) is abt 32k but am not sure
    if Linux is really
    filling the pipe. Are there any counters/stats available for this
    (netstat -s doesn't help much)?

    Also, if someone knows about other tricks to tune up Linux for
    performance, could you
    please share the same. Any pointers...

    Thanks for your time and help.

    vishal..., Dec 9, 2005
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  2. vishal...

    prg Guest

    What OS on "server"? Link speed?
    In what sense CW is the culprit? Small streams? Medium streams? Huge
    You mean Linux is hitting CW limit when _sending_? CW used only by
    sender (if smaller than advertised window size).
    If you're testing the "other" box's stack implementation, I'm not sure
    what you might want to change on the Linux box. The defaults are
    pretty good for a _client_ but not so good as a high volumn server.
    Anyway, it sounds like you would like Linux to suck up packets from the
    other box as fast as possible, perhaps also shooting a stream at the
    other box as fast as possible to see what the other box can put
    out/take in.

    Best place to start (since I've no real data points) is to look over
    the networking params and make some changes. Look here:

    You may have to read through the TCP variables several times to get a
    sense which ones are most suitable for changing _and_ how some of the
    variables interact. See especially:

    3.3.14. tcp_mem
    3.3.21. tcp_rmem
    3.3.22. tcp_sack
    3.3.29. tcp_window_scaling
    3.3.30. tcp_wmem

    You may also want to look at this:

    This last points out some 2.6 changes in TCP variables.

    Chap. 20 of _TCP/IP_Illustrated_ if you have a copy around :)

    And if you want to try some Linux benchmark/perf tools... :

    good luck,
    prg, Dec 9, 2005
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  3. vishal...

    Rick Jones Guest

    If you are using a GbE link you may want to try your tests with TSO
    disabled (ethtool -k/-K) if it isn't already disabled.

    rick jones
    Rick Jones, Dec 9, 2005
  4. vishal...

    vishal... Guest

    Thanks guys...with your pointers, I was able to resolve the issue.
    vishal..., Dec 14, 2005
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