[e2e] query reg improving TCP performane

query query.cdac at gmail.com
Thu Jul 5 07:39:02 PDT 2007

Hi All,

  I was doing some Bandwidth measurement test on a 100 mbs link with a RTT
of about 70ms.
  Based on that, I calculated the BDP as follows.

          BDP = Bandwidth * RTT
              = 13107200 bytes * .07 secs
              = 896 Kbytes
              = 900 Kbyes (approx)
              = 921600 bytes

   After that I adjusted the TCP window size as follows:

           /proc/sys/net/core/rmem_max  921600
           /proc/sys/net/core/wmem_max  921600
           /proc/sys/net/ipv4/tcp_rmem      4096    87380   921600
           /proc/sys/net/ipv4/tcp_wmem      4096    87380   921600

   These adjustmenst I had done on a Linux host with 2.6.15 kernel. The
   control algorithm , it is using BIC

   The same window adjustments I had performed on the other hand on a Linux
   with kernel 2.6.9 . It is also using BIC for congestion control.

   The Bandwidth Performnce test I am doing using iperf , a highly popular
   domain tool for measuring TCP & UDP Bandwidth Performance .

   With the default Linux 2.6 TCP window settings , I was getting a
throughput of
   nearly 10mbs which is very low for a 100 mbs link.

   So , I performed the above TCP adjustmets and I found the throughput to
be around
   55 mbs which is a significant improvement . But that is not fully
utilsing the link
   as it is a dedicated link and there was no other traffic.
   This , I proved with the next experiment where I reached a link
utilisation of
   little more than 95 mbs . That is very much O.K for a 100mbs link.

   I did the following adjustments. I increased the above calculated BDP by
   half of the value . The TCP settings now look like this.

           /proc/sys/net/core/rmem_max  175636
           /proc/sys/net/core/wmem_max  175636
           /proc/sys/net/ipv4/tcp_rmem      4096    87380   175636
           /proc/sys/net/ipv4/tcp_wmem      4096    87380   175636

    After these settings , I find the link utilisation to be nearly 95 mbs.

    According to many papers that I read , I found that the BDP should be
    to the product of Bandwidth * RTT .
    I had done that , but the link utilisation is only 50%. But when I
    to a much higher value , the link utilisation is nearly around 95 %.

    I am confused regarding my findings.Please clarify me so that I can
    the experiment correctly.

    With Thanks in Advance
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