Re: [PATCH 0/7] TCP CUBIC Hystart fixes

On 14/03/11 at 10:52 -0700, Stephen Hemminger wrote:

This is the merge of my patches and recent update Sangtae.
It addresses the problems reported by Lucas Nussbaum that Hystart causes
poor startup performance over links with lots of buffering.

Hi,

I've tested the patches, and they work fine.

Here are some results (gigabit link, RTT=11ms).

Without the patches, hystart disabled:
 Segments (cwnd, ssthresh)
   2500 ++-----------+-------------+------------+-------------+-----------++
        +            +             +            +          snd_cwnd ****** +
        |                                              snd_ssthresh ###### |
   2000 ++       *************************************************        ++
        |       **                                                         |
        |       *                                                          |
        |       *                                                          |
   1500 ++      *                                                         ++
        |       *                                                          |
        |       *                                                          |
   1000 ++      *                                                         ++
        |      **                                                          |
        |      *                                                           |
        |      *                                                           |
    500 ++     *                                                          ++
        |     **                                                           |
        +    **      +             +            +             +            +
      0 ++-***####################################################--------++
        0           0.5            1           1.5            2           2.5
                                  time (seconds)

Without the patches, hystart enabled:
 Segments (cwnd, ssthresh)
   300 ++------------+------------+-------------+------------+------------++
       +             +            +             +          snd_cwnd ****** +
       |                                            ***snd_ssthresh ###### |
   250 ++                     ***********************                     ++
       |     ******************###################################         |
       |     *                                                             |
   200 ++    *                                                            ++
       |     *                                                             |
   150 ++    *                                                            ++
       |     *                                                             |
       |     *                                                             |
   100 ++   **                                                            ++
       |    *#                                                             |
       |    *#                                                             |
    50 ++   *#                                                            ++
       |   **#                                                             |
       +  ** #       +            +             +            +             +
     0 ++-*###-------+------------+-------------+------------+------------++
       0            0.5           1            1.5           2            2.5
                                  time (seconds)

Note how slow start ends very early (~ 230 segments), resulting in poor performance.

With the patches, hystart enabled, run 1:
 Segments (cwnd, ssthresh)
   2500 ++-----+-------+------+-------+------+-------+------+-------+-----++
        +      +       +      +       +      +       +     snd_cwnd ****** +
        |                                              snd_ssthresh ###### |
   2000 ++   *********************************************************    ++
        |    *                                                             |
        |    *                                                             |
        |    *                                                             |
   1500 ++   *                                                            ++
        |    *                                                             |
        |   **                                                             |
   1000 ++  *                                                             ++
        |   *                                                              |
        |   *                                                              |
        |   *                                                              |
    500 ++  *                                                             ++
        |  **                                                              |
        +  *   +       +      +       +      +       +      +       +      +
      0 ++**##########################################################----++
        0     0.5      1     1.5      2     2.5      3     3.5      4     4.5
                                  time (seconds)

There's no perceived delay increase, but also no losses. The NIC sends data at
line rate without congestion. we don't exit slow start, but that's fine:

With the patch, hystart enabled, run 2: (that's the most frequent situation)
 Segments (cwnd, ssthresh)
   2500 ++-----+-------+------+-------+------+-------+------+-------+-----++
        +      +       +      +       +      +       +     snd_cwnd ****** +
        |                                              snd_ssthresh ###### |
   2000 ++    ********************************************************    ++
        |     *#                                                           |
        |     *#                                                           |
        |     *#                                                           |
   1500 ++    *#                                                          ++
        |     *#                                                           |
        |     *#                                                           |
   1000 ++   **#                                                          ++
        |    * #                                                           |
        |    * #                                                           |
        |    * #                                                           |
    500 ++  ** #                                                          ++
        |   *  #                                                           |
        +  **  #       +      +       +      +       +      +       +      +
      0 ++**####-------+------+-------+------+-------+------+-------+-----++
        0     0.5      1     1.5      2     2.5      3     3.5      4     4.5
                                  time (seconds)

Hystart detects a delay increase, so we exit slow start, but at a reasonable point.
Hystart works fine in that case. (no impact on performance).

With the patch, hystart enabled, run 3:
 Segments (cwnd, ssthresh)
   2500 ++-----+-------+------+-------+------+-------+------+-------+-----++
        +      +       +      +       +      +       +     snd_cwnd ****** +
        |                                              snd_ssthresh ###### |
   2000 ++     *******************************************************    ++
        |      *                                                           |
        |     **                                                           |
        |     *                                                            |
   1500 ++   **#######################################################    ++
        |    *#                                                            |
        |    *#                                                            |
   1000 ++   *#                                                           ++
        |    *#                                                            |
        |   **#                                                            |
        |   * #                                                            |
    500 ++  * #                                                           ++
        |   * #                                                            |
        +  ** #+       +      +       +      +       +      +       +      +
      0 ++**###+-------+------+-------+------+-------+------+-------+-----++
        0     0.5      1     1.5      2     2.5      3     3.5      4     4.5
                                  time (seconds)

Hystart causes slow start to end a bit too early, but late enough not to affect
performance significantly. Hystart behaves fine in that case too.


Tested-By: Lucas Nussbaum <redacted>
-- 
| Lucas Nussbaum             MCF Université Nancy 2 |
| lucas.nussbaum@loria.fr         LORIA / AlGorille |
| http://www.loria.fr/~lnussbau/  +33 3 54 95 86 19 |