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Message-ID: <20110322113456.GA21254@xanadu.blop.info>
Date: Tue, 22 Mar 2011 12:34:56 +0100
From: Lucas Nussbaum <lucas.nussbaum@...ia.fr>
To: Stephen Hemminger <shemminger@...tta.com>
Cc: "David S\. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: 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 <lucas.nussbaum@...ia.fr>
--
| Lucas Nussbaum MCF Université Nancy 2 |
| lucas.nussbaum@...ia.fr LORIA / AlGorille |
| http://www.loria.fr/~lnussbau/ +33 3 54 95 86 19 |
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