Re: [PATCH net-next] tcp: Ensure DCTCP reacts to losses

[Eric Dumazet]

On Thu, Apr 4, 2019 at 1:47 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 04/04/2019 10:26 AM, Tilmans, Olivier (Nokia - BE/Antwerp) wrote:
> > RFC8257 Â3.5 explicitly states that DCTCP should "react to loss
> > episode in the same way that a conventional TCP".
> > This is also the behavior on MS Windows.
> >
> > Currently, Linux DCTCP performs no ssthresh reduction when losses
> > are encountered. Optionally, the dctcp_clamp_alpha_on_loss resets
> > alpha to its maximal value if a RTO happens. This behavior
> > is sub-optimal for at least two reasons: i) it ignores losses
> > triggering fast retransmissions; and ii) it causes unnecessary large
> > cwnd reduction in the future if the loss was isolated as it resets
> > the historical term of DCTCP's alpha EWMA to its maximal value (i.e.,
> > denoting a total congestion). The second reason has an especially
> > noticeable effect when using DCTCP in high BDP environments, where
> > alpha normally stays at low values.
> >
> > This patch replace the clamping of alpha by setting ssthresh to
> > half of cwnd for both fast retransmissions and RTOs, at most once
> > per RTT. To reflect the change, the dctcp_clamp_alpha_on_loss option
> > has been renamed to dctcp_halve_cwnd_on_loss.
> >
> > The table below shows experimental results where we measured the
> > drop probability of a PIE AQM (not applying ECN marks) at a
> > bottleneck in the presence of a single TCP flow with either the
> > alpha-clamping option enabled or the cwnd halving proposed by this
> > patch. Results using reno or cubic are given for comparison.
> >
> >                           |  Link   |   RTT    |    Drop
> >                  TCP CC   |  speed  | base+AQM | probability
> >         ==================|=========|==========|============
> >                     CUBIC |  40Mbps |  7+20ms  |    0.21%
> >                      RENO |         |          |    0.19%
> >         DCTCP-CLAMP-ALPHA |         |          |   25.80%
> >          DCTCP-HALVE-CWND |         |          |    0.22%
> >         ------------------|---------|----------|------------
> >                     CUBIC | 100Mbps |  7+20ms  |    0.03%
> >                      RENO |         |          |    0.02%
> >         DCTCP-CLAMP-ALPHA |         |          |   23.30%
> >          DCTCP-HALVE-CWND |         |          |    0.04%
> >         ------------------|---------|----------|------------
> >                     CUBIC | 800Mbps |   1+1ms  |    0.04%
> >                      RENO |         |          |    0.05%
> >         DCTCP-CLAMP-ALPHA |         |          |   18.70%
> >          DCTCP-HALVE-CWND |         |          |    0.06%
> >
> > We see that, without halving its cwnd for all source of losses,
> > DCTCP drives the AQM to large drop probabilities in order to keep
> > the queue length under control (i.e., it repeatedly faces RTOs).
> > Instead, if DCTCP reacts to all source of losses, it can then be
> > controlled by the AQM using similar drop levels than cubic or reno.
> >
> > Signed-off-by: Koen De Schepper <koen.de_schepper@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Olivier Tilmans <olivier.tilmans@xxxxxxxxxxxxxxxxxxx>
> > Cc: Bob Briscoe <research@xxxxxxxxxxxxxx>
> > Cc: Lawrence Brakmo <brakmo@xxxxxx>
> > Cc: Florian Westphal <fw@xxxxxxxxx>
> > Cc: Daniel Borkmann <borkmann@xxxxxxxxxxxxx>
> > Cc: Yuchung Cheng <ycheng@xxxxxxxxxx>
> > Cc: Neal Cardwell <ncardwell@xxxxxxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Andrew Shewmaker <agshew@xxxxxxxxx>
> > Cc: Glenn Judd <glenn.judd@xxxxxxxxxxxxxxxxx>
> > ---
> >  net/ipv4/tcp_dctcp.c | 39 ++++++++++++++++++++++-----------------
> >  1 file changed, 22 insertions(+), 17 deletions(-)
> >
> > diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
> > index cd4814f7e962..60417243e7d7 100644
> > --- a/net/ipv4/tcp_dctcp.c
> > +++ b/net/ipv4/tcp_dctcp.c
> > @@ -67,10 +67,9 @@ static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
> >  module_param(dctcp_alpha_on_init, uint, 0644);
> >  MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
> >
> > -static unsigned int dctcp_clamp_alpha_on_loss __read_mostly;
> > -module_param(dctcp_clamp_alpha_on_loss, uint, 0644);
> > -MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss,
> > -              "parameter for clamping alpha on loss");
> > +static unsigned int dctcp_halve_cwnd_on_loss __read_mostly;
> > +module_param(dctcp_halve_cwnd_on_loss, uint, 0644);
> > +MODULE_PARM_DESC(dctcp_halve_cwnd_on_loss, "halve cwnd in case of losses");
>
> Is there a reason we still need to keep this module parameter around?
> The final RFC even says "A DCTCP sender MUST react to loss episodes in
> the same way as conventional TCP". So it's a MUST requirement in which
> case it should be enabled by default. The dctcp_clamp_alpha_on_loss was
> a bit of a hack from very early days..

I agree with Daniel and Florian

Please respin the patch removing the modparam

Also please check your SOB chain

If we see :

Signed-off-by: Koen De Schepper <koen.de_schepper@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Olivier Tilmans <olivier.tilmans@xxxxxxxxxxxxxxxxxxx>

We expect patch author is Koen De Schepper, not Olivier Tilmans

So the patch should start by 'From: Koen De Schepper
<koen.de_schepper@xxxxxxxxxxxxxxxxxxx>' if sent by Olivier.

Thanks !