Subject: Re: [RFC PATCH v5 net-next 4/4] tcp: add NV congestion control

From: Lawrence Brakmo <[email-masked]>
Date: Fri, 21 Aug 2015 21:29:55 +0000

Kenneth, thank you for your comments, I¹ve implemented most of the
improvements you've mentioned.

I¹m finishing the new patch and the updated results, they should
be done by Monday (including cdg).

On 8/5/15, 5:51 PM, "[email-masked] on behalf of Kenneth Klette
Jonassen" <[email-masked] on behalf of [email-masked]> wrote:

>On Wed, Aug 5, 2015 at 3:39 AM, Lawrence Brakmo <[email-masked]> wrote:
>> This is a request for comments.
>
>Nice to see more development on delay-based congestion control.

Thank you.

>
>It would be good to see how NV stacks up against CDG. Any chance of
>adding cdg as a congestion control parameter to your experiments?
>Experiments on NV without its temporary cwnd reductions would also be
>of interest -- to get a reference of how effective this mechanism is.

I¹m finishing with cdg experiments, they will be up on Monday together
with an update to the NV patch.
I will also have some experiments with variations in the temporary cwnd
reduction. This mechanism is meant to reduce min_rtt creep, but it is
now always successful. Its drawback is that it can increase high
percentile latency.

>
>
>> +#define NV_INIT_RTT      0xffffffff
>
>Maybe use U32_MAX?

Done

>
>
>> +static void tcpnv_init(struct sock *sk)
>> +{
>> +       struct tcpnv *ca = inet_csk_ca(sk);
>> +
>> +       tcpnv_reset(ca, sk);
>> +
>> +       ca->nv_min_rtt_reset_jiffies = jiffies + 2*HZ;
>> +       ca->nv_min_rtt = NV_INIT_RTT;
>> +       ca->nv_min_rtt_new = NV_INIT_RTT;
>> +       ca->nv_enable = nv_enable;
>
>Can this assignment be ca->nv_enable = 1? That would match the
>TCP_CA_Open case in tcpnv_state().

Done

>
>
>> +       if (nv_dec_eval_min_calls > 255)
>> +               nv_dec_eval_min_calls = 255;
>> +       if (nv_rtt_min_cnt > 63)
>> +               nv_rtt_min_cnt = 63;
>
>nv_dec_eval_min_calls can be clamped to 0-255 by changing its type to u8.
>
>nv_rtt_min_cnt can also be u8? In struct tcpnv, perhaps move
>nv_rtt_cnt to the available byte.

Done

>
>
>> +static void tcpnv_cong_avoid(struct sock *sk, u32 ack, u32 acked)
>> +{
>> +       struct tcp_sock *tp = tcp_sk(sk);
>> +       struct tcpnv *ca = inet_csk_ca(sk);
>> +
>> +       if (!tcp_is_cwnd_limited(sk))
>> +               return;
>> +
>> +       /* Only grow cwnd if NV has not detected congestion */
>> +       if (nv_enable && ca->nv_enable && !ca->nv_allow_cwnd_growth)
>> +               return;
>
>The check for ca->nv_enable might be overly harsh on some unfortunate
>sockets in TCP_CA_Disorder. Is it needed here?

TCP_CA_Disorder should not affect ca->nv_enable in the new patch

>
>
>> +static void tcpnv_acked(struct sock *sk, struct ack_sample *sample)
>
>Maybe move some of this function to tcpnv_cong_avoid()?

It needs to be here since We need the information provided in argument
sample

>
>
>> +{
>> +       const struct inet_connection_sock *icsk = inet_csk(sk);
>> +       struct tcp_sock *tp = tcp_sk(sk);
>> +       struct tcpnv *ca = inet_csk_ca(sk);
>> +       unsigned long now = jiffies;
>> +       s64 rate64 = 0;
>> +       u32 rate, max_win, cwnd_by_slope;
>> +       u32 avg_rtt;
>> +       u32 bytes_acked = 0;
>> +
>> +       /* Some calls are for duplicates without timetamps */
>> +       if (sample->rtt_us < 0)
>> +               return;
>> +
>> +       /* If not in TCP_CA_Open state, skip. */
>> +       if (icsk->icsk_ca_state != TCP_CA_Open)
>> +               return;
>
>Consider using samples in other states too, especially
>TCP_CA_Disorder. Linux 4.2 enhances RTT sampling from SACKs, so any
>non-negative RTT sample should be fully usable.

Done