Date:   Thu, 27 Sep 2018 12:39:19 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] tcp: up initial rmem to 128KB and SYN rwin to
 around 64KB

On Thu, Sep 27, 2018 at 11:21 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> Previously TCP initial receive buffer is ~87KB by default and
> the initial receive window is ~29KB (20 MSS). This patch changes
> the two numbers to 128KB and ~64KB (rounding down to the multiples
> of MSS) respectively. The patch also simplifies the calculations s.t.
> the two numbers are directly controlled by sysctl tcp_rmem[1]:
>
>   1) Initial receiver buffer budget (sk_rcvbuf): while this should
>      be configured via sysctl tcp_rmem[1], previously tcp_fixup_rcvbuf()
>      always override and set a larger size when a new connection
>      establishes.
>
>   2) Initial receive window in SYN: previously it is set to 20
>      packets if MSS <= 1460. The number 20 was based on the initial
>      congestion window of 10: the receiver needs twice amount to
>      avoid being limited by the receive window upon out-of-order
>      delivery in the first window burst. But since this only
>      applies if the receiving MSS <= 1460, connection using large MTU
>      (e.g. to utilize receiver zero-copy) may be limited by the
>      receive window.
>
> With this patch TCP memory configuration is more straight-forward and
> more properly sized to modern high-speed networks by default. Several
> popular stacks have been announcing 64KB rwin in SYNs as well.
Sorry please ignore this patch for now.

We need to adjust rbuf autotuning as well otherwise w/ larger init
rbuf it may increase too slowly during slow start. Will submit a v2

>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Wei Wang <weiwan@...gle.com>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reviewed-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> ---
>  net/ipv4/tcp.c        |  4 ++--
>  net/ipv4/tcp_input.c  | 25 ++-----------------------
>  net/ipv4/tcp_output.c | 25 ++++---------------------
>  3 files changed, 8 insertions(+), 46 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 69c236943f56..dcf51fbf5ec7 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -3896,8 +3896,8 @@ void __init tcp_init(void)
>         init_net.ipv4.sysctl_tcp_wmem[2] = max(64*1024, max_wshare);
>
>         init_net.ipv4.sysctl_tcp_rmem[0] = SK_MEM_QUANTUM;
> -       init_net.ipv4.sysctl_tcp_rmem[1] = 87380;
> -       init_net.ipv4.sysctl_tcp_rmem[2] = max(87380, max_rshare);
> +       init_net.ipv4.sysctl_tcp_rmem[1] = 131072;
> +       init_net.ipv4.sysctl_tcp_rmem[2] = max(131072, max_rshare);
>
>         pr_info("Hash tables configured (established %u bind %u)\n",
>                 tcp_hashinfo.ehash_mask + 1, tcp_hashinfo.bhash_size);
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index d703a0b3b6a2..7a59f6a96212 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -426,26 +426,7 @@ static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb)
>         }
>  }
>
> -/* 3. Tuning rcvbuf, when connection enters established state. */
> -static void tcp_fixup_rcvbuf(struct sock *sk)
> -{
> -       u32 mss = tcp_sk(sk)->advmss;
> -       int rcvmem;
> -
> -       rcvmem = 2 * SKB_TRUESIZE(mss + MAX_TCP_HEADER) *
> -                tcp_default_init_rwnd(mss);
> -
> -       /* Dynamic Right Sizing (DRS) has 2 to 3 RTT latency
> -        * Allow enough cushion so that sender is not limited by our window
> -        */
> -       if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf)
> -               rcvmem <<= 2;
> -
> -       if (sk->sk_rcvbuf < rcvmem)
> -               sk->sk_rcvbuf = min(rcvmem, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
> -}
> -
> -/* 4. Try to fixup all. It is made immediately after connection enters
> +/* 3. Try to fixup all. It is made immediately after connection enters
>   *    established state.
>   */
>  void tcp_init_buffer_space(struct sock *sk)
> @@ -454,8 +435,6 @@ void tcp_init_buffer_space(struct sock *sk)
>         struct tcp_sock *tp = tcp_sk(sk);
>         int maxwin;
>
> -       if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK))
> -               tcp_fixup_rcvbuf(sk);
>         if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
>                 tcp_sndbuf_expand(sk);
>
> @@ -485,7 +464,7 @@ void tcp_init_buffer_space(struct sock *sk)
>         tp->snd_cwnd_stamp = tcp_jiffies32;
>  }
>
> -/* 5. Recalculate window clamp after socket hit its memory bounds. */
> +/* 4. Recalculate window clamp after socket hit its memory bounds. */
>  static void tcp_clamp_window(struct sock *sk)
>  {
>         struct tcp_sock *tp = tcp_sk(sk);
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index fe7855b090e4..059b67af28b1 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -195,21 +195,6 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts,
>         inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
>  }
>
> -
> -u32 tcp_default_init_rwnd(u32 mss)
> -{
> -       /* Initial receive window should be twice of TCP_INIT_CWND to
> -        * enable proper sending of new unsent data during fast recovery
> -        * (RFC 3517, Section 4, NextSeg() rule (2)). Further place a
> -        * limit when mss is larger than 1460.
> -        */
> -       u32 init_rwnd = TCP_INIT_CWND * 2;
> -
> -       if (mss > 1460)
> -               init_rwnd = max((1460 * init_rwnd) / mss, 2U);
> -       return init_rwnd;
> -}
> -
>  /* Determine a window scaling and initial window to offer.
>   * Based on the assumption that the given amount of space
>   * will be offered. Store the results in the tp structure.
> @@ -244,7 +229,10 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
>         if (sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows)
>                 (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
>         else
> -               (*rcv_wnd) = space;
> +               (*rcv_wnd) = min_t(u32, space, U16_MAX);
> +
> +       if (init_rcv_wnd)
> +               *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
>
>         (*rcv_wscale) = 0;
>         if (wscale_ok) {
> @@ -257,11 +245,6 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
>                         (*rcv_wscale)++;
>                 }
>         }
> -
> -       if (!init_rcv_wnd) /* Use default unless specified otherwise */
> -               init_rcv_wnd = tcp_default_init_rwnd(mss);
> -       *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
> -
>         /* Set the clamp no higher than max representable value */
>         (*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);
>  }
> --
> 2.19.0.605.g01d371f741-goog
>

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