Re: [PATCH net-next v2] tcp: up initial rmem to 128KB and SYN rwin to around 64KB

[David Miller <davem@davemloft.net>]

From: Yuchung Cheng <ycheng@google.com>
Date: Fri, 28 Sep 2018 13:09:02 -0700

> 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.
> 
> This patch also lowers the initial bytes expected to receive in
> the receiver buffer autotuning algorithm - otherwise the receiver
> may take two to three rounds to increase the buffer to the
> appropriate level (2x sender congestion 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.
> 
> Signed-off-by: Yuchung Cheng <ycheng@google.com>
> Signed-off-by: Wei Wang <weiwan@google.com>
> Signed-off-by: Neal Cardwell <ncardwell@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>

Applied, thanks.