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From:   Neal Cardwell <ncardwell@google.com>
Date:   Tue, 10 Oct 2023 16:06:53 -0400
Message-ID: <CADVnQy=tPcP+sRRVwvqober3cmi_3=LukzXC3-YcWudbf1e0HA@mail.gmail.com>
Subject: Re: [PATCH net-next,v2] tcp: Set pingpong threshold via sysctl
To:     Haiyang Zhang <haiyangz@microsoft.com>
Cc:     linux-hyperv@vger.kernel.org, netdev@vger.kernel.org,
        kys@microsoft.com, davem@davemloft.net, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com, corbet@lwn.net,
        dsahern@kernel.org, ycheng@google.com, kuniyu@amazon.com,
        morleyd@google.com, mfreemon@cloudflare.com, mubashirq@google.com,
        linux-doc@vger.kernel.org, weiwan@google.com,
        linux-kernel@vger.kernel.org
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On Tue, Oct 10, 2023 at 3:24=E2=80=AFPM Haiyang Zhang <haiyangz@microsoft.c=
om> wrote:
>
> TCP pingpong threshold is 1 by default. But some applications, like SQL D=
B
> may prefer a higher pingpong threshold to activate delayed acks in quick
> ack mode for better performance.
>
> The pingpong threshold and related code were changed to 3 in the year
> 2019 in:
>   commit 4a41f453bedf ("tcp: change pingpong threshold to 3")
> And reverted to 1 in the year 2022 in:
>   commit 4d8f24eeedc5 ("Revert "tcp: change pingpong threshold to 3"")
>
> There is no single value that fits all applications.
> Add net.ipv4.tcp_pingpong_thresh sysctl tunable, so it can be tuned for
> optimal performance based on the application needs.
>
> Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
> ---
> v2: Make it per-namesapce setting, and other updates suggested by Neal Ca=
rdwell,
> and Kuniyuki Iwashima.
>
> ---
>  Documentation/networking/ip-sysctl.rst |  8 ++++++++
>  include/net/inet_connection_sock.h     | 16 ++++++++++++----
>  include/net/netns/ipv4.h               |  1 +
>  net/ipv4/sysctl_net_ipv4.c             |  8 ++++++++
>  net/ipv4/tcp_ipv4.c                    |  2 ++
>  net/ipv4/tcp_output.c                  |  4 ++--
>  6 files changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/netwo=
rking/ip-sysctl.rst
> index 5bfa1837968c..c0308b65dc2f 100644
> --- a/Documentation/networking/ip-sysctl.rst
> +++ b/Documentation/networking/ip-sysctl.rst
> @@ -1183,6 +1183,14 @@ tcp_plb_cong_thresh - INTEGER
>
>         Default: 128
>
> +tcp_pingpong_thresh - INTEGER
> +       TCP pingpong threshold is 1 by default, but some application may =
need a
> +       higher threshold for optimal performance.
> +
> +       Possible Values: 1 - 255
> +
> +       Default: 1
> +

It would be good to document what the meaning of the parameter is.
Perhaps consider something like:

'The number of estimated data replies sent for estimated incoming data
requests that must happen before TCP estimates that a connection is a
"ping-pong" (request-response) connection for which delayed
acknowledgments can provide benefits. This threshold is 1 by default,
but some applications may need a higher threshold for optimal
performance.'

Thanks for the patch!

best,
neal