netdev - Re: Add sysctl for tcp_delayed

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Message-ID: <CANn89iL1gca+f=3pJaNqnVk6yRd7q+8g-33NhfXbCzp9TLGLUw@mail.gmail.com>
Date: Mon, 10 Mar 2025 08:02:37 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Andrew Easton <Andrew@...ton24.com>
Cc: "David S. Miller" <davem@...emloft.net>, "David S. Ahern" <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	"Linux Kernel Mailing List, Network Subsystem" <netdev@...r.kernel.org>, 
	"Linux Kernel Mailing List, Sysctl API ABI" <linux-api@...r.kernel.org>
Subject: Re: Add sysctl for tcp_delayed_ack

On Mon, Mar 10, 2025 at 2:20 AM Andrew Easton <Andrew@...ton24.com> wrote:
>
> Subject: Add sysctl for tcp_delayed_ack
>
> Hi everyone,

Hi Andrew

>
> this is a proposed patch for adding a sysctl for
> disabling TCP delayed ACK (IETF RFC 1122)  without
> having to patch software to constantly poke sockets
> with TCP_QUICKACK which apparently resets on
> subsequent operations, see tcp(7).
>
> For my personal computer networks experimenting with
> globally disabling TCP delayed ACK across two other
> operating systems seems to have considerably improved
> congestion control.  (While I propose only anecdotal
> evidence, there is more to it.  Am open to the
> ensuing technical discussion, but only if that turns
> out to be a good use of other people's time.)
>
> This is my first proposed kernel patch and it is
> likely missing a whole bunch of details.  For
> example:
>
> 1. Where is the TCP ACK delay computed for IPv6?
> Could not identify this in file net/ipv6/tcp_ipv6.c .

No need to change tcp_ipv6.c

Generating ACK is generic, thus code is in net/ipv4

>
> 2. Perhaps, adding kernel configuration options for
> the ncurses interface is desireable.  What is a good
> example to learn from?

No need. per net-ns sysctl and/or per-socket options are far better
for this case.

>
> 3. Perhaps, setting constants in file
> include/uapi/linux/sysctl.h may be unnecessary, but I
> have not found any guidelines on when these CTL
> numbers are necessary.  Likely, because I did not
> read the documentation carefully enough.  Any
> pointers are appreciated.

sysctl.h is absolutely deprecated.
No need for NET_IPV4_TCP_DELAYED_ACK,
No ctl_name in 'struct ctl_table'

>
> 4. The default should probably be a value like
> net.ipv4.tcp_delayed_ack=1 that preserves the current
> behavior and hence is backwards compatible for user
> space.  A value of net.ipv4.tcp_delayed_ack=0 should
> globally (for IPv4) disable TCP delayed ACK.  Would
> also like to add the option for IPv6, but see point
> (1).
>
>
> In case a similar sysctl has already been proposed
> and rejected in the past, please point me to the
> mailing list archives, if that is not too
> inconvenient.
>
>
> Which questions have I failed to ask that I should
> have asked?
>
> Errors and lack of research are on me.
>

Make sure to compile/test your patch on top of net-next tree, and send
it inline,
not as an attachment, so that we can comment on it.

Also next time add benchmark results
like netperf -t TCP_RR (200 flows)
of netper/tcp_rr -F 1000

No delaying ACK for small RPC is essentially doubling the number of
packets to send and receive.

> Thank you for sharing your time.
>
> Andrew
>
>
>
>
>