netdev - Re: [PATCH] tcp: disable tcp_autocorking for socket when TCP

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Date: Sat, 9 Dec 2023 12:03:25 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Salvatore Dipietro <dipiets@...zon.com>
Cc: davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, 
	pabeni@...hat.com, netdev@...r.kernel.org, blakgeof@...zon.com, 
	alisaidi@...zon.com, benh@...zon.com, dipietro.salvatore@...il.com
Subject: Re: [PATCH] tcp: disable tcp_autocorking for socket when TCP_NODELAY
 flag is set

On Fri, Dec 8, 2023 at 7:23 PM Salvatore Dipietro <dipiets@...zon.com> wrote:
>
> Based on the tcp man page, if TCP_NODELAY is set, it disables Nagle's algorithm
> and packets are sent as soon as possible. However in the `tcp_push` function
> where autocorking is evaluated the `nonagle` value set by TCP_NODELAY is not
> considered which can trigger unexpected corking of packets and induce delays.
>
> For example, if two packets are generated as part of a server's reply, if the
> first one is not transmitted on the wire quickly enough, the second packet can
> trigger the autocorking in `tcp_push` and be delayed instead of sent as soon as
> possible. It will either wait for additional packets to be coalesced or an ACK
> from the client before transmitting the corked packet. This can interact badly
> if the receiver has tcp delayed acks enabled, introducing 40ms extra delay in
> completion times. It is not always possible to control who has delayed acks
> set, but it is possible to adjust when and how autocorking is triggered.
> Patch prevents autocorking if the TCP_NODELAY flag is set on the socket.
>
> Patch has been tested using an AWS c7g.2xlarge instance with Ubuntu 22.04 and
> Apache Tomcat 9.0.83 running the basic servlet below:
>
> import java.io.IOException;
> import java.io.OutputStreamWriter;
> import java.io.PrintWriter;
> import javax.servlet.ServletException;
> import javax.servlet.http.HttpServlet;
> import javax.servlet.http.HttpServletRequest;
> import javax.servlet.http.HttpServletResponse;
>
> Signed-off-by: Salvatore Dipietro <dipiets@...zon.com>
> ---

Very nice catch.

I suggest this patch lands in net tree, what about we add a Fixes: tag ?

Fixes: f54b311142a9 ("tcp: auto corking")
Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks !