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Message-ID: <CANn89i+98ifRj9SJQbK+QJrCde2UJvWr1h31gAZSuxt4i_U=iw@mail.gmail.com>
Date: Wed, 13 Dec 2023 15:10:00 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Salvatore Dipietro <dipiets@...zon.com>, davem@...emloft.net, dsahern@...nel.org,
kuba@...nel.org, 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 Tue, Dec 12, 2023 at 11:58 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Fri, 2023-12-08 at 10:20 -0800, Salvatore Dipietro 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;
> >
> > public class HelloWorldServlet extends HttpServlet {
> > @Override
> > protected void doGet(HttpServletRequest request, HttpServletResponse response)
> > throws ServletException, IOException {
> > response.setContentType("text/html;charset=utf-8");
> > OutputStreamWriter osw = new OutputStreamWriter(response.getOutputStream(),"UTF-8");
> > String s = "a".repeat(3096);
> > osw.write(s,0,s.length());
> > osw.flush();
> > }
> > }
> >
> > Load was applied using wrk2 (https://github.com/kinvolk/wrk2) from an AWS
> > c6i.8xlarge instance. With the current auto-corking behavior and TCP_NODELAY
> > set an additional 40ms latency from P99.99+ values are observed. With the
> > patch applied we see no occurrences of 40ms latencies. The patch has also been
> > tested with iperf and uperf benchmarks and no regression was observed.
> >
> > # No patch with tcp_autocorking=1 and TCP_NODELAY set on all sockets
> > ./wrk -t32 -c128 -d40s --latency -R10000 http://172.31.49.177:8080/hello/hello'
> > ...
> > 50.000% 0.91ms
> > 75.000% 1.12ms
> > 90.000% 1.46ms
> > 99.000% 1.73ms
> > 99.900% 1.96ms
> > 99.990% 43.62ms <<< 40+ ms extra latency
> > 99.999% 48.32ms
> > 100.000% 49.34ms
> >
> > # With patch
> > ./wrk -t32 -c128 -d40s --latency -R10000 http://172.31.49.177:8080/hello/hello'
> > ...
> > 50.000% 0.89ms
> > 75.000% 1.13ms
> > 90.000% 1.44ms
> > 99.000% 1.67ms
> > 99.900% 1.78ms
> > 99.990% 2.27ms <<< no 40+ ms extra latency
> > 99.999% 3.71ms
> > 100.000% 4.57ms
> >
> > Signed-off-by: Salvatore Dipietro <dipiets@...zon.com>
> > ---
> > net/ipv4/tcp.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index d3456cf840de..87751a2a6fff 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -716,7 +716,7 @@ void tcp_push(struct sock *sk, int flags, int mss_now,
> >
> > tcp_mark_urg(tp, flags);
> >
> > - if (tcp_should_autocork(sk, skb, size_goal)) {
> > + if (!nonagle && tcp_should_autocork(sk, skb, size_goal)) {
>
> It looks like the above disables autocorking even after the userspace
> sets TCP_CORK. Am I reading it correctly?Sal Is that expected?
>
Yes, it seems the patch went too far.
Also I wonder about these 40ms delays, TCP small queue handler should
kick when the prior skb is TX completed.
It seems the issue is on the driver side ?
Salvatore, which driver are you using ?
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