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Jason Xing 378979e94e tcp: remove 64 KByte limit for initial tp->rcv_wnd value 
Recently, we had some servers upgraded to the latest kernel and noticed
the indicator from the user side showed worse results than before. It is
caused by the limitation of tp->rcv_wnd.

In 2018 commit a337531b94 ("tcp: up initial rmem to 128KB and SYN rwin
to around 64KB") limited the initial value of tp->rcv_wnd to 65535, most
CDN teams would not benefit from this change because they cannot have a
large window to receive a big packet, which will be slowed down especially
in long RTT. Small rcv_wnd means slow transfer speed, to some extent. It's
the side effect for the latency/time-sensitive users.

To avoid future confusion, current change doesn't affect the initial
receive window on the wire in a SYN or SYN+ACK packet which are set within
65535 bytes according to RFC 7323 also due to the limit in
__tcp_transmit_skb():

    th->window      = htons(min(tp->rcv_wnd, 65535U));

In one word, __tcp_transmit_skb() already ensures that constraint is
respected, no matter how large tp->rcv_wnd is. The change doesn't violate
RFC.

Let me provide one example if with or without the patch:
Before:
client   --- SYN: rwindow=65535 ---> server
client   <--- SYN+ACK: rwindow=65535 ----  server
client   --- ACK: rwindow=65536 ---> server
Note: for the last ACK, the calculation is 512 << 7.

After:
client   --- SYN: rwindow=65535 ---> server
client   <--- SYN+ACK: rwindow=65535 ----  server
client   --- ACK: rwindow=175232 ---> server
Note: I use the following command to make it work:
ip route change default via [ip] dev eth0 metric 100 initrwnd 120
For the last ACK, the calculation is 1369 << 7.

When we apply such a patch, having a large rcv_wnd if the user tweak this
knob can help transfer data more rapidly and save some rtts.

Fixes: a337531b94 ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Link: https://lore.kernel.org/r/20240521134220.12510-1-kerneljasonxing@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
2024-05-23 12:21:17 +02:00
arch Probes updates for v6.10: 2024-05-17 18:29:30 -07:00
block SCSI misc on 20240514 2024-05-14 18:25:53 -07:00
certs This update includes the following changes: 2023-11-02 16:15:30 -10:00
crypto Hi, 2024-05-15 09:22:34 -07:00
Documentation Including fix from Andrii for the issue mentioned in our net-next PR, 2024-05-17 18:57:14 -07:00
drivers net: ti: icssg_prueth: Fix NULL pointer dereference in prueth_probe() 2024-05-23 12:11:09 +02:00
fs tracing cleanups for v6.10: 2024-05-17 18:34:27 -07:00
include tracing ring buffer updates for v6.10: 2024-05-17 18:40:37 -07:00
init Bootconfig updates for v6.10: 2024-05-17 18:23:55 -07:00
io_uring for-6.10/io_uring-20240511 2024-05-13 12:48:06 -07:00
ipc sysctl changes for v6.10-rc1 2024-05-17 17:31:24 -07:00
kernel kprobe/ftrace: fix build error due to bad function definition 2024-05-17 19:17:55 -07:00
lib Probes updates for v6.10: 2024-05-17 18:29:30 -07:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm ARM: 2024-05-15 14:46:43 -07:00
net tcp: remove 64 KByte limit for initial tp->rcv_wnd value 2024-05-23 12:21:17 +02:00
rust Timers and timekeeping updates: 2024-05-14 09:27:40 -07:00
samples Networking changes for 6.10. 2024-05-14 19:42:24 -07:00
scripts Modules changes for v6.10-rc1 2024-05-15 14:05:08 -07:00
security integrity-v6.10 2024-05-15 08:43:02 -07:00
sound ALSA: hda/realtek: Drop doubly quirk entry for 103c:8a2e 2024-05-13 12:54:46 +02:00
tools testing: net-drv: use stats64 for testing 2024-05-23 10:18:29 +02:00
usr Kbuild updates for v6.8 2024-01-18 17:57:07 -08:00
virt KVM x86 misc changes for 6.10: 2024-05-12 03:18:44 -04:00
.clang-format clang-format: Update with v6.7-rc4's for_each macro list 2023-12-08 23:54:38 +01:00
.cocciconfig
.editorconfig Add .editorconfig file for basic formatting 2023-12-28 16:22:47 +09:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: create a list of all built DTB files 2024-02-19 18:20:39 +09:00
.mailmap Another not-too-busy cycle for documentation, including: 2024-05-13 10:51:53 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Drop Gustavo Pimentel as PCI DWC Maintainer 2024-03-27 13:41:02 -05:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Including fix from Andrii for the issue mentioned in our net-next PR, 2024-05-17 18:57:14 -07:00
Makefile Linux 6.9 2024-05-12 14:12:29 -07:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.