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Quintin Par
Asked: 2012-03-03 02:30:02 +0800 CST

Increasing tcp slow start Initial Window in linux 3.0 kernel

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I was checking the IW for my site and realized that its 4.

[user~]$ sudo tcpdump -n -i any 'tcp[tcpflags] & (tcp-syn|tcp-ack) == tcp-syn and port 80'
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
10:13:20.971391 IP 175.x.x.x > 17.x.x.x.http: Flags [S], seq 3929903562, win 5840, options [mss 1460,sackOK,TS val 1701657127 ecr 0,nop,wscale 6], length 0

I run

[user~]$ uname -r
3.0.4x86_64-linode21

How can I increase the Initial Window?

Do you recommend recompiling to new kernel? (Don’t want to do) or is there another way?

An nginx HTTP cached hit takes 

[site ~]$ time curl www.site.com/ >/dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  6440    0  6440    0     0  26002      0 --:--:-- --:--:-- --:--:-- 39753

real    0m0.268s
user    0m0.004s
sys     0m0.012s

at least .3 sec.

linux tcp tcp-slow-start

2 Answers

  1. This was thoroughly discussed in a Hacker News entry recently, and the proces involves using the "ip route change" command with some specific parameters. There is more help on the first comment of the discussion, and I would suggest you to take a look there to try that solution and see if it works.

    Your kernel is pretty recent, so the initrwnd parameter that establishes the slow-start setting seems to be 10 by default... so your's being 4 is a little strange. Anyway, just test it and let us know if you noticed any improvements!

    • 3

  2. If you are on a kernel less than 3.2 then I’d suggest 

    sudo ip route change default via ip.address dev eth0 initcwnd 15

    You can figure this out ip.address with 

    ip route show

    You will need to add this to a startup script like rc.local

    • 2