Bug #4934

winsock listen backlog may only be set once, and is set to 5

Added by Greg Hazel over 4 years ago. Updated about 4 years ago.

Assignee:Akira Tanaka
ruby -v:- Backport:


This is commonly observed as a bug in Windows servers, where under light load clients begin to get ECONNREFUSED. This occurs even if the listen backlog is set to a high value, like 1024. The bug is that Ruby sets the value to 5 on creation of a TCPServer socket, and it is not valid in Winsock to set the listen backlog size a second time.

From: http://msdn.microsoft.com/en-us/library/ms739168(v=vs.85).aspx
"If the listen function is called on an already listening socket, it will return success without changing the value for the backlog parameter."

Here's where ruby sets it:

So one of two things needs to happen. Either:
- on WIN32 Ruby should avoid setting the listen backlog by default, which would allow the user to choose the value but could conceivably break applications where were being lazy and not setting it at all
- on WIN32 and maybe all platforms, Ruby should set the default listen backlog to SOMAXCONN (or something large) instead of 5. This will not break any existing code today, as an overly high listen backlog is not really a problem.

Other default listen backlog values of servers:
Mongrel tries to use 1024 (but silently fails on Windows because of this bug): https://github.com/mongrel/mongrel/blob/74290abc9ebd287542b2dcc0133fa41d33e5177b/lib/mongrel/tcphack.rb#L13
Unicorn uses 1024: https://github.com/defunkt/unicorn/blob/b3b6b0dff19f8a22a96525bba22bf061d03c3fc5/lib/unicorn/socket_helper.rb#L27
Tornado uses 128: https://github.com/facebook/tornado/blob/9d1af05323f9a7941e2336ce50972de65e18bcdd/tornado/httpserver.py#L214
EventMachine uses 100: https://github.com/eventmachine/eventmachine/blob/master/ext/em.cpp#L1539
Twisted uses 50: http://twistedmatrix.com/trac/browser/tags/releases/twisted-11.0.0/twisted/internet/tcp.py#L828

listen_backlog_increase.patch Magnifier (780 Bytes) Greg Hazel, 06/27/2011 07:53 AM


#2 Updated by Greg Hazel over 4 years ago

Here is a patch to set the default value to 1024.

#3 Updated by Usaku NAKAMURA over 4 years ago

  • Status changed from Open to Assigned
  • Category set to ext
  • Assignee set to Akira Tanaka
  • Target version set to 2.0.0

About the socketpair implimentation of ruby on windows,
I reject your patch.
It accepts only one connection internally, so there is no
need to expand the backlog.

About default value of TCPServer#initialize, I have no opinion.
akr-san, how do you think about this?

#4 Updated by Akira Tanaka over 4 years ago

  • ruby -v changed from ruby 1.9.3dev (2011-06-17 trunk 32136) [i386-mingw32] to -

2011/7/25 Usaku NAKAMURA redmine@ruby-lang.org:

About default value of TCPServer#initialize, I have no opinion.
akr-san, how do you think about this?

I feel we can challenge SOMAXCONN for trunk (but not for 1.9.3).
Tanaka Akira

#5 Updated by Akira Tanaka about 4 years ago

  • Status changed from Assigned to Closed

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