From: Bill Kelly <billk@...>
Date: 2010-04-26T13:46:54+09:00
Subject: Re: Networking: select() blocks for seconds (> timeout)

Raul Parolari wrote:
> I am using Ruby (as a prototype version) to communicate with a network
> of solar cells (driven by firmware), over Udp. It works most of the time
> perfectly (delivering messages with 1-5 milliseconds precision, even
> beyond what we need).
> 
> But once in a while, this happens (ubuntu machine) while waiting the
> response (from solar cells):
> 
>    result = select(( [ comm ], nil, [ comm ], 0.050))
> 
>    if result.nil?
>      # handle 50 msec timeout
>    else
>      # data
>    end
> 
> All is perfect for several thousands of messages, until this: we receive
> data (in the "else" branch) 5 seconds later; so, the select remained
> blocked for 5 seconds without signaling the 50 msec timeout.

Hi,


Here was an original discussion of the issue:  (something like, select may
have already succeeded, before a bad UDP checksum is detected by the kernel)

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/148443


Here is a kludge-workaround I ended up using:  (via timeout)

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/304785


These days, I use EventMachine.


Hope this helps,

Bill