From: Joshua Haberman <joshua@...>
Date: 2005-10-05T14:31:27+09:00
Subject: Re: state of blocking/nonblocking I/O


On Oct 4, 2005, at 12:13 AM, David Gurba wrote:

> Joshua Haberman wrote:
>> while true
>>     (read_ready, write_ready, err) = IO.select([A, B, C])
>>     read_ready.each { |io|
>>         output = process(io.read)
>>         [A, B, C].each { |client| client.write(output) unless  
>> client  == io }
>>     }
>> end
>>
>> Nonblocking I/O gives you more control over the execution of your   
>> program, and frees you from the worries of synchronizing between   
>> threads.  And it's simpler than using threads for programs that   
>> follow certain patterns.
>>
>> Josh
>>
>>
> This sounds really interesting, but I don't fully understand the  
> while loop. Nonblocking IO sends/recieves data when its ready/ 
> requested...eg. it doesn't block for the data, right?

I'm not sure exactly what you're asking.  Nonblocking I/O basically  
tells the OS: "when I do a read() or write(), only perform as much of  
the operation as you can without making me wait."  If the OS cannot  
perform *any* of the operation (because there is no data waiting to  
read, or no buffer space available to write), the call errors with  
EAGAIN.

IO.select is what you use to ask the OS what file descriptors are  
available for reading or writing.  IO.select is what blocks, until  
one of your fds is available, or a timeout has elapsed.  If you  
didn't use select, you'd have to busy-wait by reading from the fd  
over and over (getting EAGAIN every time).  That would waste the  
CPU.  Instead, you ask select to block until a file descriptor is  
available.

> I have written some threaded applications. A java tic-tac-toe game  
> which had players and observers of a game that all viewed a global  
> 'board' state. Methods to modify the game state were thread safe  
> with mutexes, how is what your saying different...? Any info  
> appreciated...

If you follow the pattern above, you don't have to make anything  
thread-safe.  You don't have to use mutexes.  You don't have to think  
about possibly bad interactions between threads like deadlock.   
Everything happens in the same thread.

Josh