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From Julien Vermillard <jvermill...@archean.fr>
Subject Re: [MINA 3.0] Thougts about the selectors
Date Thu, 11 Feb 2010 06:52:09 GMT
Le Thu, 04 Feb 2010 13:53:50 +0100,
Emmanuel Lecharny <elecharny@gmail.com> a écrit :

> I have reviewed the way we use the Selector in MINA 2.0. Here are
> some of the thoughts I have about teh way we use them for Sockets :
> 
> We currently have a system built on top of three elements :
> - IoAcceptor on the server side
> - IoConnector on the client side
> - IoProcessor which are processing the messages received or sent
> 
> IoAcceptor and IoConnector are just two sides of the same coin : a 
> IoService. The only difference is that the Connector initiates the 
> communication.
> 
> Nio Sockets
> ----------------
> In order to deal with incoming connections, the IoAcceptor uses a 
> Selector on which are registered the ServerSocketChannel for the 
> OP_ACCEPT event. On the client side, we have the same Selector but
> the ServerSocket is registered for the OP_CONNECT event.
> 
> In both case, once the session is connected/accepted, the associated 
> Channel is attached to another Selector, itself associated with an 
> IoProcessor.
> 
> Here, I'm questioning the fact that we use more than one Selector to 
> handle connect/accept  and read/write operations. The select()
> operation is not specially costly, even if it does a lot of things :
> - deregister the canceled channels
> - each channel which has had some operation since the last select is
> put to a set of selected keys
> - deregister the canceled channels again (for channel which has been 
> canceled while the step 2 was processed)
> - return the number of keys found ready in step 2
> 
> but all in all, this is a fast operation, as it just reads some bit 
> fields to determinate if something has changed since the last select. 
> Even if we have one million registered keys in the selector, first
> the number of active channel will be low, and second the processing
> time for this step is very minimal compared to the application
> processing time.
> 
> Now, wouldn't it be better to have only one selector, and then
> dispatch the tasks to some processor?
> 
> On the server side, we have to deal with :
> - newly added sessions
> - recently closed sessions
> - incoming data
> - outgoing data
> 
> On the client side, we have to deal with :
> - newly connected sessions
> - recently closed sessions
> - incoming data
> - outgoing data
> 
> each of those tasks can be processed by a separate thread selected in
> a thread pool. IMO, it may be better than the current architecture
> where we have a pool of IoProcessor, each one of them having its own
> Selector, and no thread to process the events. For instance, if we
> have 3 IoProcessor (the default value for a dual core processor),
> then we can only process 3 events in parallel. Pretty inefficient...
> 

If we can use only 1 selector, I would be pretty happy because it'll
simplify a lot of code. But I won't accept those concepts without a
bench :)

What would be the most efficient ? A thread polling and feeding a
pool of threads in charge of doing the costy operations
(read/write/accept). Or a pool of threads selecting and doing the costy
operations ?

On paper and without seeing the NIO/concurrency oddities I think I
can't answer this question.

-- 
Julien Vermillard

Archean Technologies
http://www.archean.fr

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