[jboss-jira] [JBoss JIRA] Commented: (JBREM-706) In socket transport, prevent client side oneway invocations from artificially reducing concurrency

[Ron Sigal (JIRA)]

    [ http://jira.jboss.com/jira/browse/JBREM-706?page=comments#action_12367979 ] 
            
Ron Sigal commented on JBREM-706:
---------------------------------

A simpler strategy has been adopted.  Instead of moving the connection usability test to a TimerTask, the test is done on the same thread that issues the invocation.  The test is simply to wait for a response to the invocation.  ServerThread has been changed to return a null response for client side oneway invocations.

There are several advantages to this strategy.

1. Simplicity
2. The test can begin immediately after the invocation is sent
3. Tests for all connections can run in parallel.

A possible disadvantage is that the oneway invocation thread is in use longer.  However, if the number of threads is equal to or greater than the maximum number of connections allowed by MicroSocketClientInvoker, then this shouldn't be a problem.  NOTE, however, that the default number of threads is 10 and the default maximum number of connections is 50.

The parameter MicroSocketClientInvoker.ONEWAY_CONNECTION_TIMEOUT is still used, though its default is now 2000 ms.

The parameter MicroSocketClientInvoker.ONEWAY_CONNECTION_DELAY is no longer used.

> In socket transport, prevent client side oneway invocations from artificially reducing concurrency
> --------------------------------------------------------------------------------------------------
>
>                 Key: JBREM-706
>                 URL: http://jira.jboss.com/jira/browse/JBREM-706
>             Project: JBoss Remoting
>          Issue Type: Task
>      Security Level: Public(Everyone can see) 
>    Affects Versions: 2.4.0.Beta1 (Pinto)
>            Reporter: Ron Sigal
>         Assigned To: Ron Sigal
>             Fix For: 2.4.0.Beta1 (Pinto)
>
>
> There is a subtle problem with the handling of client side oneway invocations in the socket transports (and descendants).  After the invocation has been marshalled to the wire, the connection is returned to the connection pool, from which it can be reused, even if the server side handler is still processing the previous invocation.  Consequently, invocations can back up and run serially instead of running concurrently.
> The following dialogue exposes the problem and possible solutions:
> ===============================================================================================================
> Ron Sigal wrote
> Hi Tom,
> I just noticed an interesting phenomenon that I wanted to tell you about.  Maybe it'll be obvious to you, but it surprised me.
> I'm writing unit tests for socket and http, since they behave differently on oneway invocations.  I've got a oneway ThreadPool with two threads and I make 4 oneway invocations, one right after the other.  The handler waits for 5 seconds before returning.  I expected that they would all run on the server simultaneously, but they don't.  What happens is something like this:  (1) invocation 1 gets made on thread 1 and socket connection 1 to ServerThread 1 is created;  (2) invocation 2 gets made on thread 2 and socket connection 2 to ServerThread 2 gets created;  (3) invocation 3 gets made on thead 1 and socket connection 1 gets reused.  But ServerThread 1 is still sleeping, so invocation 3 waits for 5 seconds.
> Not a big deal, but I might not have guessed it if I hadn't seen it.
> ===============================================================================================================
> Ron Sigal wrote
> Hi Tom,
> Well, it's stranger than I thought.  It turns out that for client side oneway invocations, socket, which returns immediately after marshaling the invocation, can be slower than http, which waits for a response.
> Consider a client side oneway threadpool with 2 threads and a queue of size 1, and suppose there are 4 invocations in quick succession.  The handler takes 5 seconds per invocation.
> http:
> 1. 1st invocation runs in the first threadpool thread.
> 2. 2nd invocation runs in the second threadpool thread
> 3. 3rd invocation goes on the queue
> 4  4th invocation runs on the main thread
> Assuming there are at least 3 server threads, invocations 1, 2, and 4 run in the first 5 seconds, and then invocation 3 runs.  Total time 10 seconds.
> socket (one possible scenario):
> 1. 1st invocation runs in the first threadpool thread and returns the socket connection to the pool
> 2. 2nd invocation runs in a threadpool thread, finds the pooled connection, marshals the invocation, and returns the connection to the pool.  On the server side, the ServerThread runs the invocations after finishing the 1st invocation.
> 3. 3rd invocation runs in a threadpool thread, finds the pooled connection, marshals the invocation, and returns the connection to the pool.  On the server side, the ServerThread runs the invocations after finishing the 2nd invocation.
> 4. 4th invocation runs in a threadpool thread, finds the pooled connection, marshals the invocation, and returns the connection to the pool.  On the server side, the ServerThread runs the invocations after finishing the 3rd invocation.
> Total time: 20 seconds.
> I've seen this happen.  The scary thing is that the story scales.  With more threads on the client side and server side, http can run more and more invocations in parallel.  Whereas socket could end up running arbitrarily many invocations serially.
> I think that having MicroSocketClientInvoker return after marshaling a client side oneway invocation isn't a good idea.  Ovidiu's JBREM-685 ( <http://jira.jboss.com/jira/browse/JBREM-685>A server needs redundant information to detect a one way invocation) <http://jira.jboss.com/jira/browse/JBREM-685> gives me a half baked idea.  Maybe these two ways of tagging a oneway invocation could be used to distinguish client side from server side oneway invocations.  Or something like that.
> By the way, I think that there's no problem with server side oneway invocations, since a single ServerInvoker can keep shoving invocations into the threadpool.
> -Ron
> ===============================================================================================================
> Tom Elrod wrote:
> Wow.  Never thought about this happening but makes sense.  So the real problem with the socket client invoker is it pools the connections when doing oneway invocations.  So it is queuing up invocations on the same connection which can only be processed serially on the server side. Think maybe your idea about using the ONEWAY_FLAG on the client can work.  Basically am thinking we'd have to check that before returning the connection to pool and if is set, then throw away the connection (i.e. don't put back in pool).  This way, the next oneway invocation would not be using the same connection and on the server side, should use another available server thread to do the process (so could be in parallel).  However, this means that incur more overhead on the client side to create a new connection for each oneway invocation, but is maybe worth the cost (too bad we can't tell if serve will take a long time or not).
> Let me know what you think.
> ===============================================================================================================
> Ron Sigal wrote:
> It seems to me that MicroSocketClientInvoker could:
> ...
> 2. in the case of client side oneway invocations, either throw away the connection and return immediately, or just not return until it gets a response from the server.  Since it's on a separate thread, maybe the latter is preferable, since then it can return the connection to the pool after it gets a response.
> ===============================================================================================================
> Tom Elrod wrote:
> Yeah, this is a tricky one.  On the one hand, if don't throw away the connection and wait for the server response AND the server takes a long time, would be possible to exhaust the pool since all threads will be stuck waiting for server to respond.  With the new change to the thread pool to use the RUN mode, would mean calling threads would then be making the remote call and would pretty much be negating the purpose of oneway client side (since client caller thread would be making the network call and blocking until gets response).
> However, if do throw away the connection, then means that in general have defeated purpose of connection pooling since will have to create a new connection each time oneway client side invocations are made.  So if the server processes the requests quickly, would be taking on the extra overhead of creating a new client connection for every call.
> Hmmm.  Could go either way if feel strongly one way or another.  However  think I am leaning towards throwing away the connection.
> ===============================================================================================================
> Ron Sigal wrote:
> This might be complicating things, but for client side oneway invocations we could set a short per invocation timeout, say 1 second or 5 seconds.  If the response comes back, put the connection back in the pool.  If it times out, throw the connection away.  Don't know how to pick the timeout, though.
>  
> ===============================================================================================================
> Tom Elrod wrote:
> I hate getting too complicated as tends to increase the chance of introducing bugs.  However, if wanted to do this, would say need to make the timeout very low, i.e. 1 second.

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