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A thread pool solution involves spawning a number of threads at server startup and then assigning incoming requests to non-busy threads as they arrive If all threads in the pool are already busy handling requests, either the request can be queued until a thread becomes available to handle it, or new threads can be created and added to the pool This model works well for servers that want to bound their request-handling resources, because all the necessary threads and queues can be allocated at program start-up One drawback to this approach is that switching requests from one thread to another via a queue can result in excessive thread context switching overhead Also, if the server allocates insufficient resources to handle the volume of requests it receives, queues could become filled, and incoming requests might have to be temporarily rejected Detailed analyses of other threading models and variations on the models described here can be found in [38] 2142 POA Multithreading Issues After the ORB core dispatches a request to the POA where the target object is located, the threading policy of the POA must be taken into account As explained in Section 1147, a POA can have either the SINGLE_THREAD_MODEL value or the ORB_CTRL_MODEL value for its ThreadPolicy How the POA completes the request dispatch to the appropriate servant depends entirely on its ThreadPolicy value When a POA has the SINGLE_THREAD_MODEL policy value, it guarantees that all servant invocations will be serialized Even if the underlying ORB and other POAs in the same server use multiple threads, a POA created with the SINGLE_THREAD_MODEL policy value never performs request dispatching on multiple servants simultaneously If the underlying ORB is multithreaded, a SINGLE_THREAD_MODEL POA must be able to switch incoming requests onto the single thread it uses for all servant dispatching Application designers should beware that switching requests from one thread to another results in thread context switching overhead Also, on some platforms SINGLE_THREAD_MODEL POAs must perform their dispatching using the main thread (the one in which the program's main function was invoked); otherwise, calling code that is unaware of multithreading (perhaps because it was not compiled with the proper options) will not work correctly In this case, it is the responsibility of the application to invoke ORB::perform_work or ORB::run to ensure that the ORB allows the POAs to get access to the main thread When you perform collocated requests from within the same application on objects in SINGLE_THREAD_MODEL POAs, be sure that your vendor's POA implementation properly conforms to the specification Even requests made locally must be dispatched on the POA's single thread rather than dispatched directly using the caller's thread If your ORB bypasses the dispatching mechanisms for collocated requests when calling objects in SINGLE_THREAD_MODEL POAs, perhaps by invoking virtual functions directly on the servant, it does not conform to the specification
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POAs created with the ORB_CTRL_MODEL threading policy value are far less constrained with respect to request dispatching than their single-threaded counterparts The ORB_CTRL_MODEL policy value only implies that the POA is allowed to dispatch multiple requests concurrently; it does not prescribe how requests are assigned to threads This means that an ORB_CTRL_MODEL POA might be implemented to use its own threading policy independent of that of the underlying ORB, or it might be implemented to fit seamlessly with the model used by the ORB The POA might use the thread pool model In that case, it has its own pool of threads and its own queue for holding requests when all its threads are busy This approach works well except for the price of switching requests from one thread to another Also, there is always the possibility that the volume of requests that the POA receives will far outstrip the dispatching capacity of its thread pool, thus requiring requests to be temporarily rejected A POA might use a thread-per-servant approach, creating a new thread for each servant added to its Active Object Map In this approach, the POA dispatches all requests for a given servant on that servant's thread This technique performs well if the set of servants in a POA is relatively fixed and small Otherwise, a POA that has either many registered servants or many servants that are only briefly registered and then destroyed may incur too much thread creation overhead, or it may try to create too many threads at once Using the thread-per-request model means that the POA creates a new thread for each incoming request This approach works well only if the POA receives a relatively low volume of long-running requests Otherwise, the overhead of creating many threads or having too many threads active simultaneously becomes too great A POA might simply continue the request dispatch on the same thread that the ORB used to dispatch the request from the network port it was received on This approach avoids the cost of switching the request processing from one thread to another, but it ties up an ORB thread for the duration of the request and prevents it from being used to do other ORB work Each POA in a group of POAs sharing a single POAManager might rely on it to supply a request-dispatching thread This technique merely pushes the multithreading issues from the POA to the POAManager, and it might also mean increased contention for threading resources if the POAManager is controlling request flow for multiple POAs Other request-dispatching strategies are also possible for ORB_CTRL_MODEL POAs Depending on their policies, different POAs in the same server application might even employ different strategies Because the POA specification does not require any particular threading model for ORB_CTRL_MODEL POAs, POA implementers are permitted to use any of these approaches, or any other approach that they deem useful Although this arrangement provides maximum flexibility for POA implementers, it makes it impossible to write portable applications that make assumptions about the underlying POA threading model
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