ACE Tutorial 013
Multiple thread pools

Let's take a look now at the new Task object. This will obviously be different from the Tasks we've created before but I think you'll be surprised at how relatively simple it actually is.

Remember that the goal of this tutorial was to use the reference counting abilities of the ACE_Data_Block. The only way to show that effectively is to have a data block passed between different threads. A thread pool isn't really going to do that so, instead, our new Task can be part of a chain of tasks. In that way, each Task can pass the data on to another and satisfy our need for moving the ACE_Data_Block around. If we've done the reference counting correctly then none of our tasks will be trying to work with deleted data and we won't have any memory leaks at the end.

There's not much to the header, so I've included it and the cpp file on this one page.

task.h

    // $Id: page06.html,v 1.7 1999/04/03 21:56:28 jcej Exp $

    #ifndef TASK_H
    #define TASK_H

    #include "ace/Task.h"

    #if !defined (ACE_LACKS_PRAGMA_ONCE)
    # pragma once
    #endif /* ACE_LACKS_PRAGMA_ONCE */

    #include "mld.h"

    /*
       This is much like the Task we've used in the past for implementing a  thread
       pool.  This time, however, I've made the Task an element in a  singly-linked
       list.  As the svc() method finishes the process() on a  unit of work, it
       will enqueue that unit of work to the next_ Task if  there is one.  If the
       Task does not have a next_ Task, it will invoke the unit of work object's
       fini() method after invoking process().
     */
    class Task : public ACE_Task < ACE_MT_SYNCH >
    {
    public:

        typedef ACE_Task < ACE_MT_SYNCH > inherited;

            // Construct ourselves and an optional number of subtasks
            // chained beyond us.
        Task (int sub_tasks = 0);
        ~Task (void);

            /*
              I really wanted this to be called open() but that was already
              claimed by the Task framework.  start() will kick off our thread
              pool for us.
            */
        int start (int threads = 1);

            // Take Unit_Of_Work objects from the thread pool and invoke
            // their process() and/or fini() as appropriate.
        int svc (void);

            // Shut down the thread pool and it's associated subtasks
        int close (u_long flags = 0);

            // Wait for the pool and subtasks to close
        int wait (void);

    protected:
        ACE_Barrier * barrier_;
        Task *next_;
        MLD;
    };

    #endif

task.cpp

    // $Id: page06.html,v 1.7 1999/04/03 21:56:28 jcej Exp $

    #include "task.h"
    #include "block.h"
    #include "work.h"

    /*
       Construct the Task with zero or more subtasks.  If subtasks are requested,
       we assign our next_ pointer to the first of those and let it worry about
       any remaining subtasks.
     */
    Task::Task (int sub_tasks)
            : barrier_ (0)
             ,next_ (0)
    {
        ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task ctor 0x%x\n", (void *) this));
        if (sub_tasks)
        {
            next_ = new Task (--sub_tasks);
        }
    }

    /*
       Delete our barrier object and any subtasks we may have.
     */
    Task::~Task (void)
    {
        ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task dtor 0x%x\n", (void *) this));

        delete barrier_;
        delete next_;
    }

    /*
       Open our thread pool with the requested number of threads.  If subtasks are
       enabled, they inherit the thread-pool size.  Make sure that the subtasks can
       be opened before we open our own threadpool.
     */
    int Task::start (int threads)
    {
        if (next_)
        {
            if (next_->start (threads) == -1)
            {
                return -1;
            }
        }

        barrier_ = new ACE_Barrier (threads);
        return this->activate (THR_NEW_LWP, threads);
    }

    /*
       Close ourselves and any subtasks.  This just prints a message so that we can
       assure ourselves things are cleaned up correctly.
     */
    int Task::close (u_long flags)
    {
        ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task close 0x%x\n", (void *) this));
        if (next_)
        {
            next_->close (flags);
        }

        return (0);
    }

    /*
       Wait for all of the threads in our pool to exit and then wait for any
       subtasks.  When called from the front of the task chain, this won't return
       until all thread pools in the chain have exited.
     */
    int Task::wait (void)
    {
        inherited::wait ();
        if (next_)
        {
            next_->wait ();
        }
        return (0);
    }

    /*
       Like the thread-pools before, this is where all of the work is done.
     */
    int Task::svc (void)
    {
            // Wait for all threads to get this far before continuing.
        this->barrier_->wait ();

        ACE_DEBUG ((LM_DEBUG, "(%P|%t) Task 0x%x starts in thread %u\n", (void *) this, ACE_Thread::self ()));

            // getq() wants an ACE_Message_Block so we'll start out with one
            // of those.  We could do some casting (or even auto-casting) to
            // avoid the extra variable but I prefer to be clear about our actions.
        ACE_Message_Block *message;

            // What we really put into the queue was our Message_Block.
            // After we get the message from the queue, we'll cast it to this
            // so that we know how to work on it.
        Message_Block *message_block;

            // And, of course, our Message_Block contains our Data_Block
            // instead of the typical ACE_Data_Block
        Data_Block *data_block;

          // Even though we put Work objects into the queue, we take them
          // out using the baseclass pointer.  This allows us to create new
          // derivatives without having to change this svc() method.
        Unit_Of_Work *work;

        while (1)
        {
                // Get the ACE_Message_Block
            if (this->getq (message) == -1)
            {
                ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "getq"), -1);
            }

                // "Convert" it to our Message_Block
            message_block = (Message_Block *) message;

                // Get the ACE_Data_Block and "convert" to Data_Block in one step.
            data_block = (Data_Block *) (message_block->data_block ());

                // Get the unit of work from the data block
            work = data_block->data ();

                // Show the object's instance value and "type name"
            work->who_am_i ();
            work->what_am_i ();

                // If there is a hangup we need to tell our pool-peers as
                // well as any subtasks.
            if (message_block->msg_type () == ACE_Message_Block::MB_HANGUP)
            {
                    // duplicate()ing the message block will increment the
                    // reference counts on the data blocks.  This allows us
                    // to safely release() the message block.  The rule of
                    // thumb is that if you pass a message block to a new
                    // owner, duplicate() it.  Then you can release() when
                    // you're done and not worry about memory leaks.
                if (this->putq (message_block->duplicate ()) == -1)
                {
                    ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "putq"), -1);
                }

                    // If we have a subtask, duplicate() the message block
                    // again and pass it to that task's queue
                if (next_ && next_->putq (message_block->duplicate ()) == -1)
                {
                    ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "putq"), -1);
                }

                    // We're now done with our copy of the block, so we can
                    // release it.  Our peers/subtasks have their own message
                    // block to access the shared data blocks.
                message_block->release ();

                break;
            }

                // If this isn't a hangup/shutdown message then we tell the
                // unit of work to process() for a while.
            work->process ();

            if (next_)
            {
                    // If we have subtasks, we pass the block on to them.  Notice
                    // that I don't bother to duplicate() the block since I won't
                    // release it in this case.  I could have invoked
                    // duplicate() in the puq() and then release()
                    // afterwards.  Either is acceptable.
                if (next_->putq (message_block) == -1)
                    ACE_ERROR_RETURN ((LM_ERROR, "%p\n", "putq"), -1);
            }
            else
            {
                    // If we don't have subtasks then invoke fini() to tell
                    // the unit of work that we won't be invoking process()
                    // any more.  Then release() the block.  This release()
                    // would not change if we duplicate()ed in the above conditional
                work->fini ();
                message_block->release ();
            }

                // Pretend that the work takes some time...
            ACE_OS::sleep (ACE_Time_Value (0, 250));
        }

        return (0);
    }

So you see... it wasn't really that much more complicated. We really just have to remember to pass to next_ when we finish working on the data. If your Unit_Of_Work derivative is going to implement a state machine be sure that you also implement a fini() method or ensure that your chain of subtasks is large enough for all possible states.

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