threadpool-dynamic.hpp

_status :
    HANGUP, // nothing to do
    INCING, // increasing threads
    DECING, // decreasing threads


manager_thread_proc {
    while (_run) {
        CondtionLock guard(ctrlCond);
        while (_run && _status != INCING) { // no event
            ctrlCond.wait();
        }
        if (!_run) break;
        if (_status == INCIING) { // create new threads
            for (i <- 0 to threadCnt) {
                new Thread()
                sleep(1000) // to make cpu smooth
            }
        }
    }
}

// called by worker thread after `doWork'
inc_active {
    CondtionLock guard(ctrlCond);
    if (_status == HANGUP) {
        _status = INCING;
        ctrlCond.signal(); 
    }
}

pushWorker {
    if (queue.size > maxQueueSize / 2) { // busy queue
        freeStartTime = 0;
    } else if (freeStartTime == 0)
        freeStartTime = now();
    // ...
}

pop {
    if (_decCount > 0 && freeStartTime + threhold < now()) { // free for a while
        CondtionLock guard(ctrlCond);
        if (_status == HANGUP) {
            // which means it's safe to decreasing threads
            _status = DECING;
            _decCount = DEC_COUNT;
        }         
    }
    if (_decCount-- > 0) {
        // return a special flag to tell the calling thread to exit after processed the queue item
    } else {
        CondtionLock guard(ctrlCond);
        // decreasing done
        _status = HANGUP;  
    }
    // other normal stuff...
}

worker_thread_proc {
    while (_run) {
        // signal.wait
        item = pop(&type);
        doWork(item);
        if (type == exit)
            break; // exit this worker thread
    }
}