Question

I have a totally thread-safe FIFO structure( TaskList ) to store task classes, multiple number of threads, some of which creates and stores task and the others processes the tasks. TaskList class has a pop_front() method which returns the first task if there is at least one. Otherwise it returns NULL.
Here is an example of processing function:

TaskList tlist;

unsigned _stdcall ThreadFunction(void * qwe)
{
    Task * task;
    while(!WorkIsOver) // a global bool to end all threads.
    {
        while(task = tlist.pop_front())
        {
            // process Task
        }
    }
    return 0;
}

My problem is, sometimes, there is no new task in the task list, so the processing threads enters in an endless loop (while(!WorkIsOver)) and CPU load increases. Somehow I have to make the threads wait until a new task is stored in the list. I think about Suspending and Resuming but then I need extra info about which threads are suspending or running which brings a greater complexity to coding.

Any ideas?

PS. I am using winapi, not Boost or TBB for threading. Because sometimes I have to terminate threads that process for too long, and create new ones immediately. This is critical for me. Please do not suggest any of these two.

Thanks

Answer 1

Assuming you are developing this in DevStudio, you can get the control you want using [IO Completion Ports]. Scary name, for a simple tool.

• First, create an IOCompletion Port: CreateIOCompletionPort
• Create your pool of worker threads using _beginthreadex / CreateThread
• In each worker thread, implement a loop that calls GetQueuedCompletionStatus – The returned lpCompletionKey will be pointing to a work item to process.
• Now, whenever you get a work item to process: call PostQueuedCompletionStatus from any thread – passing in the pointer to your work item as the completion key parameter.

Thats it. 3 API calls and you have implemented a thread pooling mechanism based on a kernel implemented queue object. Each call to PostQueuedCompletionStatus will automatically be deserialized onto a thread pool thread thats blocking on GetQueuedCompletionStatus. The pool of worker threads is created, and maintained – by you – so you can call TerminateThread on any worker threads that are taking too long. Even better – depending on how it is set up the kernel will only wake up as many threads as needed to ensure that each CPU core is running at ~100% load.

NB. TerminateThread is really not an appropriate API to use. Unless you really know what you are doing the threads are going to leak their stacks, none of the memory allocated by code on the thread will be deallocated and so on. TerminateThread is really only useful during process shutdown. There are some articles on the net detailing how to release the known OS resources that are leaked each time TerminateThread is called – if you persist in this approach you really need to find and read them if you haven’t already.