Threadpool queueuserworkitem with many threads

Question

Heads up: I am not very familiar with working with threadpool, which might be obvious from the following code. I am under the impression that I could push many values into this queue and then it would wait for one thread to complete and then move onto the next and the system would handle the synchronization of how many threads to be running.

I am trying to use ThreadPool::QueueUserWorkItem(waitcallback, num) where the value of num is iterated up to a dynamic value depending on some prior algorithm. The problem I am coming across is the program crashes when it gets too high.

WaitCallback^ wcb = gcnew WaitCallBack(this, &createImage);
for(int i = 0; i < numBlocks; i++)
{
    ThreadPool::QueueUserWorkItem(wcb, i);
}

I get the message "Runtime Error! This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information."

My most resent run through had numBlocks = 644.

Answer 1

It's hard to say what caused the program to crash. Most likely, an exception was thrown in one of the threads, and that brought the program down. You'll have to determine where in your code the exception was thrown.

As you know, ThreadPool::QueueUserWorkItem queues an item to be processed by the threadpool. But there can be multiple threads processing items from that queue. For example, you could have 20 pool threads, with 15 of them processing the work items that you queued.

If you really have that many items to process and you want them done one at a time, why not just queue one thread to do them one at a time. I've never done managed C++, so I won't try to write an example with it. But perhaps you can translate this C# code:

void ProcessInBackground(object state)
{
    int numBlocks = (int)state;
    for (int i = 0; i < numBlocks; ++i)
    {
        createImage(i);
    }
}

And then you can call it with:

ThreadPool::QueueUserWorkItem(ProcessInBackground, numBlocks);

That creates a single thread that will process the items in order.

I suspect you can convert that to managed C++ fairly easily.