Mutithreading thread control

Question

Hello everyone,

How do I control the number of threads that my program is working on?

I have a program that is now ready for mutithreading but one problem is that the program is extremely memory intensive and i have to limit the number of threads running so that i don't run out of ram. The main program goes through and creates a whole bunch of handles and associated threads in suspended state.

I want the program to activate a set number of threads and when one thread finishes, it will automatically unsuspended the next thread in line until all the work has been completed. How do i do this?

Someone has once mentioned something about using a thread handler, but I can't seem to find any information about how to write one or exactly how it would work.

If anyone can help, it would be greatly appreciated.

Using windows and visual c++. Note: i don't need to worry about the traditional problems of access with the threads, each one is completely independent of each other, its more of like batch processing rather than true mutithreading of a program.

Thanks,

-Faken

Answer 1

Don't create threads explicitly. Create a thread pool, see Thread Pools and queue up your work using QueueUserWorkItem. The thread pool size should be determined by the number of hardware threads available (number of cores and ratio of hyperthreading) and the ratio of CPU vs. IO your work items do. By controlling the size of the thread pool you control the number of maximum concurrent threads.

Answer 2

There is so much to say here.
There are a few ways
You should only create as many thread handles as you plan on running at the same time, then reuse them when they complete. (Look up thread pool).
This guarantees that you can never have too many running at the same time. This raises the question of funding out when a thread completes. You can have a callback be called just before a thread terminates where a parameter in that callback is the thread handle that just finished. Use Boost bind and boost signals for that. When the callback is called, look for another task for that thread handle and restart the thread. That way all you have to do is add to the "tasks to do" list and the callback will remove the tasks for you. No polling needed, and no worries about too many threads.

Answer 3

A Suspended thread doesn't use CPU resources, but it still consumes memory, so you really shouldn't be creating more threads than you want to run simultaneously.

It is better to have only as many threads as your maximum number of simultaneous tasks, and to use a queue to pass units of work to the pool of worker threads.

You can give work to the standard pool of threads created by Windows using the Windows Thread Pool API.

Be aware that you will share these threads and the queue used to submit work to them with all of the code in your process. If, for some reason, you don't want to share your worker threads with other code in your process, then you can create a FIFO queue, create as many threads as you want to run simultaneously and have each of them pull work items out of the queue. If the queue is empty they will block until work items are added to the queue.