CODEWITHSUNDEEP
c#.netmultithreadingperformance
Reid
Reid

Reputation: 4534

How to "Free" a thread

I have 20 threads running at a time in my program, (create 20 wait for them to finish, start another 20), after a while my program slows way down. Do I need to free the tasks or do anything special? If so how, if not is there a common reason why a program like this would slow down?

Upvotes: 4

Views: 3983

Answers (2)

Ed Bayiates
Ed Bayiates

Reputation: 11210

Reed's answer is probably the best way to deal with your issue; however, if you do want to manage the threads yourself, and not use the ThreadPool or TPL, I'd have to ask why you would let 20 threads die and create 20 more. Creating threads is an expensive process, which is why the thread pool exists. If you continually have the same number of parallel tasks, or a maximum number, they should be created once and reused. You can use locking constructs such as semaphore and mutex and have the threads wait when they are done, and just give them new data to work with and release them to proceed again. Waiting on a lock is a very inexpensive operation -- orders of magnitude cheaper than recreating a thread.

So for example, a thread might look like this (pseudocode):

while (program_not_ending)
{
    wait_for_new_data_release;     // Wait on thread's personal mutex

    process_new_data;

    resignal_my_mutex;             // Cause the beginning of loop to wait again

    release_semaphore_saying_I_am_done;   // Increment parent semaphore count
}

The parent would then wait for its semaphore to fill up that 20 threads completed, reset the data buckets, and clear all of the thread mutexes.

Upvotes: 1

Reed Copsey
Reed Copsey

Reputation: 564413

You might want to consider using the ThreadPool, either directly, or via the Task Parallel Library (my preferred option). This is likely a better, simpler, and cleaner design than spawning your own threads and blocking on them repeatedly.

That being said, if your program is getting progressively slower, this is something where a profiler can help dramatically. Without seeing code, it's very difficult to diagnose. For example, depending on the work that you're doing, you may be causing the GC to become less efficient over time, which could cause the % of time spent in GC to climb as the program continues its execution. Profiling should give you a good indication of what is taking time as your program executes.

Upvotes: 12

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