C# Splitting loop on multiple threads

Question

I have a task that essentially loops through a collection and does an operation on them in pairs (for int i = 0; i < limit; i+=2 etc.) And so, most suggestions I see on threading loops use some sort of foreach mechanism. But that seems a bit tricky to me, seeing as how I use this approach of operating in pairs.

So what I would want to do is essentially replace:

DoOperation(list.Take(numberToProcess));

with

Thread lowerHalf = new Thread(() => => DoOperation(list.Take(numberToProcess/2)));

Thread lowerHalf = new Thread(() => => DoOperation(list.getRange(numberToProcess/2, numberToProcess));

lowerHalf.Start();
upperHalf.Start();

And this seems to get the work done, but it's VERY slow. Every iteration is slower than the previous one, and when I debug, the Thread view shows a growing list of Threads.

But I was under the impression that Threads terminated themselves upon completion? And yes, the threads do complete. The DoOperation() method is pretty much just a for loop.

So what am I not understanding here?

You could just do .Take(halfCount) for the lower half, then .Skip(halfCount) on the upper half. — misthema, Nov 09 '16 at 13:04

score 5 · Accepted Answer · answered Jun 08 '14 at 15:26

5

Try Parallel.For It will save lot of work.

answered Jun 08 '14 at 15:26

Pranit Kothari

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I can't seem to get it to work. I try to run a loop that would trigger this threading behaviour 18 times, but after 1-7 (very fast) runs, the program seems to be stuck somewhere. – Christofer Ohlsson Jun 08 '14 at 21:55
Seriously, this x 100000 – sircodesalot Jun 09 '14 at 13:27

score 2 · Answer 2 · edited May 23 '17 at 12:17

2

To explain pranitkothari's answer a little bit more and give a different example you can use

list.AsParallel().ForAll(delegate([ListContainingType] item) {
    // do stuff to a single item here (whatever is done in DoOperation() in your code
    // except applied to a single item rather than several)
});

For instance, if I had a list string, it would be

List<String> list = new List<String>();
list.AsParallel().ForAll(delegate(String item) {
    // do stuff to a single item here (whatever is done in DoOperation() in your code
    // except applied to a single item rather than several)
});

This will let you perform an operation for each item in the list on a separate thread. It's simpler in that it handles all the "multi-threadedness" for you.

This is a good post that explains one of the differences in them

edited May 23 '17 at 12:17

Community

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answered Jun 08 '14 at 15:29

Ben Black

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No luck so far with this approach. Lots of squiggly red lines. It says the type for the argument to the ForAll() call cannot be inferred. Is it because it is a custom object and not a primitive? – Christofer Ohlsson Jun 08 '14 at 21:59
That shouldn't matter, as long as you replace `[ListContainingType]` with the type that is contained in the list, then it should work. – Ben Black Jun 09 '14 at 13:23
I did that. But don't have the code available to me at the moment. Will have a look later on. Maybe I made some other silly mistake. – Christofer Ohlsson Jun 09 '14 at 13:42
Gotcha, when you get it, go ahead and update your question with what you've tried so we can see what exactly is happening. – Ben Black Jun 09 '14 at 13:44

C# Splitting loop on multiple threads

2 Answers2