Parallel Operation Batching
For better getting your head around you should get the Patterns for Parallel Programming: Understanding and Applying Parallel Patterns with the .NET Framework 4. It's a great source and explains the common ways on how to use the TPL.
Take a look at page 26 (Very small loop bodies). There you'll find this example:
Parallel.ForEach(Partitioner.Create(from, to), range =>
{
for (int i = range.Item1; i < range.Item2; i++)
{
// ... process i
}
});
So the missing piece you're searching is the System.Concurrent.Collections.Partitioner
.
Comments
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Tim Lloyd almost 2 years
Is there built-in support in the TPL (Task-Parallel-Library) for batching operations?
I was recently playing with a routine to carry out character replacement on a character array using a lookup table i.e. transliteration:
for (int i = 0; i < chars.Length; i++) { char replaceChar; if (lookup.TryGetValue(chars[i], out replaceChar)) { chars[i] = replaceChar; } }
I could see that this could be trivially parallelized, so jumped in with a first stab which I knew would perform worse as the tasks were too fine-grained:
Parallel.For(0, chars.Length, i => { char replaceChar; if (lookup.TryGetValue(chars[i], out replaceChar)) { chars[i] = replaceChar; } });
I then reworked the algorithm to use batching so that the work could be chunked onto different threads in less fine-grained batches. This made use of threads as expected and I got some near linear speed up.
I'm sure that there must be built-in support for batching in the TPL. What is the syntax, and how do I use it?
const int CharBatch = 100; int charLen = chars.Length; Parallel.For(0, ((charLen / CharBatch) + 1), i => { int batchUpper = ((i + 1) * CharBatch); for (int j = i * CharBatch; j < batchUpper && j < charLen; j++) { char replaceChar; if (lookup.TryGetValue(chars[j], out replaceChar)) { chars[j] = replaceChar; } } });
Update
After using @Oliver's answer and replacing
Parallel.For
with a Parallel.ForEach and a Partitioner the code is as follows:const int CharBatch = 100; Parallel.ForEach(Partitioner.Create(0, chars.Length, CharBatch), range => { for (int i = range.Item1; i < range.Item2; i++) { char replaceChar; if (lookup.TryGetValue(chars[i], out replaceChar)) { chars[i] = replaceChar; } } });
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Tim Lloyd over 13 yearsThanks for the link, but do you have a specific answer to my question?
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Tim Lloyd over 13 yearsThat's brilliant thanks. Works perfectly and a very nice API too :) I have updated my question with an example based on your answer.