how to chose concurrency level of a concurrent dictionary

Question

I'm using a concurrent dictionary to store about two million records and want to know what to initialize the concurrency level of my dictionary to.

The MSDN page has the following comment in its example code:

The higher the concurrencyLevel, the higher the theoretical number of operations that could be performed concurrently on the ConcurrentDictionary. However, global operations like resizing the dictionary take longer as the concurrencyLevel rises.

This is the best I could find that explains what the concurrencyLevel means and yet it is still very vague.

Answer 1

The most important thing to understand is that even if you have more concurrent accesses than the concurrencyLevel, operations will still be thread-safe. That is, setting concurrencyLevel is a matter of performance, not correctness.

concurrencyLevel specifies the number of independent locks which are available for map operations. If you have n threads concurrently accessing the dictionary, setting concurrencyLevel higher than n will not yield any additional benefit. In general, the optimal value for concurrencyLevel will be significantly lower than the number of worker threads, though, since a given worker won't spend the majority of its time accessing the dictionary.

Profiling your application is really the only way to determine the optimal concurrencyLevel. Setting it to the expected number of worker threads is a good start, though.