c# - Why isn't Parallel.ForEach running multiple threads?

Question

Today i tried do some optimization to foreach statement, that works on XDocument.

Before optimization:

foreach (XElement elem in xDoc.Descendants("APSEvent").ToList())
{
    //some operations
}

After optimization:

Parallel.ForEach(xDoc.Descendants("APSEvent").ToList(), elem =>
{
    //same operations
});

I saw that .NET in Parallel.ForEach(...) opened ONLY one thread! As a result the timespan of Parallel was bigger than standard foreach.

Why do you think .NET only opened 1 thread? Because of locking of file? Thanks

Answer 1

It's by design that Parallel.ForEach may use fewer threads than requested to achieve better performance. According to MSDN [link]:

By default, the Parallel.ForEach and Parallel.For methods can use a variable number of tasks. That's why, for example, the ParallelOptions class has a MaxDegreeOfParallelism property instead of a "MinDegreeOfParallelism" property. The idea is that the system can use fewer threads than requested to process a loop.

The .NET thread pool adapts dynamically to changing workloads by allowing the number of worker threads for parallel tasks to change over time. At run time, the system observes whether increasing the number of threads improves or degrades overall throughput and adjusts the number of worker threads accordingly.