C# Parallel.ForEach in Parallel.ForEach. Does it make sense?

Question

C++ / C#

Good afternoon. The question is purely theoretical - there is such a piece of code:

ParallelOptions _parallel_options = new ParallelOptions();
         _parallel_options.MaxDegreeOfParallelism = _ret.threads_count;

         Parallel.ForEach<string>(File.ReadLines(_ret.file_name, _codepage)
                                  , _parallel_options
                                  , (_line, _parallel_state, _index) =>
         {
            //some code
            Parallel.For(0, _idx, (_field_index) =>
            {
            //some code
            });
         });

Is it possible to get some profit from parallel - a construction inside parallel, if you do not limit the MaxDegreeOfParallelism of the first loop? Or if you limit?

Answer 1

It seems to me that in this case you will run into reading from disk

Answer 2

There was a task, I decided to do it in a similar way. As a result, when using such a structure without creating a second thread for the internal Parallel.ForEach, a performance drop was noticed when executing a piece of code. As a result, for my task, it was decided to use only one Parallel.ForEach.