如何在 C# 中高效地并行运行代码？

Question

I thought that my problem is the following -- how to prepare for parallel execution?

My current code resemble this pattern:

  var worker = new Worker();
  while (true)
    worker.RandomTest();

RandomTest is mostly CPU-bound. On rare occasions, workers will write to a file (after obtaining a lock on it), but this is so infrequent that I/O delays shouldn't need to be considered.

I would like to run this in parallel, I could use Parallel.For and use some big number as an upper limit (instead while ), but I have problems how to prepare for such execution, because I don't want to create worker at each iteration.

To prepare I would need to know parallel pool size in advance and also inside loop the index of the thread/task/job so I would know how to associate current execution path with the worker.

---

As it turns out my problem is more fundamental. I couldn't figure out how to make preparation, so since I have endless loop I though why not:

  Parallel.Loop(0,1000,(i,_) => {
    var worker = new Worker();
    while (true)
      worker.RandomTest();
  }

Till now I assumed the launched executions will be in sane limits in comparison to available CPU cores. But no -- Parallel creates new iterations like crazy, so already created jobs are basically stalled because of the incoming flood of the new ones.

Of course I can hardcode the fixed number how many parallel jobs to run, but then the responsibility of figuring out how much is not too much and not too little is on me.

I know how to put a fixed number, but how to put a good number? Ie when I run program on different machine or in different conditions (for example with CPU-demanding process in the background)?

Here Worker is not thread safe type, thus I create as many worker as there are jobs to do.

Answer 1

One way to solve this problem is to start a number of threads equal to the number of cores, and do an infinite while loop in each thread:

for (int i = 0; i < Environment.ProcessorCount; i++)
{
    _ = new Thread(() =>
    {
        Worker worker = new();
        while (true) worker.RandomTest();
    });
}

These threads are foreground threads, so they'll keep the program alive even after exiting the Main method. In case any of the loops fail, the whole program will crash immediately with an unhandled exception. This primitive behavior might be closer to what you want to do, than the sophisticated Parallel class with its obscure partitioners.

Regarding the optimal degree of parallelism, for CPU-bound workloads it is usually Environment.ProcessorCount . With this configuration the parallel execution will use all the cores of your machine, without too much thread-switching.