Why decrementing one variable modifies incrementing of another one in C# Parallel.ForEach loop?

Question

Running the code from Parallel.ForEach keeps spawning new threads with a few my modifications

The output with commented line:

//threadsRemaining = Interlocked.Decrement(ref concurrentThreads);

is "obvious", i.e. expected one:

[00:00] Job 0 complete. 2 threads remaining. unsafeCount=2
[00:00] Job 1 complete. 1 threads remaining. unsafeCount=1
[00:00] Job 2 complete. 3 threads remaining. unsafeCount=3
[00:00] Job 3 complete. 4 threads remaining. unsafeCount=4
[00:00] Job 4 complete. 5 threads remaining. unsafeCount=5
[00:00] Job 5 complete. 6 threads remaining. unsafeCount=6
[00:01] Job 6 complete. 7 threads remaining. unsafeCount=7
[00:01] Job 8 complete. 8 threads remaining. unsafeCount=8
[00:01] Job 7 complete. 9 threads remaining. unsafeCount=9
[00:01] Job 9 complete. 10 threads remaining. unsafeCount=10

While the output of the same code upon UNcommenting the above line is:

[00:00] Job 0 complete. 1 threads remaining. unsafeCount=1
[00:00] Job 1 complete. 0 threads remaining. unsafeCount=0
[00:00] Job 3 complete. 0 threads remaining. unsafeCount=0
[00:00] Job 2 complete. 1 threads remaining. unsafeCount=1
[00:00] Job 4 complete. 1 threads remaining. unsafeCount=1
[00:00] Job 5 complete. 1 threads remaining. unsafeCount=1
[00:01] Job 6 complete. 1 threads remaining. unsafeCount=1
[00:01] Job 8 complete. 1 threads remaining. unsafeCount=1
[00:01] Job 9 complete. 1 threads remaining. unsafeCount=1
[00:01] Job 7 complete. 0 threads remaining. unsafeCount=0

Can you explain me why decrementing one variable threadsRemainin stops (or prevents) incrementing another one unsafeCount ?

The code of console app:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace seParallelForEachKeepsSpawningNewThreads
{
  public class Node
  {
    public Node Previous { get; private set; }
    public Node(Node previous)
    {
      Previous = previous;
    }
  }
  public class Program
  {
    public static void Main(string[] args)
    {
      DateTime startMoment = DateTime.Now;
      int concurrentThreads = 0;
      int unsafeCount = 0;

      var jobs = Enumerable.Range(0, 10);
      ParallelOptions po = new ParallelOptions
      {
        MaxDegreeOfParallelism = Environment.ProcessorCount
      };
      Parallel.ForEach(jobs, po, delegate(int jobNr)
      {
        int threadsRemaining = Interlocked.Increment(ref concurrentThreads);
        unsafeCount++;

        int heavyness = jobNr % 9;

        //Give the processor and the garbage collector something to do...
        List<Node> nodes = new List<Node>();
        Node current = null;
        //for (int y = 0; y < 1024 * 1024 * heavyness; y++)
        for (int y = 0; y < 1024 * 4 * heavyness; y++)
        {
          current = new Node(current);
          nodes.Add(current);
        }

        TimeSpan elapsed = DateTime.Now - startMoment;
//*****************
         //threadsRemaining = Interlocked.Decrement(ref concurrentThreads);
        Console.WriteLine("[{0:mm\\:ss}] Job {1} complete. {2} threads remaining. unsafeCount={2}", 
          elapsed, jobNr, threadsRemaining, unsafeCount);
      });
      Console.WriteLine("FINISHED");
      Console.ReadLine();
    }
  }
}

score 5 · Accepted Answer · answered Mar 13 '13 at 08:23

5

This is the problem:

Console.WriteLine(
    "[{0:mm\\:ss}] Job {1} complete. {2} threads remaining. unsafeCount={2}", 
    elapsed, jobNr, threadsRemaining, unsafeCount);

The final part should be {3}, not {2}. You're just printing out threadsRemaining twice at the moment...

answered Mar 13 '13 at 08:23

Jon Skeet

1,261,211
792
8,724
8,929

1

dammit, the one bit of code I *wasn't* looking at; easily done, though. – Marc Gravell Mar 13 '13 at 08:24
Oops...thanks, that was fast, can accept answer in 6 min though – Fulproof Mar 13 '13 at 08:28

Why decrementing one variable modifies incrementing of another one in C# Parallel.ForEach loop?

1 Answers1