Thread with the lowest priority is invoked more times

Question

The following code 5 threads with different priorities are competing for access to the CPU with 8 cores (Mac OS X 10.8.5, Mono). Each thread increases its counter.

using System;
using System.Threading;

      class PriorityTesting 
     { 
       static long[] counts; 
       static bool finish;

       static void ThreadFunc(object iThread) 
       { 
         while(true) 
         { 
           if(finish) 
              break; 
           counts[(int)iThread]++; 
         } 
       }

       static void Main() 
       { 
         counts = new long[5]; 
         Thread[] t = new Thread[5]; 
         for(int i=0; i<t.Length; i++)  
         { 
           t[i] = new Thread(ThreadFunc); 
           t[i].Priority = (ThreadPriority)i; 
         } 
         // Запускаем потоки 
         for(int i=0; i<t.Length; i++) 
           t[i].Start(i);

         // Даём потокам возможность поработать 10 c 
         Thread.Sleep(10000);

         // Сигнал о завершении 
         finish = true;

         // Ожидаем завершения всех потоков 
         for(int i=0; i<t.Length; i++) 
           t[i].Join(); 
         // Вывод результатов 
         for(int i=0; i<t.Length; i++) 
           Console.WriteLine("Thread with priority {0, 15}, Counts: {1}", (ThreadPriority)i, counts[i]); 
       }   
     }

Compilation:

$ mcs PriorityTesting.cs
$ mono PriorityTesting.exe

Output:

Thread with priority          Lowest, Counts: 178544880
Thread with priority     BelowNormal, Counts: 167783608
Thread with priority          Normal, Counts: 160593225
Thread with priority     AboveNormal, Counts: 79123315
Thread with priority         Highest, Counts: 81623159

How is it that the thread with the lowest priority is invoked more times than threads with the highest priority?

UPD:

The same code on the CPU with 2 cores gives (Windows, .NET):

 Thread with priority         Lowest, Counts:    7608195 
 Thread with priority    BelowNormal, Counts:   10457706 
 Thread with priority         Normal, Counts:   17852629 
 Thread with priority    AboveNormal, Counts:  297729812 
 Thread with priority        Highest, Counts:  302506232

Why the difference?

Answer 1

The Priority support is not implemented in Mono, so the behaviour you're seeing about the Lowest one being invoked more times may be mere luck.

There's a pull request from a contributor implementing this property, in github, so you may want to join him in asking for a review for it.

UPDATE: This answer may be outdated. As Mono advances rapidly, it's better that you re-test your program, maybe Priority works nowadays.