Parallel.ForEach should not be used for IO bound tasks

docs/standard/parallel-programming/how-to-write-a-simple-parallel-foreach-loop.md

@@ -1,7 +1,7 @@
 ---
 title: Write a simple parallel program using Parallel.ForEach
 description: In this article, learn how to enable data parallelism in .NET. Write a Parallel.ForEach loop over any IEnumerable or IEnumerable<T> data source.
-ms.date: 02/14/2019
+ms.date: 02/23/2021
 dev_langs:
   - "csharp"
   - "vb"
@@ -19,7 +19,7 @@ This example shows how to use a <xref:System.Threading.Tasks.Parallel.ForEach%2A
 
 ## Example
 
-This example assumes you have several .jpg files in a *C:\Users\Public\Pictures\Sample Pictures* folder and creates a new sub-folder named *Modified*. When you run the example, it rotates each .jpg image in *Sample Pictures* and saves it to *Modified*. You can modify the two paths as necessary.
+This example showcases the power of Parallel.ForEach for CPU intensive operations. Only for demonstration purpose we have picked up operation around Prime numbers. When you run the example, it randomly generates 2 million numbers and tries to filter only Prime Numbers. In first case we iterate over the collection via simple for loop, in second case we iterate over the collection via Parallel.ForEach. In the end we display the time taken in both the cases.
 
 [!code-csharp[TPL_Parallel#03](../../../samples/snippets/csharp/VS_Snippets_Misc/tpl_parallel/cs/simpleforeach.cs#03)]
 [!code-vb[TPL_Parallel#03](../../../samples/snippets/visualbasic/VS_Snippets_Misc/tpl_parallel/vb/simpleforeach.vb#03)]
@@ -43,14 +43,6 @@ In Visual Studio, there are Visual Basic and C# console application templates fo
 
 From the command line, you can use either the .NET Core CLI commands (for example, `dotnet new console` or `dotnet new console -lang vb`), or you can create the file and use the command-line compiler for a .NET Framework application.
 
-For a .NET Core project, you must reference the **System.Drawing.Common** NuGet package. In Visual Studio, use the NuGet Package Manager to install the package. Alternatively, you can add a reference to the package in your \*.csproj or \*.vbproj file:
-
-```xml
-<ItemGroup>
-     <PackageReference Include="System.Drawing.Common" Version="4.5.1" />
-</ItemGroup>
-```
-
 To run a .NET Core console application from the command line, use `dotnet run` from the folder that contains your application.
 
 To run your console application from Visual Studio, press **F5**.

samples/snippets/csharp/VS_Snippets_Misc/tpl_parallel/cs/simpleforeach.cs

@@ -1,40 +1,112 @@
 // <snippet03>
 using System;
-using System.IO;
-using System.Threading;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
 using System.Threading.Tasks;
-using System.Drawing;
 
-public class Example
+namespace ParallelExample
 {
-    public static void Main()
+    class Program
     {
-        // A simple source for demonstration purposes. Modify this path as necessary.
-        string[] files = Directory.GetFiles(@"C:\Users\Public\Pictures\Sample Pictures", "*.jpg");
-        string newDir = @"C:\Users\Public\Pictures\Sample Pictures\Modified";
-        Directory.CreateDirectory(newDir);
-
-        // Method signature: Parallel.ForEach(IEnumerable<TSource> source, Action<TSource> body)
-        Parallel.ForEach(files, (currentFile) =>
-                                {
-                                    // The more computational work you do here, the greater
-                                    // the speedup compared to a sequential foreach loop.
-                                    string filename = Path.GetFileName(currentFile);
-                                    var bitmap = new Bitmap(currentFile);
-
-                                    bitmap.RotateFlip(RotateFlipType.Rotate180FlipNone);
-                                    bitmap.Save(Path.Combine(newDir, filename));
-
-                                    // Peek behind the scenes to see how work is parallelized.
-                                    // But be aware: Thread contention for the Console slows down parallel loops!!!
-
-                                    Console.WriteLine($"Processing {filename} on thread {Thread.CurrentThread.ManagedThreadId}");
-                                    //close lambda expression and method invocation
-                                });
-
-        // Keep the console window open in debug mode.
-        Console.WriteLine("Processing complete. Press any key to exit.");
-        Console.ReadKey();
+        static void Main(string[] args)
+        {
+            // 2 million
+            int limit = 2 * 1000000;
+            var inputs = new List<int>(limit);
+            Random radomGenerator = new Random();
+            for (int index = 0; index < limit; index++)
+            {
+                inputs.Add(radomGenerator.Next());
+            }
+
+            var watch = new Stopwatch();
+            watch.Start();
+            var primeNumbers = GetPrimeList(inputs);
+            watch.Stop();
+
+            var watchForParallel = new Stopwatch();
+            watchForParallel.Start();
+            var primeNumbersFromParallel = GetPrimeListWithParallel(inputs);
+            watchForParallel.Stop();
+
+            Console.WriteLine($"Classical For loop    | Total prime numbers : {primeNumbersFromParallel.Count} | Time Taken : {watch.ElapsedMilliseconds} ms.");
+            Console.WriteLine($"Parallel.ForEach loop | Total prime numbers : {primeNumbersFromParallel.Count} | Time Taken : {watchForParallel.ElapsedMilliseconds} ms.");
+
+            Console.WriteLine("Press any key to exit.");
+            Console.ReadLine();
+        }
+
+        /// <summary>
+        /// GetPrimeList returns Prime numbers by using sequential ForEach
+        /// </summary>
+        /// <param name="inputs"></param>
+        /// <returns></returns>
+        static IList<int> GetPrimeList(IList<int> inputs)
+        {
+            var primeNumbers = new List<int>();
+
+            foreach (var item in inputs)
+            {
+                if (IsPrime(item))
+                {
+                    primeNumbers.Add(item);
+                }
+            }
+
+            return primeNumbers;
+        }
+
+        /// <summary>
+        /// GetPrimeListWithParallel returns Prime numbers by using Parallel.ForEach
+        /// </summary>
+        /// <param name="inputs"></param>
+        /// <returns></returns>
+        static IList<int> GetPrimeListWithParallel(IList<int> inputs)
+        {
+            var primeNumbers = new ConcurrentBag<int>();
+
+            Parallel.ForEach(inputs, item =>
+            {
+                if (IsPrime(item))
+                {
+                    primeNumbers.Add(item);
+                }
+            });
+
+            return primeNumbers.ToList();
+        }
+
+        /// <summary>
+        /// IsPrime returns true if number is Prime, else false.(https://en.wikipedia.org/wiki/Prime_number)
+        /// </summary>
+        /// <param name="number"></param>
+        /// <returns></returns>
+        static bool IsPrime(int number)
+        {
+
+            if (number <= 1)
+            {
+                return false;
+            }
+
+            if (number == 2 || number % 2 == 0)
+            {
+                return true;
+            }
+
+            int limit = (int)Math.Floor(Math.Sqrt(number));
+
+            for (int index = 3; index <= limit; index += 2)
+            {
+                if (number % index == 0)
+                {
+                    return false;
+                }
+            }
+            return true;
+        }
     }
 }
 // </snippet03>

samples/snippets/visualbasic/VS_Snippets_Misc/tpl_parallel/vb/simpleforeach.vb

@@ -1,37 +1,83 @@
 '<snippet03>
-Imports System.IO
-Imports System.Threading
-Imports System.Threading.Tasks
-Imports System.Drawing
+Imports System.Collections.Concurrent
 
-Module ForEachDemo
+Namespace ParallelExample
+    Class Program
+        Shared Sub Main()
 
-    Sub Main()
-        ' A simple source for demonstration purposes. Modify this path as necessary.
-        Dim files As String() = Directory.GetFiles("C:\Users\Public\Pictures\Sample Pictures", "*.jpg")
-        Dim newDir As String = "C:\Users\Public\Pictures\Sample Pictures\Modified"
-        Directory.CreateDirectory(newDir)
+            '2 million
+            Dim limit As Integer = 2 * 1000000
+            Dim inputs = New List(Of Integer)(limit)
+            Dim radomGenerator As Random = New Random()
 
-        Parallel.ForEach(files, Sub(currentFile)
-                                    ' The more computational work you do here, the greater 
-                                    ' the speedup compared to a sequential foreach loop.
-                                    Dim filename As String = Path.GetFileName(currentFile)
-                                    Dim bitmap As New Bitmap(currentFile)
+            For index As Integer = 0 To limit - 1
+                inputs.Add(radomGenerator.[Next]())
+            Next
 
-                                    bitmap.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone)
-                                    bitmap.Save(Path.Combine(newDir, filename))
+            Dim watch = New Stopwatch()
+            watch.Start()
+            Dim primeNumbers = GetPrimeList(inputs)
+            watch.Stop()
 
-                                    ' Peek behind the scenes to see how work is parallelized.
-                                    ' But be aware: Thread contention for the Console slows down parallel loops!!!
+            Dim watchForParallel = New Stopwatch()
+            watchForParallel.Start()
+            Dim primeNumbersFromParallel = GetPrimeListWithParallel(inputs)
+            watchForParallel.Stop()
 
-                                    Console.WriteLine($"Processing {filename} on thread {Thread.CurrentThread.ManagedThreadId}")
-                                    'close lambda expression and method invocation
-                                End Sub)
+            Console.WriteLine($"Classical For loop    | Total prime numbers : {primeNumbersFromParallel.Count} | Time Taken : {watch.ElapsedMilliseconds} ms.")
+            Console.WriteLine($"Parallel.ForEach loop | Total prime numbers : {primeNumbersFromParallel.Count} | Time Taken : {watchForParallel.ElapsedMilliseconds} ms.")
 
+            Console.WriteLine("Press any key to exit.")
+            Console.ReadLine()
+        End Sub
 
-        ' Keep the console window open in debug mode.
-        Console.WriteLine("Processing complete. Press any key to exit.")
-        Console.ReadKey()
-    End Sub
-End Module
+        ' GetPrimeList returns Prime numbers by using sequential ForEach
+        Private Shared Function GetPrimeList(inputs As IList(Of Integer)) As IList(Of Integer)
+            Dim primeNumbers = New List(Of Integer)()
+
+            For Each item In inputs
+
+                If IsPrime(item) Then
+                    primeNumbers.Add(item)
+                End If
+            Next
+
+            Return primeNumbers
+        End Function
+
+        ' GetPrimeListWithParallel returns Prime numbers by using Parallel.ForEach
+        Private Shared Function GetPrimeListWithParallel(inputs As IList(Of Integer)) As IList(Of Integer)
+            Dim primeNumbers = New ConcurrentBag(Of Integer)()
+            Parallel.ForEach(inputs, Sub(item)
+
+                                         If IsPrime(item) Then
+                                             primeNumbers.Add(item)
+                                         End If
+                                     End Sub)
+            Return primeNumbers.ToList()
+        End Function
+
+        ' IsPrime returns true if number is Prime, else false.(https://en.wikipedia.org/wiki/Prime_number)
+        Private Shared Function IsPrime(number As Integer) As Boolean
+            If number <= 1 Then
+                Return False
+            End If
+
+            If number = 2 OrElse number Mod 2 = 0 Then
+                Return True
+            End If
+
+            Dim limit As Integer = CInt(Math.Floor(Math.Sqrt(number)))
+
+            For index As Integer = 3 To limit Step 2
+
+                If number Mod index = 0 Then
+                    Return False
+                End If
+            Next
+
+            Return True
+        End Function
+    End Class
+End Namespace
 '</snippet03>