Procedura: utilizzare Parallel.Invoke per eseguire operazioni in parallelo

Articolo
08/11/2011

Questo esempio mostra come parallelizzare operazioni tramite ParallelInvoke() in Task Parallel Library. Tre operazioni vengono eseguite in un'origine dati condivisa. Poiché nessuna delle operazioni modifica l'origine, possono essere eseguite in parallelo in modo semplice.

Nota
Questa documentazione utilizza espressioni lambda per definire delegati in TPL.Se non si ha familiarità con le espressioni lambda in C# o Visual Basic, vedere Espressioni lambda in PLINQ e TPL.

Esempio

' How to: Use Parallel.Invoke to Execute Parallel Operations
Option Explicit On
Option Strict On

Imports System.Threading.Tasks
Imports System.Net

Module ParallelTasks

    Sub Main()
        ' Retrieve Darwin's "Origin of the Species" from Gutenberg.org.
        Dim words As String() = CreateWordArray("http://www.gutenberg.org/files/2009/2009.txt")

        '#Region "ParallelTasks"
        ' Perform three tasks in parallel on the source array
        Parallel.Invoke(Sub()
                            Console.WriteLine("Begin first task...")
                            GetLongestWord(words)
                            ' close first Action
                        End Sub,
            Sub()
                Console.WriteLine("Begin second task...")
                GetMostCommonWords(words)
                'close second Action
            End Sub,
            Sub()
                Console.WriteLine("Begin third task...")
                GetCountForWord(words, "species")
                'close third Action
            End Sub)
        'close parallel.invoke
        Console.WriteLine("Returned from Parallel.Invoke")
        '#End Region

        Console.WriteLine("Press any key to exit")
        Console.ReadKey()
    End Sub

#Region "HelperMethods"
    Sub GetCountForWord(ByVal words As String(), ByVal term As String)
        Dim findWord = From word In words _
            Where word.ToUpper().Contains(term.ToUpper()) _
            Select word

        Console.WriteLine("Task 3 -- The word ""{0}"" occurs {1} times.", term, findWord.Count())
    End Sub

    Sub GetMostCommonWords(ByVal words As String())
        Dim frequencyOrder = From word In words _
            Where word.Length > 6 _
            Group By word
            Into wordGroup = Group, Count()
            Order By wordGroup.Count() Descending _
            Select wordGroup

        Dim commonWords = From grp In frequencyOrder
                          Select grp
                          Take (10)

        Dim s As String
        s = "Task 2 -- The most common words are:" & vbCrLf
        For Each v In commonWords
            s = s & v(0) & vbCrLf
        Next
        Console.WriteLine(s)
    End Sub

    Function GetLongestWord(ByVal words As String()) As String
        Dim longestWord = (From w In words _
            Order By w.Length Descending _
            Select w).First()

        Console.WriteLine("Task 1 -- The longest word is {0}", longestWord)
        Return longestWord
    End Function


    ' An http request performed synchronously for simplicity.
    Function CreateWordArray(ByVal uri As String) As String()
        Console.WriteLine("Retrieving from {0}", uri)

        ' Download a web page the easy way.
        Dim s As String = New WebClient().DownloadString(uri)

        ' Separate string into an array of words, removing some common punctuation.
        Return s.Split(New Char() {" "c, ControlChars.Lf, ","c, "."c, ";"c, ":"c, _
        "-"c, "_"c, "/"c}, StringSplitOptions.RemoveEmptyEntries)
    End Function
#End Region


    ' Output (May vary on each execution):
    ' Retrieving from http://www.gutenberg.org/dirs/etext99/otoos610.txt
    ' Response stream received.
    ' Begin first task...
    ' Begin second task...
    ' Task 2 -- The most common words are:
    ' species
    ' selection
    ' varieties
    ' natural
    ' animals
    ' between
    ' different
    ' distinct
    ' several
    ' conditions
    '
    ' Begin third task...
    ' Task 1 -- The longest word is characteristically
    ' Task 3 -- The word "species" occurs 1927 times.
    ' Returned from Parallel.Invoke
    ' Press any key to exit 
    ' 
End Module

namespace ParallelTasks
{
    using System;
    using System.IO;
    using System.Linq;
    using System.Text;
    using System.Threading;
    using System.Threading.Tasks;
    using System.Net;

    class ParallelInvoke
    {
        static void Main()
        {
            // Retrieve Darwin's "Origin of the Species" from Gutenberg.org.
            string[] words = CreateWordArray(@"http://www.gutenberg.org/files/2009/2009.txt");

            #region ParallelTasks
            // Perform three tasks in parallel on the source array
            Parallel.Invoke(() =>
                             {
                                 Console.WriteLine("Begin first task...");
                                 GetLongestWord(words);
                             },  // close first Action

                             () =>
                             {
                                 Console.WriteLine("Begin second task...");
                                 GetMostCommonWords(words);
                             }, //close second Action

                             () =>
                             {
                                 Console.WriteLine("Begin third task...");
                                 GetCountForWord(words, "species");
                             } //close third Action
                         ); //close parallel.invoke

            Console.WriteLine("Returned from Parallel.Invoke");
            #endregion

            Console.WriteLine("Press any key to exit");
            Console.ReadKey();
        }

        #region HelperMethods
        private static void GetCountForWord(string[] words, string term)
        {
            var findWord = from word in words
                           where word.ToUpper().Contains(term.ToUpper())
                           select word;

            Console.WriteLine(@"Task 3 -- The word ""{0}"" occurs {1} times.",
                term, findWord.Count());
        }

        private static void GetMostCommonWords(string[] words)
        {
            var frequencyOrder = from word in words
                                 where word.Length > 6
                                 group word by word into g
                                 orderby g.Count() descending
                                 select g.Key;

            var commonWords = frequencyOrder.Take(10);

            StringBuilder sb = new StringBuilder();
            sb.AppendLine("Task 2 -- The most common words are:");
            foreach (var v in commonWords)
            {
                sb.AppendLine("  " + v);
            }
            Console.WriteLine(sb.ToString());
        }

        private static string GetLongestWord(string[] words)
        {
            var longestWord = (from w in words
                               orderby w.Length descending
                               select w).First();

            Console.WriteLine("Task 1 -- The longest word is {0}", longestWord);
            return longestWord;
        }


        // An http request performed synchronously for simplicity.
        static string[] CreateWordArray(string uri)
        {
            Console.WriteLine("Retrieving from {0}", uri);

            // Download a web page the easy way.
            string s = new WebClient().DownloadString(uri);

            // Separate string into an array of words, removing some common punctuation.
            return s.Split(
                new char[] { ' ', '\u000A', ',', '.', ';', ':', '-', '_', '/' },
                StringSplitOptions.RemoveEmptyEntries);
        }
        #endregion
    }

    /* Output (May vary on each execution):
        Retrieving from http://www.gutenberg.org/dirs/etext99/otoos610.txt
        Response stream received.
        Begin first task...
        Begin second task...
        Task 2 -- The most common words are:
          species
          selection
          varieties
          natural
          animals
          between
          different
          distinct
          several
          conditions

        Begin third task...
        Task 1 -- The longest word is characteristically
        Task 3 -- The word "species" occurs 1927 times.
        Returned from Parallel.Invoke
        Press any key to exit  
     */
}

Notare che con Invoke() basta indicare quali azioni si desidera eseguire simultaneamente e il runtime gestisce tutti i dettagli di pianificazione dei thread, compreso l'adeguamento automatico al numero di core nel computer host.

In questo esempio viene eseguita la parallelizzazione delle operazioni, non dei dati. Un approccio alternativo consiste nel parallelizzare le query LINQ tramite PLINQ ed eseguire le query in sequenza. Un altro approccio alternativo e parallelizzare i dati tramite PLINQ. Infine, un'altra opzione è parallelizzare sia le query sia le attività. Benché l'overhead risultante possa comportare una riduzione delle prestazioni nei computer host aventi un numero di processori relativamente ridotto, tale approccio risulta decisamente più efficiente nei computer con molti processori.