ガベージコレクションの通知

[アーティクル]
08/09/2011

状況によっては、共通言語ランタイムによるフルガベージコレクションによって、パフォーマンスに悪影響が出ることがあります。これは、特に大量の要求を処理するサーバーで問題になります。このような環境では、長時間のガベージコレクションにより要求がタイムアウトします。重要な処理を実行している最中にフルガベージコレクションが実行されないようにするために、フルガベージコレクションが近づいていることが通知されます。これを受けて、作業負荷を別のサーバーインスタンスにリダイレクトするためのアクションを実行できます。現在のサーバーインスタンスが要求を処理する必要がなければ、ガベージコレクションをユーザーが強制的に実行することもできます。

メモ
この機能は、同時実行ガベージコレクションが無効になっている場合にのみ使用できます。既定では、ホストされた環境を使用中で、ホスト側で構成が変更されていない限り、同時実行ガベージコレクションは有効になっています。サーバーのガベージコレクションを使用する場合、同時実行ガベージコレクションは使用できません。同時実行ガベージコレクションの実行中もメモリ割り当てが可能なため、この機能では、同時実行ガベージコレクションがサポートされていません。同時実行ガベージコレクションを無効にする方法については、ランタイム設定 <gcConcurrent> を参照してください。

この機能は、同時実行ガベージコレクションが無効になっている場合にのみ使用できます。既定では、ホストされた環境を使用中で、ホスト側で構成が変更されていない限り、同時実行ガベージコレクションは有効になっています。サーバーのガベージコレクションを使用する場合、同時実行ガベージコレクションは使用できません。同時実行ガベージコレクションの実行中もメモリ割り当てが可能なため、この機能では、同時実行ガベージコレクションがサポートされていません。同時実行ガベージコレクションを無効にする方法については、ランタイム設定 <gcConcurrent> を参照してください。

RegisterForFullGCNotification メソッドは、フルガベージコレクションが近づいていることをランタイムが検知したときに通知を発生するように登録します。この通知には、フルガベージコレクションが近づいている場合と、フルガベージコレクションが完了した場合の 2 つがあります。

通知が発生したことを判断するには、WaitForFullGCApproach メソッドおよび WaitForFullGCComplete メソッドを使用します。一般に、これらのメソッドを while ループ内で使用し、通知の状態を表す GCNotificationStatus 列挙型を継続的に取得します。その値が Succeeded であれば、以下のいずれかの処理を実行します。

WaitForFullGCApproach メソッドで取得した通知を受け、作業負荷をリダイレクトします。ガベージコレクションを手動で強制的に実行することもできます。
WaitForFullGCComplete メソッドで取得した通知を受け、現在のサーバーインスタンスで再度要求を処理できるようにします。情報を収集することもできます。たとえば、CollectionCount メソッドを使用して、ガベージコレクションの回数を記録できます。

WaitForFullGCApproach メソッドと WaitForFullGCComplete メソッドは、連携して動作するように設計されています。どちらか一方だけを使用すると、結果が不定になります。

フルガベージコレクション

ランタイムでフルガベージコレクションが発生するのは、以下に示すいくつかのシナリオに該当する場合です。

十分なメモリがジェネレーション 2 に移動され、次のジェネレーション 2 のガベージコレクションが発生する場合。
十分なメモリが、大きいオブジェクトのヒープに移動され、次のジェネレーション 2 のガベージコレクションが発生する場合。
その他の要因により、ジェネレーション 1 のガベージコレクションがジェネレーション 2 のガベージコレクションに移行する場合。

RegisterForFullGCNotification メソッドで指定するしきい値は、最初の 2 つのシナリオに該当します。ただし、最初のシナリオでは、以下の 2 つの理由から、指定したしきい値に比例したタイミングで常に通知を受け取るとは限りません。

ランタイムは、パフォーマンス上の理由から、小さなオブジェクトの割り当てをチェックしません。
メモリがジェネレーション 2 に移動されるのは、ジェネレーション 1 のガベージコレクションが実行された場合だけです。

3 番目のシナリオも、通知を受け取るタイミングに不確定性が生じる原因になります。保証されるわけではないものの、この間要求をリダイレクトしたり、都合の良いときに自分でガベージコレクションを強制的に実行したりして、タイミングの悪いフルガベージコレクションの影響を緩和することは有効な方法です。

通知しきい値パラメーター

RegisterForFullGCNotification メソッドには、ジェネレーション 2 のオブジェクトと大きなオブジェクトのヒープのしきい値を指定する 2 つのパラメーターがあります。これらの値を超えると、ガベージコレクションの通知が発行されます。次の表でこれらのパラメーターについて説明します。

パラメーター	説明
maxGenerationThreshold	ジェネレーション 2 に移動されたオブジェクトに基づいて通知を発行するタイミングを指定する、1 ～ 99 の数値。
largeObjectHeapThreshold	大きなオブジェクトのヒープに割り当てられたオブジェクトに基づいて通知を発行するタイミングを指定する、1 ～ 99 の数値。

指定した値が大きすぎると、通知を受け取る可能性が高まりますが、ランタイムでガベージコレクションが実行されるまでに長い時間待つことになります。ガベージコレクションを手動で強制的に実行すると、ランタイムでガベージコレクションが実行される場合よりも多くのオブジェクトが解放されます。

指定した値が小さすぎると、通知を受け取るのに十分な時間がないうちにガベージコレクションが実行される可能性があります。

例

説明

次の例では、サーバーのグループが受信した Web 要求を処理します。要求の処理による作業負荷をシミュレートするため、バイト配列が List<T> コレクションに追加されています。各サーバーはガベージコレクションの通知を登録し、ユーザーメソッド WaitForFullGCProc でスレッドを開始して、WaitForFullGCApproach メソッドと WaitForFullGCComplete メソッドで返される GCNotificationStatus 列挙型を継続的に監視します。

WaitForFullGCApproach メソッドと WaitForFullGCComplete メソッドは、通知が発行されると、それぞれのイベント処理用ユーザーメソッドを呼び出します。

OnFullGCApproachNotify

このメソッドはユーザーメソッド RedirectRequests を呼び出し、要求をキューに格納するサーバーに対して、このサーバーへの要求の送信を一時停止するよう指示します。この動作は、それ以上オブジェクトが割り当てられないように、クラスレベル変数 bAllocate に false を設定することでシミュレートされます。

次に、ユーザーメソッド FinishExistingRequests が呼び出され、保留中のサーバー要求の処理を終了します。この動作は、List<T> コレクションをクリアすることでシミュレートされます。

最後に、作業負荷が軽いため、ガベージコレクションが強制的に実行されます。
OnFullGCCompleteNotify

サーバーがフルガベージコレクションの影響を受ける可能性が低くなったため、ユーザーメソッド AcceptRequests を呼び出し、要求の受付を再開します。この動作は、bAllocate 変数に true を設定し、オブジェクトの List<T> コレクションへの追加を再開することでシミュレートされます。

この例の Main メソッドのコードを次に示します。


    ' Variables for continual checking in the
    ' While loop in the WaitForFullGcProc method.
    Private Shared checkForNotify As Boolean = False

    ' Variable for suspending work 
    ' (such as servicing allocated server requests)
    ' after a notification is received and then 
    ' resuming allocation after inducing a garbage collection.
    Private Shared bAllocate As Boolean = False

    ' Variable for ending the example.
    Private Shared finalExit As Boolean = False

    ' Collection for objects that  
    ' simulate the server request workload.
    Private Shared load As New List(Of Byte())


    Public Shared Sub Main(ByVal args() As String)
        Try
            ' Register for a notification. 
            GC.RegisterForFullGCNotification(10, 10)
            Console.WriteLine("Registered for GC notification.")

            bAllocate = True
            checkForNotify = True

            ' Start a thread using WaitForFullGCProc.
            Dim thWaitForFullGC As Thread = _
                New Thread(New ThreadStart(AddressOf WaitForFullGCProc))
            thWaitForFullGC.Start()

            ' While the thread is checking for notifications in
            ' WaitForFullGCProc, create objects to simulate a server workload.
            Try
                Dim lastCollCount As Integer = 0
                Dim newCollCount As Integer = 0


                While (True)
                    If bAllocate = True Then

                        load.Add(New Byte(1000) {})
                        newCollCount = GC.CollectionCount(2)
                        If (newCollCount <> lastCollCount) Then
                            ' Show collection count when it increases:
                            Console.WriteLine("Gen 2 collection count: {0}", _
                                              GC.CollectionCount(2).ToString)
                            lastCollCount = newCollCount
                        End If

                        ' For ending the example (arbitrary).
                        If newCollCount = 500 Then
                            finalExit = True
                            checkForNotify = False
                            bAllocate = False
                            Exit While
                        End If

                    End If
                End While

            Catch outofMem As OutOfMemoryException
                Console.WriteLine("Out of memory.")
            End Try

            finalExit = True
            checkForNotify = False
            GC.CancelFullGCNotification()

        Catch invalidOp As InvalidOperationException
            Console.WriteLine("GC Notifications are not supported while concurrent GC is enabled." _
                              & vbLf & invalidOp.Message)
        End Try
    End Sub


// Variable for continual checking in the 
// While loop in the WaitForFullGCProc method.
static bool checkForNotify = false;

// Variable for suspending work 
// (such servicing allocated server requests)
// after a notification is received and then 
// resuming allocation after inducing a garbage collection.
static bool bAllocate = false;

// Variable for ending the example.
static bool finalExit = false;

// Collection for objects that  
// simulate the server request workload.
static List<byte[]> load = new List<byte[]>();


public static void Main(string[] args)
{
    try
    {
        // Register for a notification. 
        GC.RegisterForFullGCNotification(10, 10);
        Console.WriteLine("Registered for GC notification.");

        checkForNotify = true;
        bAllocate = true;

        // Start a thread using WaitForFullGCProc.
        Thread thWaitForFullGC = new Thread(new ThreadStart(WaitForFullGCProc));
        thWaitForFullGC.Start();

        // While the thread is checking for notifications in
        // WaitForFullGCProc, create objects to simulate a server workload.
        try
        {

            int lastCollCount = 0;
            int newCollCount = 0;


            while (true)
            {
                if (bAllocate)
                {
                    load.Add(new byte[1000]);
                    newCollCount = GC.CollectionCount(2);
                    if (newCollCount != lastCollCount)
                    {
                        // Show collection count when it increases:
                        Console.WriteLine("Gen 2 collection count: {0}", GC.CollectionCount(2).ToString());
                        lastCollCount = newCollCount;
                    }

                    // For ending the example (arbitrary).
                    if (newCollCount == 500)
                    {
                        finalExit = true;
                        checkForNotify = false;
                        break;
                    }
                }
            }

        }
        catch (OutOfMemoryException)
        {
            Console.WriteLine("Out of memory.");
        }


        finalExit = true;
        checkForNotify = false;
        GC.CancelFullGCNotification();

    }
    catch (InvalidOperationException invalidOp)
    {

        Console.WriteLine("GC Notifications are not supported while concurrent GC is enabled.\n"
            + invalidOp.Message);
    }
}

private:
    // Variable for continual checking in the
    // While loop in the WaitForFullGCProc method.
    static bool checkForNotify = false;

    // Variable for suspending work
    // (such servicing allocated server requests)
    // after a notification is received and then
    // resuming allocation after inducing a garbage collection.
    static bool bAllocate = false;

    // Variable for ending the example.
    static bool finalExit = false;

    // Collection for objects that
    // simulate the server request workload.
    static List<array<Byte>^>^ load = gcnew List<array<Byte>^>();


public:
    static void Main()
    {
        try
        {
            // Register for a notification.
            GC::RegisterForFullGCNotification(10, 10);
            Console::WriteLine("Registered for GC notification.");

            checkForNotify = true;
            bAllocate = true;

            // Start a thread using WaitForFullGCProc.
            Thread^ thWaitForFullGC = gcnew Thread(gcnew ThreadStart(&WaitForFullGCProc));
            thWaitForFullGC->Start();

            // While the thread is checking for notifications in
            // WaitForFullGCProc, create objects to simulate a server workload.
            try
            {
                int lastCollCount = 0;
                int newCollCount = 0;


                while (true)
                {
                    if (bAllocate)
                    {
                        load->Add(gcnew array<Byte>(1000));
                        newCollCount = GC::CollectionCount(2);
                        if (newCollCount != lastCollCount)
                        {
                            // Show collection count when it increases:
                            Console::WriteLine("Gen 2 collection count: {0}", GC::CollectionCount(2).ToString());
                            lastCollCount = newCollCount;
                        }

                        // For ending the example (arbitrary).
                        if (newCollCount == 500)
                        {
                            finalExit = true;
                            checkForNotify = false;
                            break;
                        }
                    }
                }

            }
            catch (OutOfMemoryException^)
            {
                Console::WriteLine("Out of memory.");
            }


            finalExit = true;
            checkForNotify = false;
            GC::CancelFullGCNotification();

        }
        catch (InvalidOperationException^ invalidOp)
        {

            Console::WriteLine("GC Notifications are not supported while concurrent GC is enabled.\n"
                + invalidOp->Message);
        }
    }

次のコードは、ユーザーメソッド WaitForFullGCProc のコードであり、ガベージコレクションの通知を継続的にチェックする while ループが含まれています。

    Public Shared Sub WaitForFullGCProc()

        While True
            ' CheckForNotify is set to true and false in Main.

            While checkForNotify
                ' Check for a notification of an approaching collection.
                Dim s As GCNotificationStatus = GC.WaitForFullGCApproach
                If (s = GCNotificationStatus.Succeeded) Then
                    Console.WriteLine("GC Notifiction raised.")
                    OnFullGCApproachNotify()
                ElseIf (s = GCNotificationStatus.Canceled) Then
                    Console.WriteLine("GC Notification cancelled.")
                    Exit While
                Else
                    ' This can occur if a timeout period
                    ' is specified for WaitForFullGCApproach(Timeout) 
                    ' or WaitForFullGCComplete(Timeout)  
                    ' and the time out period has elapsed. 
                    Console.WriteLine("GC Notification not applicable.")
                    Exit While
                End If

                ' Check for a notification of a completed collection.
                s = GC.WaitForFullGCComplete
                If (s = GCNotificationStatus.Succeeded) Then
                    Console.WriteLine("GC Notifiction raised.")
                    OnFullGCCompleteEndNotify()
                ElseIf (s = GCNotificationStatus.Canceled) Then
                    Console.WriteLine("GC Notification cancelled.")
                    Exit While
                Else
                    ' Could be a time out.
                    Console.WriteLine("GC Notification not applicable.")
                    Exit While
                End If

            End While
            Thread.Sleep(500)
            ' FinalExit is set to true right before  
            ' the main thread cancelled notification.
            If finalExit Then
                Exit While
            End If

        End While
    End Sub

public static void WaitForFullGCProc()
{
    while (true)
    {
        // CheckForNotify is set to true and false in Main.
        while (checkForNotify)
        {
            // Check for a notification of an approaching collection.
            GCNotificationStatus s = GC.WaitForFullGCApproach();
            if (s == GCNotificationStatus.Succeeded)
            {
                Console.WriteLine("GC Notifiction raised.");
                OnFullGCApproachNotify();
            }
            else if (s == GCNotificationStatus.Canceled)
            {
                Console.WriteLine("GC Notification cancelled.");
                break;
            }
            else
            {
                // This can occur if a timeout period
                // is specified for WaitForFullGCApproach(Timeout) 
                // or WaitForFullGCComplete(Timeout)  
                // and the time out period has elapsed. 
                Console.WriteLine("GC Notification not applicable.");
                break;
            }

            // Check for a notification of a completed collection.
            s = GC.WaitForFullGCComplete();
            if (s == GCNotificationStatus.Succeeded)
            {
                Console.WriteLine("GC Notifiction raised.");
                OnFullGCCompleteEndNotify();
            }
            else if (s == GCNotificationStatus.Canceled)
            {
                Console.WriteLine("GC Notification cancelled.");
                break;
            }
            else
            {
                // Could be a time out.
                Console.WriteLine("GC Notification not applicable.");
                break;
            }
        }


        Thread.Sleep(500);
        // FinalExit is set to true right before  
        // the main thread cancelled notification.
        if (finalExit)
        {
            break;
        }
    }

}

public:
    static void WaitForFullGCProc()
    {
        while (true)
        {
            // CheckForNotify is set to true and false in Main.
            while (checkForNotify)
            {
                // Check for a notification of an approaching collection.
                GCNotificationStatus s = GC::WaitForFullGCApproach();
                if (s == GCNotificationStatus::Succeeded)
                {
                    Console::WriteLine("GC Notifiction raised.");
                    OnFullGCApproachNotify();
                }
                else if (s == GCNotificationStatus::Canceled)
                {
                    Console::WriteLine("GC Notification cancelled.");
                    break;
                }
                else
                {
                    // This can occur if a timeout period
                    // is specified for WaitForFullGCApproach(Timeout)
                    // or WaitForFullGCComplete(Timeout)
                    // and the time out period has elapsed.
                    Console::WriteLine("GC Notification not applicable.");
                    break;
                }

                // Check for a notification of a completed collection.
                s = GC::WaitForFullGCComplete();
                if (s == GCNotificationStatus::Succeeded)
                {
                    Console::WriteLine("GC Notifiction raised.");
                    OnFullGCCompleteEndNotify();
                }
                else if (s == GCNotificationStatus::Canceled)
                {
                    Console::WriteLine("GC Notification cancelled.");
                    break;
                }
                else
                {
                    // Could be a time out.
                    Console::WriteLine("GC Notification not applicable.");
                    break;
                }
            }


            Thread::Sleep(500);
            // FinalExit is set to true right before
            // the main thread cancelled notification.
            if (finalExit)
            {
                break;
            }
        }
    }

次のコードは、OnFullGCApproachNotify メソッドのコードです。

WaitForFullGCProc メソッド

    Public Shared Sub OnFullGCApproachNotify()
        Console.WriteLine("Redirecting requests.")

        ' Method that tells the request queuing  
        ' server to not direct requests to this server. 
        RedirectRequests()

        ' Method that provides time to 
        ' finish processing pending requests. 
        FinishExistingRequests()

        ' This is a good time to induce a GC collection
        ' because the runtime will induce a ful GC soon.
        ' To be very careful, you can check precede with a
        ' check of the GC.GCCollectionCount to make sure
        ' a full GC did not already occur since last notified.
        GC.Collect()
        Console.WriteLine("Induced a collection.")
    End Sub

public static void OnFullGCApproachNotify()
{

    Console.WriteLine("Redirecting requests.");

    // Method that tells the request queuing  
    // server to not direct requests to this server. 
    RedirectRequests();

    // Method that provides time to 
    // finish processing pending requests. 
    FinishExistingRequests();

    // This is a good time to induce a GC collection
    // because the runtime will induce a full GC soon.
    // To be very careful, you can check precede with a
    // check of the GC.GCCollectionCount to make sure
    // a full GC did not already occur since last notified.
    GC.Collect();
    Console.WriteLine("Induced a collection.");

}

public:
    static void OnFullGCApproachNotify()
    {
        Console::WriteLine("Redirecting requests.");

        // Method that tells the request queuing
        // server to not direct requests to this server.
        RedirectRequests();

        // Method that provides time to
        // finish processing pending requests.
        FinishExistingRequests();

        // This is a good time to induce a GC collection
        // because the runtime will induce a full GC soon.
        // To be very careful, you can check precede with a
        // check of the GC.GCCollectionCount to make sure
        // a full GC did not already occur since last notified.
        GC::Collect();
        Console::WriteLine("Induced a collection.");

    }

次のコードは、OnFullGCApproachComplete メソッドのコードです。

WaitForFullGCProc メソッド

    Public Shared Sub OnFullGCCompleteEndNotify()
        ' Method that informs the request queuing server
        ' that this server is ready to accept requests again.
        AcceptRequests()
        Console.WriteLine("Accepting requests again.")
    End Sub

public static void OnFullGCCompleteEndNotify()
{
    // Method that informs the request queuing server
    // that this server is ready to accept requests again.
    AcceptRequests();
    Console.WriteLine("Accepting requests again.");
}

public:
    static void OnFullGCCompleteEndNotify()
    {
        // Method that informs the request queuing server
        // that this server is ready to accept requests again.
        AcceptRequests();
        Console::WriteLine("Accepting requests again.");
    }

次のコードは、OnFullGCApproachNotify メソッドと OnFullGCCompleteNotify メソッドから呼び出されるユーザーメソッドです。このユーザーメソッドは、要求のリダイレクト、既存の要求の終了、フルガベージコレクションの実行後の要求の受け付け再開を実行します。

private static void RedirectRequests()
{
    // Code that sends requests
    // to other servers.

    // Suspend work.
    bAllocate = false;

}

private static void FinishExistingRequests()
{
    // Code that waits a period of time
    // for pending requests to finish.

    // Clear the simulated workload.
    load.Clear();

}

private static void AcceptRequests()
{
    // Code that resumes processing
    // requests on this server.

    // Resume work.
    bAllocate = true;

}

private:
    static void RedirectRequests()
    {
        // Code that sends requests
        // to other servers.

        // Suspend work.
        bAllocate = false;

    }

    static void FinishExistingRequests()
    {
        // Code that waits a period of time
        // for pending requests to finish.

        // Clear the simulated workload.
        load->Clear();

    }

    static void AcceptRequests()
    {
        // Code that resumes processing
        // requests on this server.

        // Resume work.
        bAllocate = true;
    }

コード例全体を次に示します。

Imports System
Imports System.Collections.Generic
Imports System.Threading
Imports Microsoft.VisualBasic



Class Program


    ' Variables for continual checking in the
    ' While loop in the WaitForFullGcProc method.
    Private Shared checkForNotify As Boolean = False

    ' Variable for suspending work 
    ' (such as servicing allocated server requests)
    ' after a notification is received and then 
    ' resuming allocation after inducing a garbage collection.
    Private Shared bAllocate As Boolean = False

    ' Variable for ending the example.
    Private Shared finalExit As Boolean = False

    ' Collection for objects that  
    ' simulate the server request workload.
    Private Shared load As New List(Of Byte())


    Public Shared Sub Main(ByVal args() As String)
        Try
            ' Register for a notification. 
            GC.RegisterForFullGCNotification(10, 10)
            Console.WriteLine("Registered for GC notification.")

            bAllocate = True
            checkForNotify = True

            ' Start a thread using WaitForFullGCProc.
            Dim thWaitForFullGC As Thread = _
                New Thread(New ThreadStart(AddressOf WaitForFullGCProc))
            thWaitForFullGC.Start()

            ' While the thread is checking for notifications in
            ' WaitForFullGCProc, create objects to simulate a server workload.
            Try
                Dim lastCollCount As Integer = 0
                Dim newCollCount As Integer = 0


                While (True)
                    If bAllocate = True Then

                        load.Add(New Byte(1000) {})
                        newCollCount = GC.CollectionCount(2)
                        If (newCollCount <> lastCollCount) Then
                            ' Show collection count when it increases:
                            Console.WriteLine("Gen 2 collection count: {0}", _
                                              GC.CollectionCount(2).ToString)
                            lastCollCount = newCollCount
                        End If

                        ' For ending the example (arbitrary).
                        If newCollCount = 500 Then
                            finalExit = True
                            checkForNotify = False
                            bAllocate = False
                            Exit While
                        End If

                    End If
                End While

            Catch outofMem As OutOfMemoryException
                Console.WriteLine("Out of memory.")
            End Try

            finalExit = True
            checkForNotify = False
            GC.CancelFullGCNotification()

        Catch invalidOp As InvalidOperationException
            Console.WriteLine("GC Notifications are not supported while concurrent GC is enabled." _
                              & vbLf & invalidOp.Message)
        End Try
    End Sub

    Public Shared Sub OnFullGCApproachNotify()
        Console.WriteLine("Redirecting requests.")

        ' Method that tells the request queuing  
        ' server to not direct requests to this server. 
        RedirectRequests()

        ' Method that provides time to 
        ' finish processing pending requests. 
        FinishExistingRequests()

        ' This is a good time to induce a GC collection
        ' because the runtime will induce a ful GC soon.
        ' To be very careful, you can check precede with a
        ' check of the GC.GCCollectionCount to make sure
        ' a full GC did not already occur since last notified.
        GC.Collect()
        Console.WriteLine("Induced a collection.")
    End Sub

    Public Shared Sub OnFullGCCompleteEndNotify()
        ' Method that informs the request queuing server
        ' that this server is ready to accept requests again.
        AcceptRequests()
        Console.WriteLine("Accepting requests again.")
    End Sub

    Public Shared Sub WaitForFullGCProc()

        While True
            ' CheckForNotify is set to true and false in Main.

            While checkForNotify
                ' Check for a notification of an approaching collection.
                Dim s As GCNotificationStatus = GC.WaitForFullGCApproach
                If (s = GCNotificationStatus.Succeeded) Then
                    Console.WriteLine("GC Notifiction raised.")
                    OnFullGCApproachNotify()
                ElseIf (s = GCNotificationStatus.Canceled) Then
                    Console.WriteLine("GC Notification cancelled.")
                    Exit While
                Else
                    ' This can occur if a timeout period
                    ' is specified for WaitForFullGCApproach(Timeout) 
                    ' or WaitForFullGCComplete(Timeout)  
                    ' and the time out period has elapsed. 
                    Console.WriteLine("GC Notification not applicable.")
                    Exit While
                End If

                ' Check for a notification of a completed collection.
                s = GC.WaitForFullGCComplete
                If (s = GCNotificationStatus.Succeeded) Then
                    Console.WriteLine("GC Notifiction raised.")
                    OnFullGCCompleteEndNotify()
                ElseIf (s = GCNotificationStatus.Canceled) Then
                    Console.WriteLine("GC Notification cancelled.")
                    Exit While
                Else
                    ' Could be a time out.
                    Console.WriteLine("GC Notification not applicable.")
                    Exit While
                End If

            End While
            Thread.Sleep(500)
            ' FinalExit is set to true right before  
            ' the main thread cancelled notification.
            If finalExit Then
                Exit While
            End If

        End While
    End Sub

    Private Shared Sub RedirectRequests()
        ' Code that sends requests
        ' to other servers.

        ' Suspend work.
        bAllocate = False
    End Sub

    Private Shared Sub FinishExistingRequests()
        ' Code that waits a period of time
        ' for pending requests to finish.

        ' Clear the simulated workload.
        load.Clear()

    End Sub

    Private Shared Sub AcceptRequests()
        ' Code that resumes processing
        ' requests on this server.

        ' Resume work.
        bAllocate = True
    End Sub
End Class

using System;
using System.Collections.Generic;
using System.Threading;

namespace GCNotify
{
    class Program
    {

        // Variable for continual checking in the 
        // While loop in the WaitForFullGCProc method.
        static bool checkForNotify = false;

        // Variable for suspending work 
        // (such servicing allocated server requests)
        // after a notification is received and then 
        // resuming allocation after inducing a garbage collection.
        static bool bAllocate = false;

        // Variable for ending the example.
        static bool finalExit = false;

        // Collection for objects that  
        // simulate the server request workload.
        static List<byte[]> load = new List<byte[]>();


        public static void Main(string[] args)
        {
            try
            {
                // Register for a notification. 
                GC.RegisterForFullGCNotification(10, 10);
                Console.WriteLine("Registered for GC notification.");

                checkForNotify = true;
                bAllocate = true;

                // Start a thread using WaitForFullGCProc.
                Thread thWaitForFullGC = new Thread(new ThreadStart(WaitForFullGCProc));
                thWaitForFullGC.Start();

                // While the thread is checking for notifications in
                // WaitForFullGCProc, create objects to simulate a server workload.
                try
                {

                    int lastCollCount = 0;
                    int newCollCount = 0;


                    while (true)
                    {
                        if (bAllocate)
                        {
                            load.Add(new byte[1000]);
                            newCollCount = GC.CollectionCount(2);
                            if (newCollCount != lastCollCount)
                            {
                                // Show collection count when it increases:
                                Console.WriteLine("Gen 2 collection count: {0}", GC.CollectionCount(2).ToString());
                                lastCollCount = newCollCount;
                            }

                            // For ending the example (arbitrary).
                            if (newCollCount == 500)
                            {
                                finalExit = true;
                                checkForNotify = false;
                                break;
                            }
                        }
                    }

                }
                catch (OutOfMemoryException)
                {
                    Console.WriteLine("Out of memory.");
                }


                finalExit = true;
                checkForNotify = false;
                GC.CancelFullGCNotification();

            }
            catch (InvalidOperationException invalidOp)
            {

                Console.WriteLine("GC Notifications are not supported while concurrent GC is enabled.\n"
                    + invalidOp.Message);
            }
        }




        public static void OnFullGCApproachNotify()
        {

            Console.WriteLine("Redirecting requests.");

            // Method that tells the request queuing  
            // server to not direct requests to this server. 
            RedirectRequests();

            // Method that provides time to 
            // finish processing pending requests. 
            FinishExistingRequests();

            // This is a good time to induce a GC collection
            // because the runtime will induce a full GC soon.
            // To be very careful, you can check precede with a
            // check of the GC.GCCollectionCount to make sure
            // a full GC did not already occur since last notified.
            GC.Collect();
            Console.WriteLine("Induced a collection.");

        }


        public static void OnFullGCCompleteEndNotify()
        {
            // Method that informs the request queuing server
            // that this server is ready to accept requests again.
            AcceptRequests();
            Console.WriteLine("Accepting requests again.");
        }

        public static void WaitForFullGCProc()
        {
            while (true)
            {
                // CheckForNotify is set to true and false in Main.
                while (checkForNotify)
                {
                    // Check for a notification of an approaching collection.
                    GCNotificationStatus s = GC.WaitForFullGCApproach();
                    if (s == GCNotificationStatus.Succeeded)
                    {
                        Console.WriteLine("GC Notifiction raised.");
                        OnFullGCApproachNotify();
                    }
                    else if (s == GCNotificationStatus.Canceled)
                    {
                        Console.WriteLine("GC Notification cancelled.");
                        break;
                    }
                    else
                    {
                        // This can occur if a timeout period
                        // is specified for WaitForFullGCApproach(Timeout) 
                        // or WaitForFullGCComplete(Timeout)  
                        // and the time out period has elapsed. 
                        Console.WriteLine("GC Notification not applicable.");
                        break;
                    }

                    // Check for a notification of a completed collection.
                    s = GC.WaitForFullGCComplete();
                    if (s == GCNotificationStatus.Succeeded)
                    {
                        Console.WriteLine("GC Notifiction raised.");
                        OnFullGCCompleteEndNotify();
                    }
                    else if (s == GCNotificationStatus.Canceled)
                    {
                        Console.WriteLine("GC Notification cancelled.");
                        break;
                    }
                    else
                    {
                        // Could be a time out.
                        Console.WriteLine("GC Notification not applicable.");
                        break;
                    }
                }


                Thread.Sleep(500);
                // FinalExit is set to true right before  
                // the main thread cancelled notification.
                if (finalExit)
                {
                    break;
                }
            }

        }

        private static void RedirectRequests()
        {
            // Code that sends requests
            // to other servers.

            // Suspend work.
            bAllocate = false;

        }

        private static void FinishExistingRequests()
        {
            // Code that waits a period of time
            // for pending requests to finish.

            // Clear the simulated workload.
            load.Clear();

        }

        private static void AcceptRequests()
        {
            // Code that resumes processing
            // requests on this server.

            // Resume work.
            bAllocate = true;

        }
    }
}

using namespace System;
using namespace System::Collections::Generic;
using namespace System::Threading;

namespace GCNotify
{
    ref class Program
    {
    private:
        // Variable for continual checking in the
        // While loop in the WaitForFullGCProc method.
        static bool checkForNotify = false;

        // Variable for suspending work
        // (such servicing allocated server requests)
        // after a notification is received and then
        // resuming allocation after inducing a garbage collection.
        static bool bAllocate = false;

        // Variable for ending the example.
        static bool finalExit = false;

        // Collection for objects that
        // simulate the server request workload.
        static List<array<Byte>^>^ load = gcnew List<array<Byte>^>();


    public:
        static void Main()
        {
            try
            {
                // Register for a notification.
                GC::RegisterForFullGCNotification(10, 10);
                Console::WriteLine("Registered for GC notification.");

                checkForNotify = true;
                bAllocate = true;

                // Start a thread using WaitForFullGCProc.
                Thread^ thWaitForFullGC = gcnew Thread(gcnew ThreadStart(&WaitForFullGCProc));
                thWaitForFullGC->Start();

                // While the thread is checking for notifications in
                // WaitForFullGCProc, create objects to simulate a server workload.
                try
                {
                    int lastCollCount = 0;
                    int newCollCount = 0;


                    while (true)
                    {
                        if (bAllocate)
                        {
                            load->Add(gcnew array<Byte>(1000));
                            newCollCount = GC::CollectionCount(2);
                            if (newCollCount != lastCollCount)
                            {
                                // Show collection count when it increases:
                                Console::WriteLine("Gen 2 collection count: {0}", GC::CollectionCount(2).ToString());
                                lastCollCount = newCollCount;
                            }

                            // For ending the example (arbitrary).
                            if (newCollCount == 500)
                            {
                                finalExit = true;
                                checkForNotify = false;
                                break;
                            }
                        }
                    }

                }
                catch (OutOfMemoryException^)
                {
                    Console::WriteLine("Out of memory.");
                }


                finalExit = true;
                checkForNotify = false;
                GC::CancelFullGCNotification();

            }
            catch (InvalidOperationException^ invalidOp)
            {

                Console::WriteLine("GC Notifications are not supported while concurrent GC is enabled.\n"
                    + invalidOp->Message);
            }
        }

    public:
        static void OnFullGCApproachNotify()
        {
            Console::WriteLine("Redirecting requests.");

            // Method that tells the request queuing
            // server to not direct requests to this server.
            RedirectRequests();

            // Method that provides time to
            // finish processing pending requests.
            FinishExistingRequests();

            // This is a good time to induce a GC collection
            // because the runtime will induce a full GC soon.
            // To be very careful, you can check precede with a
            // check of the GC.GCCollectionCount to make sure
            // a full GC did not already occur since last notified.
            GC::Collect();
            Console::WriteLine("Induced a collection.");

        }


    public:
        static void OnFullGCCompleteEndNotify()
        {
            // Method that informs the request queuing server
            // that this server is ready to accept requests again.
            AcceptRequests();
            Console::WriteLine("Accepting requests again.");
        }

    public:
        static void WaitForFullGCProc()
        {
            while (true)
            {
                // CheckForNotify is set to true and false in Main.
                while (checkForNotify)
                {
                    // Check for a notification of an approaching collection.
                    GCNotificationStatus s = GC::WaitForFullGCApproach();
                    if (s == GCNotificationStatus::Succeeded)
                    {
                        Console::WriteLine("GC Notifiction raised.");
                        OnFullGCApproachNotify();
                    }
                    else if (s == GCNotificationStatus::Canceled)
                    {
                        Console::WriteLine("GC Notification cancelled.");
                        break;
                    }
                    else
                    {
                        // This can occur if a timeout period
                        // is specified for WaitForFullGCApproach(Timeout)
                        // or WaitForFullGCComplete(Timeout)
                        // and the time out period has elapsed.
                        Console::WriteLine("GC Notification not applicable.");
                        break;
                    }

                    // Check for a notification of a completed collection.
                    s = GC::WaitForFullGCComplete();
                    if (s == GCNotificationStatus::Succeeded)
                    {
                        Console::WriteLine("GC Notifiction raised.");
                        OnFullGCCompleteEndNotify();
                    }
                    else if (s == GCNotificationStatus::Canceled)
                    {
                        Console::WriteLine("GC Notification cancelled.");
                        break;
                    }
                    else
                    {
                        // Could be a time out.
                        Console::WriteLine("GC Notification not applicable.");
                        break;
                    }
                }


                Thread::Sleep(500);
                // FinalExit is set to true right before
                // the main thread cancelled notification.
                if (finalExit)
                {
                    break;
                }
            }
        }

    private:
        static void RedirectRequests()
        {
            // Code that sends requests
            // to other servers.

            // Suspend work.
            bAllocate = false;

        }

        static void FinishExistingRequests()
        {
            // Code that waits a period of time
            // for pending requests to finish.

            // Clear the simulated workload.
            load->Clear();

        }

        static void AcceptRequests()
        {
            // Code that resumes processing
            // requests on this server.

            // Resume work.
            bAllocate = true;
        }
    };
}

int main()
{
    GCNotify::Program::Main();
}

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