Garbage Collection Notifications

There are situations in which a full garbage collection (that is, a generation 2 collection) by the common language runtime may adversely affect performance. This can be an issue particularly with servers that process large volumes of requests; in this case, a long garbage collection can cause a request time-out. To prevent a full collection from occurring during a critical period, you can be notified that a full garbage collection is approaching and then take action to redirect the workload to another server instance. You can also induce a collection yourself, provided that the current server instance does not need to process requests.

The RegisterForFullGCNotification method registers for a notification to be raised when the runtime senses that a full garbage collection is approaching. There are two parts to this notification: when the full garbage collection is approaching and when the full garbage collection has completed.

Upozorenje

When the <gcConcurrent> configuration element is enabled, WaitForFullGCComplete may return NotApplicable GCNotificationStatus if the full GC was done as a background GC.

To determine when a notification has been raised, use the WaitForFullGCApproach and WaitForFullGCComplete methods. Typically, you use these methods in a while loop to continually obtain a GCNotificationStatus enumeration that shows the status of the notification. If that value is Succeeded, you can do the following:

• In response to a notification obtained with the WaitForFullGCApproach method, you can redirect the workload and possibly induce a collection yourself.

• In response to a notification obtained with the WaitForFullGCComplete method, you can make the current server instance available to process requests again. You can also gather information. For example, you can use the CollectionCount method to record the number of collections.

The WaitForFullGCApproach and the WaitForFullGCComplete methods are designed to work together. Using one without the other can produce indeterminate results.

Full Garbage Collection

The runtime causes a full garbage collection when any of the following scenarios are true:

• Enough memory has been promoted into generation 2 to cause the next generation 2 collection.

• Enough memory has been promoted into the large object heap to cause the next generation 2 collection.

• A collection of generation 1 is escalated to a collection of generation 2 due to other factors.

The thresholds you specify in the RegisterForFullGCNotification method apply to the first two scenarios. However, in the first scenario you will not always receive the notification at the time proportional to the threshold values you specify for two reasons:

• The runtime does not check each small object allocation (for performance reasons).

• Only generation 1 collections promote memory into generation 2.

The third scenario also contributes to the uncertainty of when you will receive the notification. Although this is not a guarantee, it does prove to be a useful way to mitigate the effects of an inopportune full garbage collection by redirecting the requests during this time or inducing the collection yourself when it can be better accommodated.

Notification Threshold Parameters

The RegisterForFullGCNotification method has two parameters to specify the threshold values of the generation 2 objects and the large object heap. When those values are met, a garbage collection notification should be raised. The following table describes these parameters.

Parameter	Description
maxGenerationThreshold	A number between 1 and 99 that specifies when the notification should be raised based on the objects promoted in generation 2.
largeObjectHeapThreshold	A number between 1 and 99 that specifies when the notification should be raised based on the objects that are allocated in the large object heap.

If you specify a value that is too high, there is a high probability that you will receive a notification, but it could be too long a period to wait before the runtime causes a collection. If you induce a collection yourself, you may reclaim more objects than would be reclaimed if the runtime causes the collection.

If you specify a value that is too low, the runtime may cause the collection before you have had sufficient time to be notified.

Example

Description

In the following example, a group of servers service incoming Web requests. To simulate the workload of processing requests, byte arrays are added to a List<T> collection. Each server registers for a garbage collection notification and then starts a thread on the WaitForFullGCProc user method to continuously monitor the GCNotificationStatus enumeration that is returned by the WaitForFullGCApproach and the WaitForFullGCComplete methods.

The WaitForFullGCApproach and the WaitForFullGCComplete methods call their respective event-handling user methods when a notification is raised:

• OnFullGCApproachNotify

  This method calls the RedirectRequests user method, which instructs the request queuing server to suspend sending requests to the server. This is simulated by setting the class-level variable bAllocate to false so that no more objects are allocated.

  Next, the FinishExistingRequests user method is called to finish processing the pending server requests. This is simulated by clearing the List<T> collection.

  Finally, a garbage collection is induced because the workload is light.

• OnFullGCCompleteNotify

  This method calls the user method AcceptRequests to resume accepting requests because the server is no longer susceptible to a full garbage collection. This action is simulated by setting the bAllocate variable to true so that objects can resume being added to the List<T> collection.

The following code contains the Main method of the example.

C#

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);
    }
}

The following code contains the WaitForFullGCProc user method, that contains a continuous while loop to check for garbage collection notifications.

C#

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 Notification 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.
            GCNotificationStatus status = GC.WaitForFullGCComplete();
            if (status == GCNotificationStatus.Succeeded)
            {
                Console.WriteLine("GC Notification raised.");
                OnFullGCCompleteEndNotify();
            }
            else if (status == 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;
        }
    }
}

The following code contains the OnFullGCApproachNotify method as called from the

WaitForFullGCProc method.

C#

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.");
}

The following code contains the OnFullGCApproachComplete method as called from the

WaitForFullGCProc method.

C#

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.");
}

The following code contains the user methods that are called from the OnFullGCApproachNotify and OnFullGCCompleteNotify methods. The user methods redirect requests, finish existing requests, and then resume requests after a full garbage collection has occurred.

C#

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;
}

The entire code sample is as follows:

C#

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 Notification 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.
                    GCNotificationStatus status = GC.WaitForFullGCComplete();
                    if (status == GCNotificationStatus.Succeeded)
                    {
                        Console.WriteLine("GC Notification raised.");
                        OnFullGCCompleteEndNotify();
                    }
                    else if (status == 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;
        }
    }
}

See also

• Garbage Collection