gc.java

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/*
 * @(#)GC.java	1.15 06/10/10
 *
 * Copyright  1990-2008 Sun Microsystems, Inc. All Rights Reserved.  
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER  
 *   
 * This program is free software; you can redistribute it and/or  
 * modify it under the terms of the GNU General Public License version  
 * 2 only, as published by the Free Software Foundation.   
 *   
 * This program is distributed in the hope that it will be useful, but  
 * WITHOUT ANY WARRANTY; without even the implied warranty of  
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU  
 * General Public License version 2 for more details (a copy is  
 * included at /legal/license.txt).   
 *   
 * You should have received a copy of the GNU General Public License  
 * version 2 along with this work; if not, write to the Free Software  
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  
 * 02110-1301 USA   
 *   
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa  
 * Clara, CA 95054 or visit www.sun.com if you need additional  
 * information or have any questions. 
 *
 */

package sun.misc;

import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.SortedSet;
import java.util.TreeSet;
import sun.misc.ThreadRegistry;


/**
 * Support for garbage-collection latency requests.
 *
 * @version  1.8, 00/05/16
 * @author   Mark Reinhold
 * @since    JDK1.2
 */

public class GC {

    private GC() { }		/* To prevent instantiation */


    /* Latency-target value indicating that there's no active target
     */
    private static final long NO_TARGET = Long.MAX_VALUE;

    /* The current latency target, or NO_TARGET if there is no target
     */
    private static long latencyTarget = NO_TARGET;

    /* The daemon thread that implements the latency-target mechanism,
     * or null if there is presently no daemon thread
     */
    private static Thread daemon = null;

    /* The lock object for the latencyTarget and daemon fields.  The daemon
     * thread, if it exists, waits on this lock for notification that the
     * latency target has changed.
     */
    private static class LatencyLock extends Object { };
    private static Object lock = new LatencyLock();


    /**
     * Returns the maximum <em>object-inspection age</em>, which is the number
     * of real-time milliseconds that have elapsed since the
     * least-recently-inspected heap object was last inspected by the garbage
     * collector.
     *
     * <p> For simple stop-the-world collectors this value is just the time
     * since the most recent collection.  For generational collectors it is the
     * time since the oldest generation was most recently collected.  Other
     * collectors are free to return a pessimistic estimate of the elapsed
     * time, or simply the time since the last full collection was performed.
     *
     * <p> Note that in the presence of reference objects, a given object that
     * is no longer strongly reachable may have to be inspected multiple times
     * before it can be reclaimed.
     */
    public static native long maxObjectInspectionAge();


    private static class Daemon extends Thread {

	public void run() {
	    while (!ThreadRegistry.exitRequested()) {
		long l;
		synchronized (lock) {

		    l = latencyTarget;
		    if (l == NO_TARGET) {
			/* No latency target, so exit */
			GC.daemon = null;
			return;
		    }

		    long d = maxObjectInspectionAge();
		    if (d >= l) {
			/* Do a full collection.  There is a remote possibility
			 * that a full collection will occurr between the time
			 * we sample the inspection age and the time the GC
			 * actually starts, but this is sufficiently unlikely
			 * that it doesn't seem worth the more expensive JVM
			 * interface that would be required.
			 */
		        System.gc();
			d = 0;
		    }

		    /* Wait for the latency period to expire,
                     * or for notification that the period has changed
		     */
		    try {
			lock.wait(l - d);
		    } catch (InterruptedException x) {
			continue;
		    }
		}
	    }
	}

	private Daemon(ThreadGroup tg) {
	    super(tg, "GC Daemon");
	}

	/* Create a new daemon thread in the root thread group */
	public static void create() {
	    PrivilegedAction pa = new PrivilegedAction() {
		public Object run() {
		    ThreadGroup tg = Thread.currentThread().getThreadGroup();
		    for (ThreadGroup tgn = tg;
			 tgn != null;
			 tg = tgn, tgn = tg.getParent());
		    Daemon d = new Daemon(tg);
		    d.setDaemon(true);
		    d.setPriority(Thread.MIN_PRIORITY + 1);
		    d.start();
		    GC.daemon = d;
		    return null;
		}};
	    AccessController.doPrivileged(pa);
	}

    }


    /* Sets the latency target to the given value.
     * Must be invoked while holding the lock.
     */
    private static void setLatencyTarget(long ms) {
	latencyTarget = ms;
	if (daemon == null) {
	    /* Create a new daemon thread */
	    Daemon.create();
	} else {
	    /* Notify the existing daemon thread
             * that the lateency target has changed
	     */
	    lock.notify();
	}
    }


    /**
     * Represents an active garbage-collection latency request.  Instances of
     * this class are created by the <code>{@link #requestLatency}</code>
     * method.  Given a request, the only interesting operation is that of
     * cancellation.
     */
    public static class LatencyRequest implements Comparable {

	/* Instance counter, used to generate unique identifers */
	private static long counter = 0;

	/* Sorted set of active latency requests */
	private static SortedSet requests = null;

	/* Examine the request set and reset the latency target if necessary.
         * Must be invoked while holding the lock.
         */
	private static void adjustLatencyIfNeeded() {
	    if ((requests == null) || requests.isEmpty()) {
		if (latencyTarget != NO_TARGET) {
		    setLatencyTarget(NO_TARGET);
		}
	    } else {
		LatencyRequest r = (LatencyRequest)requests.first();
		if (r.latency != latencyTarget) {
		    setLatencyTarget(r.latency);
		}
	    }
	}

	/* The requested latency, or NO_TARGET
         * if this request has been cancelled
         */
	private long latency;

	/* Unique identifier for this request */
	private long id;

	private LatencyRequest(long ms) {
	    if (ms <= 0) {
		throw new IllegalArgumentException("Non-positive latency: "
                                                   + ms);
	    }
	    this.latency = ms;
	    synchronized (lock) {
		this.id = ++counter;
		if (requests == null) {
		    requests = new TreeSet();
		}
		requests.add(this);
		adjustLatencyIfNeeded();
	    }
	}

	/**
         * Cancels this latency request.
         *
         * @throws  IllegalStateException
         *          If this request has already been cancelled
         */
	public void cancel() {
	    synchronized (lock) {
		if (this.latency == NO_TARGET) {
		    throw new IllegalStateException("Request already"
						    + " cancelled");
		}
		if (!requests.remove(this)) {
		    throw new InternalError("Latency request "
                                            + this + " not found");
		}
		if (requests.isEmpty()) requests = null;
		this.latency = NO_TARGET;
		adjustLatencyIfNeeded();
	    }
	}

	public int compareTo(Object o) {
	    LatencyRequest r = (LatencyRequest)o;
	    long d = this.latency - r.latency;
	    if (d == 0) d = this.id - r.id;
	    return (d < 0) ? -1 : ((d > 0) ? +1 : 0);
	}

	public String toString() {
	    return (LatencyRequest.class.getName()
		    + "[" + latency + "," + id + "]");
	}

    }


    /**
     * Makes a new request for a garbage-collection latency of the given
     * number of real-time milliseconds.  A low-priority daemon thread makes a
     * best effort to ensure that the maximum object-inspection age never
     * exceeds the smallest of the currently active requests.
     *
     * @param   latency
     *          The requested latency
     * 
     * @throws  IllegalArgumentException
     *          If the given <code>latency</code> is non-positive
     */
    public static LatencyRequest requestLatency(long latency) {
	return new LatencyRequest(latency);
    }


    /**
     * Returns the current smallest garbage-collection latency request, or zero
     * if there are no active requests.
     */
    public static long currentLatencyTarget() {
	long t = latencyTarget;
	return (t == NO_TARGET) ? 0 : t;
    }

}