/**
 * Copyright (C) 2009-2013 FoundationDB, LLC
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

package com.foundationdb.util;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.lang.management.MemoryMXBean;
import java.lang.management.MemoryUsage;
import java.util.ArrayList;
import java.util.List;

public class GCMonitor extends Thread {
    private static final Logger LOG = LoggerFactory.getLogger(GCMonitor.class);
    private static final String THREAD_NAME = "GC_MONITOR";
    private static final String LOG_MSG_FORMAT = "%s spent %s ms on %s collections, heap at %.2f%%";

    private static class GCInfo {
        public final GarbageCollectorMXBean mxBean;
        public long lastCollectionCount = 0;
        public long lastCollectionTime = 0;

        public GCInfo(GarbageCollectorMXBean mxBean) {
            this.mxBean = mxBean;
        }

        public long updateCollectionTime() {
            long prev = lastCollectionTime;
            lastCollectionTime = mxBean.getCollectionTime();
            return lastCollectionTime - prev;
        }

        public long updateCollectionCount() {
            long prev = lastCollectionCount;
            lastCollectionCount = mxBean.getCollectionCount();
            return lastCollectionCount - prev;
        }
    }

    private final int interval;
    private final int logThreshold;
    private final List<GCInfo> gcInfo = new ArrayList<>();
    private final MemoryMXBean memoryBean = ManagementFactory.getMemoryMXBean();
    private volatile boolean running;


    public GCMonitor(int interval, int logThreshold) {
        super(THREAD_NAME);
        this.interval = interval;
        this.logThreshold = logThreshold;
        for(GarbageCollectorMXBean gcBean : ManagementFactory.getGarbageCollectorMXBeans()) {
            gcInfo.add(new GCInfo(gcBean));
        }
    }

    @Override
    public void run() {
        running = !gcInfo.isEmpty();
        while(running) {
            runInternal();
            try {
                Thread.sleep(interval);
            } catch(InterruptedException e) {
                LOG.debug("Interrupted, exiting", e);
                running = false;
            }
        }
    }

    public void stopRunning() {
        this.running = false;
        interrupt();
    }

    private void runInternal() {
        for(GCInfo info : gcInfo) {
            long elapsed = info.updateCollectionTime();
            long collections = info.updateCollectionCount();

            // Skip if there were no collections or very few (so as to not log on every startup)
            if(collections == 0 || info.lastCollectionCount <= 3) {
                continue;
            }

            long timePerCollection = elapsed / collections;
            boolean doWarn = (timePerCollection >= logThreshold);
            boolean doDebug = LOG.isDebugEnabled();

            if(doWarn || doDebug) {
                MemoryUsage usage = memoryBean.getHeapMemoryUsage();
                double percent = 100 * ((double)usage.getUsed() / (double)usage.getMax());
                String message =  String.format(LOG_MSG_FORMAT, info.mxBean.getName(), elapsed, collections, percent);
                if(doWarn) {
                    LOG.warn(message);
                } else {
                    LOG.debug(message);
                }
            }

            // Anything we can do to try and reduce pressure?
        }
    }
}