/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.solr.util;
import java.lang.invoke.MethodHandles;
import java.lang.ref.WeakReference;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.TreeSet;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.ReentrantLock;
import org.apache.solr.common.util.Cache;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* A LFU cache implementation based upon ConcurrentHashMap.
* <p>
* This is not a terribly efficient implementation. The tricks used in the
* LRU version were not directly usable, perhaps it might be possible to
* rewrite them with LFU in mind.
* <p>
* <b>This API is experimental and subject to change</b>
*
* @since solr 1.6
*/
public class ConcurrentLFUCache<K, V> implements Cache<K,V> {
private static final Logger log = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
private final ConcurrentHashMap<Object, CacheEntry<K, V>> map;
private final int upperWaterMark, lowerWaterMark;
private final ReentrantLock markAndSweepLock = new ReentrantLock(true);
private boolean isCleaning = false; // not volatile... piggybacked on other volatile vars
private final boolean newThreadForCleanup;
private volatile boolean islive = true;
private final Stats stats = new Stats();
@SuppressWarnings("unused")
private final int acceptableWaterMark;
private long lowHitCount = 0; // not volatile, only accessed in the cleaning method
private final EvictionListener<K, V> evictionListener;
private CleanupThread cleanupThread;
private final boolean timeDecay;
public ConcurrentLFUCache(int upperWaterMark, final int lowerWaterMark, int acceptableSize,
int initialSize, boolean runCleanupThread, boolean runNewThreadForCleanup,
EvictionListener<K, V> evictionListener, boolean timeDecay) {
if (upperWaterMark < 1) throw new IllegalArgumentException("upperWaterMark must be > 0");
if (lowerWaterMark >= upperWaterMark)
throw new IllegalArgumentException("lowerWaterMark must be < upperWaterMark");
map = new ConcurrentHashMap<>(initialSize);
newThreadForCleanup = runNewThreadForCleanup;
this.upperWaterMark = upperWaterMark;
this.lowerWaterMark = lowerWaterMark;
this.acceptableWaterMark = acceptableSize;
this.evictionListener = evictionListener;
this.timeDecay = timeDecay;
if (runCleanupThread) {
cleanupThread = new CleanupThread(this);
cleanupThread.start();
}
}
public ConcurrentLFUCache(int size, int lowerWatermark) {
this(size, lowerWatermark, (int) Math.floor((lowerWatermark + size) / 2),
(int) Math.ceil(0.75 * size), false, false, null, true);
}
public void setAlive(boolean live) {
islive = live;
}
@Override
public V get(K key) {
CacheEntry<K, V> e = map.get(key);
if (e == null) {
if (islive) stats.missCounter.incrementAndGet();
return null;
}
if (islive) {
e.lastAccessed = stats.accessCounter.incrementAndGet();
e.hits.incrementAndGet();
}
return e.value;
}
@Override
public V remove(K key) {
CacheEntry<K, V> cacheEntry = map.remove(key);
if (cacheEntry != null) {
stats.size.decrementAndGet();
return cacheEntry.value;
}
return null;
}
@Override
public V put(K key, V val) {
if (val == null) return null;
CacheEntry<K, V> e = new CacheEntry<>(key, val, stats.accessCounter.incrementAndGet());
CacheEntry<K, V> oldCacheEntry = map.put(key, e);
int currentSize;
if (oldCacheEntry == null) {
currentSize = stats.size.incrementAndGet();
} else {
currentSize = stats.size.get();
}
if (islive) {
stats.putCounter.incrementAndGet();
} else {
stats.nonLivePutCounter.incrementAndGet();
}
// Check if we need to clear out old entries from the cache.
// isCleaning variable is checked instead of markAndSweepLock.isLocked()
// for performance because every put invokation will check until
// the size is back to an acceptable level.
//
// There is a race between the check and the call to markAndSweep, but
// it's unimportant because markAndSweep actually aquires the lock or returns if it can't.
//
// Thread safety note: isCleaning read is piggybacked (comes after) other volatile reads
// in this method.
if (currentSize > upperWaterMark && !isCleaning) {
if (newThreadForCleanup) {
new Thread(this::markAndSweep).start();
} else if (cleanupThread != null) {
cleanupThread.wakeThread();
} else {
markAndSweep();
}
}
return oldCacheEntry == null ? null : oldCacheEntry.value;
}
/**
* Removes items from the cache to bring the size down to the lowerWaterMark.
*/
private void markAndSweep() {
if (!markAndSweepLock.tryLock()) return;
try {
long lowHitCount = this.lowHitCount;
isCleaning = true;
this.lowHitCount = lowHitCount; // volatile write to make isCleaning visible
int sz = stats.size.get();
if (sz <= upperWaterMark) {
/* SOLR-7585: Even though we acquired a lock, multiple threads might detect a need for calling this method.
* Locking keeps these from executing at the same time, so they run sequentially. The second and subsequent
* sequential runs of this method don't need to be done, since there are no elements to remove.
*/
return;
}
int wantToRemove = sz - lowerWaterMark;
TreeSet<CacheEntry<K, V>> tree = new TreeSet<>();
for (CacheEntry<K, V> ce : map.values()) {
// set hitsCopy to avoid later Atomic reads. Primitive types are faster than the atomic get().
ce.hitsCopy = ce.hits.get();
ce.lastAccessedCopy = ce.lastAccessed;
if (timeDecay) {
ce.hits.set(ce.hitsCopy >>> 1);
}
if (tree.size() < wantToRemove) {
tree.add(ce);
} else {
/*
* SOLR-7585: Before doing this part, make sure the TreeSet actually has an element, since the first() method
* fails with NoSuchElementException if the set is empty. If that test passes, check hits. This test may
* never actually fail due to the upperWaterMark check above, but we'll do it anyway.
*/
if (tree.size() > 0) {
/* If hits are not equal, we can remove before adding which is slightly faster. I can no longer remember
* why removing first is faster, but I vaguely remember being sure about it!
*/
if (ce.hitsCopy < tree.first().hitsCopy) {
tree.remove(tree.first());
tree.add(ce);
} else if (ce.hitsCopy == tree.first().hitsCopy) {
tree.add(ce);
tree.remove(tree.first());
}
}
}
}
for (CacheEntry<K, V> e : tree) {
evictEntry(e.key);
}
} finally {
isCleaning = false; // set before markAndSweep.unlock() for visibility
markAndSweepLock.unlock();
}
}
private void evictEntry(K key) {
CacheEntry<K, V> o = map.remove(key);
if (o == null) return;
stats.size.decrementAndGet();
stats.evictionCounter.incrementAndGet();
if (evictionListener != null) evictionListener.evictedEntry(o.key, o.value);
}
/**
* Returns 'n' number of least used entries present in this cache.
* <p>
* This uses a TreeSet to collect the 'n' least used items ordered by ascending hitcount
* and returns a LinkedHashMap containing 'n' or less than 'n' entries.
*
* @param n the number of items needed
* @return a LinkedHashMap containing 'n' or less than 'n' entries
*/
public Map<K, V> getLeastUsedItems(int n) {
Map<K, V> result = new LinkedHashMap<>();
if (n <= 0)
return result;
TreeSet<CacheEntry<K, V>> tree = new TreeSet<>();
// we need to grab the lock since we are changing the copy variables
markAndSweepLock.lock();
try {
for (Map.Entry<Object, CacheEntry<K, V>> entry : map.entrySet()) {
CacheEntry<K, V> ce = entry.getValue();
ce.hitsCopy = ce.hits.get();
ce.lastAccessedCopy = ce.lastAccessed;
if (tree.size() < n) {
tree.add(ce);
} else {
// If the hits are not equal, we can remove before adding
// which is slightly faster
if (ce.hitsCopy < tree.first().hitsCopy) {
tree.remove(tree.first());
tree.add(ce);
} else if (ce.hitsCopy == tree.first().hitsCopy) {
tree.add(ce);
tree.remove(tree.first());
}
}
}
} finally {
markAndSweepLock.unlock();
}
for (CacheEntry<K, V> e : tree) {
result.put(e.key, e.value);
}
return result;
}
/**
* Returns 'n' number of most used entries present in this cache.
* <p>
* This uses a TreeSet to collect the 'n' most used items ordered by descending hitcount
* and returns a LinkedHashMap containing 'n' or less than 'n' entries.
*
* @param n the number of items needed
* @return a LinkedHashMap containing 'n' or less than 'n' entries
*/
public Map<K, V> getMostUsedItems(int n) {
Map<K, V> result = new LinkedHashMap<>();
if (n <= 0)
return result;
TreeSet<CacheEntry<K, V>> tree = new TreeSet<>();
// we need to grab the lock since we are changing the copy variables
markAndSweepLock.lock();
try {
for (Map.Entry<Object, CacheEntry<K, V>> entry : map.entrySet()) {
CacheEntry<K, V> ce = entry.getValue();
ce.hitsCopy = ce.hits.get();
ce.lastAccessedCopy = ce.lastAccessed;
if (tree.size() < n) {
tree.add(ce);
} else {
// If the hits are not equal, we can remove before adding
// which is slightly faster
if (ce.hitsCopy > tree.last().hitsCopy) {
tree.remove(tree.last());
tree.add(ce);
} else if (ce.hitsCopy == tree.last().hitsCopy) {
tree.add(ce);
tree.remove(tree.last());
}
}
}
} finally {
markAndSweepLock.unlock();
}
for (CacheEntry<K, V> e : tree) {
result.put(e.key, e.value);
}
return result;
}
public int size() {
return stats.size.get();
}
@Override
public void clear() {
map.clear();
}
public Map<Object, CacheEntry<K, V>> getMap() {
return map;
}
public static class CacheEntry<K, V> implements Comparable<CacheEntry<K, V>> {
K key;
V value;
volatile AtomicLong hits = new AtomicLong(0);
long hitsCopy = 0;
volatile long lastAccessed = 0;
long lastAccessedCopy = 0;
public CacheEntry(K key, V value, long lastAccessed) {
this.key = key;
this.value = value;
this.lastAccessed = lastAccessed;
}
@Override
public int compareTo(CacheEntry<K, V> that) {
if (this.hitsCopy == that.hitsCopy) {
if (this.lastAccessedCopy == that.lastAccessedCopy) {
return 0;
}
return this.lastAccessedCopy < that.lastAccessedCopy ? 1 : -1;
}
return this.hitsCopy < that.hitsCopy ? 1 : -1;
}
@Override
public int hashCode() {
return value.hashCode();
}
@Override
public boolean equals(Object obj) {
return value.equals(obj);
}
@Override
public String toString() {
return "key: " + key + " value: " + value + " hits:" + hits.get();
}
}
private boolean isDestroyed = false;
public void destroy() {
try {
if (cleanupThread != null) {
cleanupThread.stopThread();
}
} finally {
isDestroyed = true;
}
}
public Stats getStats() {
return stats;
}
public static class Stats {
private final AtomicLong accessCounter = new AtomicLong(0),
putCounter = new AtomicLong(0),
nonLivePutCounter = new AtomicLong(0),
missCounter = new AtomicLong();
private final AtomicInteger size = new AtomicInteger();
private AtomicLong evictionCounter = new AtomicLong();
public long getCumulativeLookups() {
return (accessCounter.get() - putCounter.get() - nonLivePutCounter.get()) + missCounter.get();
}
public long getCumulativeHits() {
return accessCounter.get() - putCounter.get() - nonLivePutCounter.get();
}
public long getCumulativePuts() {
return putCounter.get();
}
public long getCumulativeEvictions() {
return evictionCounter.get();
}
public int getCurrentSize() {
return size.get();
}
public long getCumulativeNonLivePuts() {
return nonLivePutCounter.get();
}
public long getCumulativeMisses() {
return missCounter.get();
}
public void add(Stats other) {
accessCounter.addAndGet(other.accessCounter.get());
putCounter.addAndGet(other.putCounter.get());
nonLivePutCounter.addAndGet(other.nonLivePutCounter.get());
missCounter.addAndGet(other.missCounter.get());
evictionCounter.addAndGet(other.evictionCounter.get());
size.set(Math.max(size.get(), other.size.get()));
}
}
public static interface EvictionListener<K, V> {
public void evictedEntry(K key, V value);
}
private static class CleanupThread extends Thread {
private WeakReference<ConcurrentLFUCache> cache;
private boolean stop = false;
public CleanupThread(ConcurrentLFUCache c) {
cache = new WeakReference<>(c);
}
@Override
public void run() {
while (true) {
synchronized (this) {
if (stop) break;
try {
this.wait();
} catch (InterruptedException e) {
}
}
if (stop) break;
ConcurrentLFUCache c = cache.get();
if (c == null) break;
c.markAndSweep();
}
}
void wakeThread() {
synchronized (this) {
this.notify();
}
}
void stopThread() {
synchronized (this) {
stop = true;
this.notify();
}
}
}
@Override
protected void finalize() throws Throwable {
try {
if (!isDestroyed) {
log.error("ConcurrentLFUCache was not destroyed prior to finalize(), indicates a bug -- POSSIBLE RESOURCE LEAK!!!");
destroy();
}
} finally {
super.finalize();
}
}
}