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/**
* 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.hadoop.util;
import java.util.Comparator;
import java.util.Iterator;
import java.util.PriorityQueue;
import org.apache.hadoop.HadoopIllegalArgumentException;
import org.apache.hadoop.classification.InterfaceAudience;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
/**
* A low memory footprint Cache which extends {@link LightWeightGSet}.
* An entry in the cache is expired if
* (1) it is added to the cache longer than the creation-expiration period, and
* (2) it is not accessed for the access-expiration period.
* When an entry is expired, it may be evicted from the cache.
* When the size limit of the cache is set, the cache will evict the entries
* with earliest expiration time, even if they are not expired.
*
* It is guaranteed that number of entries in the cache is less than or equal
* to the size limit. However, It is not guaranteed that expired entries are
* evicted from the cache. An expired entry may possibly be accessed after its
* expiration time. In such case, the expiration time may be updated.
*
* This class does not support null entry.
*
* This class is not thread safe.
*
* @param Key type for looking up the entries
* @param Entry type, which must be
* (1) a subclass of K, and
* (2) implementing {@link Entry} interface, and
*/
@InterfaceAudience.Private
public class LightWeightCache extends LightWeightGSet {
/** Limit the number of entries in each eviction. */
private static final int EVICTION_LIMIT = 1 << 16;
/**
* Entries of {@link LightWeightCache}.
*/
public static interface Entry extends LinkedElement {
/** Set the expiration time. */
public void setExpirationTime(long timeNano);
/** Get the expiration time. */
public long getExpirationTime();
}
/** Comparator for sorting entries by expiration time in ascending order. */
private static final Comparator expirationTimeComparator
= new Comparator() {
@Override
public int compare(Entry left, Entry right) {
final long l = left.getExpirationTime();
final long r = right.getExpirationTime();
return l > r? 1: l < r? -1: 0;
}
};
/** A clock for measuring time so that it can be mocked in unit tests. */
static class Clock {
/** @return the current time. */
long currentTime() {
return System.nanoTime();
}
}
private static int updateRecommendedLength(int recommendedLength,
int sizeLimit) {
return sizeLimit > 0 && sizeLimit < recommendedLength?
(sizeLimit/4*3) // 0.75 load factor
: recommendedLength;
}
/*
* The memory footprint for java.util.PriorityQueue is low but the
* remove(Object) method runs in linear time. We may improve it by using a
* balanced tree. However, we do not yet have a low memory footprint balanced
* tree implementation.
*/
private final PriorityQueue queue;
private final long creationExpirationPeriod;
private final long accessExpirationPeriod;
private final int sizeLimit;
private final Clock clock;
/**
* @param recommendedLength Recommended size of the internal array.
* @param sizeLimit the limit of the size of the cache.
* The limit is disabled if it is <= 0.
* @param creationExpirationPeriod the time period C > 0 in nanoseconds that
* the creation of an entry is expired if it is added to the cache
* longer than C.
* @param accessExpirationPeriod the time period A >= 0 in nanoseconds that
* the access of an entry is expired if it is not accessed
* longer than A.
*/
public LightWeightCache(final int recommendedLength,
final int sizeLimit,
final long creationExpirationPeriod,
final long accessExpirationPeriod) {
this(recommendedLength, sizeLimit,
creationExpirationPeriod, accessExpirationPeriod, new Clock());
}
@VisibleForTesting
LightWeightCache(final int recommendedLength,
final int sizeLimit,
final long creationExpirationPeriod,
final long accessExpirationPeriod,
final Clock clock) {
super(updateRecommendedLength(recommendedLength, sizeLimit));
this.sizeLimit = sizeLimit;
if (creationExpirationPeriod <= 0) {
throw new IllegalArgumentException("creationExpirationPeriod = "
+ creationExpirationPeriod + " <= 0");
}
this.creationExpirationPeriod = creationExpirationPeriod;
if (accessExpirationPeriod < 0) {
throw new IllegalArgumentException("accessExpirationPeriod = "
+ accessExpirationPeriod + " < 0");
}
this.accessExpirationPeriod = accessExpirationPeriod;
this.queue = new PriorityQueue(
sizeLimit > 0? sizeLimit + 1: 1 << 10, expirationTimeComparator);
this.clock = clock;
}
void setExpirationTime(final Entry e, final long expirationPeriod) {
e.setExpirationTime(clock.currentTime() + expirationPeriod);
}
boolean isExpired(final Entry e, final long now) {
return now > e.getExpirationTime();
}
private E evict() {
@SuppressWarnings("unchecked")
final E polled = (E)queue.poll();
final E removed = super.remove(polled);
Preconditions.checkState(removed == polled);
return polled;
}
/** Evict expired entries. */
private void evictExpiredEntries() {
final long now = clock.currentTime();
for(int i = 0; i < EVICTION_LIMIT; i++) {
final Entry peeked = queue.peek();
if (peeked == null || !isExpired(peeked, now)) {
return;
}
final E evicted = evict();
Preconditions.checkState(evicted == peeked);
}
}
/** Evict entries in order to enforce the size limit of the cache. */
private void evictEntries() {
if (sizeLimit > 0) {
for(int i = size(); i > sizeLimit; i--) {
evict();
}
}
}
@Override
public E get(K key) {
final E entry = super.get(key);
if (entry != null) {
if (accessExpirationPeriod > 0) {
// update expiration time
final Entry existing = (Entry)entry;
Preconditions.checkState(queue.remove(existing));
setExpirationTime(existing, accessExpirationPeriod);
queue.offer(existing);
}
}
return entry;
}
@Override
public E put(final E entry) {
if (!(entry instanceof Entry)) {
throw new HadoopIllegalArgumentException(
"!(entry instanceof Entry), entry.getClass()=" + entry.getClass());
}
evictExpiredEntries();
final E existing = super.put(entry);
if (existing != null) {
queue.remove(existing);
}
final Entry e = (Entry)entry;
setExpirationTime(e, creationExpirationPeriod);
queue.offer(e);
evictEntries();
return existing;
}
@Override
public E remove(K key) {
evictExpiredEntries();
final E removed = super.remove(key);
if (removed != null) {
Preconditions.checkState(queue.remove(removed));
}
return removed;
}
@Override
public Iterator iterator() {
final Iterator iter = super.iterator();
return new Iterator() {
@Override
public boolean hasNext() {
return iter.hasNext();
}
@Override
public E next() {
return iter.next();
}
@Override
public void remove() {
// It would be tricky to support this because LightWeightCache#remove
// may evict multiple elements via evictExpiredEntries.
throw new UnsupportedOperationException("Remove via iterator is " +
"not supported for LightWeightCache");
}
};
}
}
