/*
* Copyright 2015 Ben Manes. All Rights Reserved.
*
* Licensed 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 com.github.benmanes.caffeine.cache.simulator.policy.linked;
import static java.util.Objects.requireNonNull;
import static java.util.stream.Collectors.toSet;
import java.util.Set;
import org.apache.commons.lang3.StringUtils;
import com.github.benmanes.caffeine.cache.simulator.BasicSettings;
import com.github.benmanes.caffeine.cache.simulator.admission.Admission;
import com.github.benmanes.caffeine.cache.simulator.admission.Admittor;
import com.github.benmanes.caffeine.cache.simulator.policy.Policy;
import com.github.benmanes.caffeine.cache.simulator.policy.PolicyStats;
import com.google.common.base.MoreObjects;
import com.typesafe.config.Config;
import it.unimi.dsi.fastutil.longs.Long2ObjectMap;
import it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap;
/**
* Least/Most Frequency Used in O(1) time as described in <a href="http://dhruvbird.com/lfu.pdf"> An
* O(1) algorithm for implementing the LFU cache eviction scheme</a>.
*
* @author ben.manes@gmail.com (Ben Manes)
*/
public final class FrequentlyUsedPolicy implements Policy {
final PolicyStats policyStats;
final Long2ObjectMap<Node> data;
final EvictionPolicy policy;
final FrequencyNode freq0;
final Admittor admittor;
final int maximumSize;
public FrequentlyUsedPolicy(Admission admission, EvictionPolicy policy, Config config) {
this.policyStats = new PolicyStats(admission.format("linked." + policy.label()));
this.admittor = admission.from(config, policyStats);
BasicSettings settings = new BasicSettings(config);
this.data = new Long2ObjectOpenHashMap<>();
this.maximumSize = settings.maximumSize();
this.policy = requireNonNull(policy);
this.freq0 = new FrequencyNode();
}
/** Returns all variations of this policy based on the configuration parameters. */
public static Set<Policy> policies(Config config, EvictionPolicy policy) {
BasicSettings settings = new BasicSettings(config);
return settings.admission().stream().map(admission ->
new FrequentlyUsedPolicy(admission, policy, config)
).collect(toSet());
}
@Override
public PolicyStats stats() {
return policyStats;
}
@Override
public void record(long key) {
policyStats.recordOperation();
Node node = data.get(key);
admittor.record(key);
if (node == null) {
onMiss(key);
} else {
onHit(node);
}
}
/** Moves the entry to the next higher frequency list, creating it if necessary. */
private void onHit(Node node) {
policyStats.recordHit();
int newCount = node.freq.count + 1;
FrequencyNode freqN = (node.freq.next.count == newCount)
? node.freq.next
: new FrequencyNode(newCount, node.freq);
node.remove();
if (node.freq.isEmpty()) {
node.freq.remove();
}
node.freq = freqN;
node.append();
}
/** Adds the entry, creating an initial frequency list of 1 if necessary, and evicts if needed. */
private void onMiss(long key) {
FrequencyNode freq1 = (freq0.next.count == 1)
? freq0.next
: new FrequencyNode(1, freq0);
Node node = new Node(key, freq1);
policyStats.recordMiss();
data.put(key, node);
node.append();
evict(node);
}
/** Evicts while the map exceeds the maximum capacity. */
private void evict(Node candidate) {
if (data.size() > maximumSize) {
Node victim = nextVictim(candidate);
boolean admit = admittor.admit(candidate.key, victim.key);
if (admit) {
evictEntry(victim);
} else {
evictEntry(candidate);
}
policyStats.recordEviction();
}
}
/**
* Returns the next victim, excluding the newly added candidate. This exclusion is required so
* that a candidate has a fair chance to be used, rather than always rejected due to existing
* entries having a high frequency from the distant past.
*/
Node nextVictim(Node candidate) {
if (policy == EvictionPolicy.MFU) {
// highest, never the candidate
return freq0.prev.nextNode.next;
}
// find the lowest that is not the candidate
Node victim = freq0.next.nextNode.next;
if (victim == candidate) {
victim = (victim.next == victim.prev)
? victim.freq.next.nextNode.next
: victim.next;
}
return victim;
}
/** Removes the entry. */
private void evictEntry(Node node) {
data.remove(node.key);
node.remove();
if (node.freq.isEmpty()) {
node.freq.remove();
}
}
public enum EvictionPolicy {
LFU, MFU;
public String label() {
return StringUtils.capitalize(name().toLowerCase());
}
}
/** A frequency count and associated chain of cache entries. */
static final class FrequencyNode {
final int count;
final Node nextNode;
FrequencyNode prev;
FrequencyNode next;
public FrequencyNode() {
nextNode = new Node(this);
this.prev = this;
this.next = this;
this.count = 0;
}
public FrequencyNode(int count, FrequencyNode prev) {
nextNode = new Node(this);
this.prev = prev;
this.next = prev.next;
prev.next = this;
next.prev = this;
this.count = count;
}
public boolean isEmpty() {
return (nextNode == nextNode.next);
}
/** Removes the node from the list. */
public void remove() {
prev.next = next;
next.prev = prev;
next = prev = null;
}
@Override
public String toString() {
return MoreObjects.toStringHelper(this)
.add("count", count)
.toString();
}
}
/** A cache entry on the frequency node's chain. */
static final class Node {
final long key;
FrequencyNode freq;
Node prev;
Node next;
public Node(FrequencyNode freq) {
this.key = Long.MIN_VALUE;
this.freq = freq;
this.prev = this;
this.next = this;
}
public Node(long key, FrequencyNode freq) {
this.next = null;
this.prev = null;
this.freq = freq;
this.key = key;
}
/** Appends the node to the tail of the list. */
public void append() {
prev = freq.nextNode.prev;
next = freq.nextNode;
prev.next = this;
next.prev = this;
}
/** Removes the node from the list. */
public void remove() {
prev.next = next;
next.prev = prev;
next = prev = null;
}
@Override
public String toString() {
return MoreObjects.toStringHelper(this)
.add("key", key)
.add("freq", freq)
.toString();
}
}
}