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/**
* Copyright 2003-2010 Terracotta, Inc.
*
* 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 net.sf.ehcache.pool.impl;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.IdentityHashMap;
import java.util.List;
import java.util.Map;
import net.sf.ehcache.pool.PoolEvictor;
/**
* Abstract implementation of a global 'cache value' maximizing pool eviction algorithm.
* <p>
*
* @author Chris Dennis
*
* @param <T> type of store handled by this evictor
*/
public abstract class AbstractBalancedAccessEvictor<T> implements PoolEvictor<T> {
private static final double ALPHA = 1.0;
private static final int SAMPLE_SIZE = 5;
/**
* Comparator used to rank the stores in order of eviction 'cost'.
*/
private final class EvictionCostComparator implements Comparator<T> {
private final long unloadedSize;
private final Map<T, Float> evictionCostCache;
public EvictionCostComparator(long unloadedSize, int collectionSize) {
this.unloadedSize = unloadedSize;
this.evictionCostCache = new IdentityHashMap<T, Float>(collectionSize);
}
public int compare(T s1, T s2) {
Float f1 = evictionCostCache.get(s1);
if (f1 == null) {
f1 = evictionCost(s1, unloadedSize);
evictionCostCache.put(s1, f1);
}
Float f2 = evictionCostCache.get(s2);
if (f2 == null) {
f2 = evictionCost(s2, unloadedSize);
evictionCostCache.put(s2, f2);
}
return Float.compare(f1, f2);
}
}
/**
* Evict the specified number of bytes or the hinted number of elements from the specified store
*
* @param store store to evict from
* @param count number of elements to evict
* @param bytes number of bytes to evict
* @return {@code true} if the eviction succeeded
*/
protected abstract boolean evict(T store, int count, long bytes);
/**
* Return the hit rate for the supplied store.
*
* @param store store to query
* @return hit rate
*/
protected abstract float hitRate(T store);
/**
* Return the miss rate for the supplied store.
*
* @param store store to query
* @return miss rate
*/
protected abstract float missRate(T store);
/**
* Return the number of mappings in the supplied store.
*
* @param store store to size
* @return mapping count
*/
protected abstract long countSize(T store);
/**
* Return the size in bytes of the supplied store.
*
* @param store store to size
* @return size in bytes
*/
protected abstract long byteSize(T store);
/**
* {@inheritDoc}
*/
public boolean freeSpace(Collection<T> from, long bytes) {
if (from == null || from.isEmpty()) {
return false;
}
List<T> random = new ArrayList<T>(from);
Collections.shuffle(random);
for (int i = 0; i < random.size(); i += SAMPLE_SIZE) {
List<T> sorted = random.subList(i, Math.min(SAMPLE_SIZE + i, random.size()));
Collections.sort(sorted, new EvictionCostComparator(getDesiredUnloadedSize(sorted), sorted.size() + 1));
for (T store : sorted) {
int count;
long byteSize = byteSize(store);
long countSize = countSize(store);
if (countSize == 0 || byteSize == 0) {
count = 1;
} else {
count = (int) Math.max((bytes * countSize) / byteSize, 1L);
}
if (evict(store, count, bytes)) {
return true;
}
}
}
return false;
}
private float evictionCost(T store, long unloadedSize) {
/*
* The code below is a simplified version of this:
*
* float meanEntrySize = byteSize / countSize;
* float accessRate = hitRate + missRate;
* float fillLevel = hitRate / accessRate;
* float deltaFillLevel = fillLevel / byteSize;
*
* return meanEntrySize * accessRate * deltaFillLevel * hitDistributionFunction(fillLevel);
*/
float hitRate = hitRate(store);
float missRate = missRate(store);
long countSize = countSize(store);
float accessRate = hitRate + missRate;
if (accessRate == 0.0f) {
if (byteSize(store) > unloadedSize) {
return Float.NEGATIVE_INFINITY;
} else {
return Float.POSITIVE_INFINITY;
}
} else if (hitRate == 0.0f) {
return Float.POSITIVE_INFINITY;
} else {
float cost = (hitRate / countSize) * hitDistributionFunction(hitRate / accessRate);
if (Float.isNaN(cost)) {
throw new AssertionError(String.format("NaN Eviction Cost [hit:%f miss:%f size:%d]", hitRate, missRate, countSize));
} else {
return cost;
}
}
}
private static float hitDistributionFunction(float fillLevel) {
return (float) Math.pow(fillLevel, -ALPHA);
}
private long getDesiredUnloadedSize(Collection<T> from) {
long unloadedSize = 0L;
for (T s : from) {
unloadedSize += byteSize(s);
}
return unloadedSize / from.size();
}
}
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