org.h2.dev.cache

Class CacheLIRS<K,V>

java.lang.Object

java.util.AbstractMap<K,V>

org.h2.dev.cache.CacheLIRS<K,V>

Type Parameters:

K - the key type

V - the value type

All Implemented Interfaces:

java.util.Map<K,V>

public class CacheLIRS<K,V>
extends java.util.AbstractMap<K,V>

A scan resistant cache. It is meant to cache objects that are relatively costly to acquire, for example file content.

This implementation is multi-threading safe and supports concurrent access. Null keys or null values are not allowed. The map fill factor is at most 75%.

Each entry is assigned a distinct memory size, and the cache will try to use at most the specified amount of memory. The memory unit is not relevant, however it is suggested to use bytes as the unit.

This class implements an approximation of the the LIRS replacement algorithm invented by Xiaodong Zhang and Song Jiang as described in http://www.cse.ohio-state.edu/~zhang/lirs-sigmetrics-02.html with a few smaller changes: An additional queue for non-resident entries is used, to prevent unbound memory usage. The maximum size of this queue is at most the size of the rest of the stack. About 6.25% of the mapped entries are cold.

Internally, the cache is split into a number of segments, and each segment is an individual LIRS cache.

Accessed entries are only moved to the top of the stack if at least a number of other entries have been moved to the front (8 per segment by default). Write access and moving entries to the top of the stack is synchronized per segment.

Nested Class Summary

Nested classes/interfaces inherited from class java.util.AbstractMap

java.util.AbstractMap.SimpleEntry<K,V>, java.util.AbstractMap.SimpleImmutableEntry<K,V>

Constructor Summary

Constructors

Constructor and Description
CacheLIRS(long maxMemory) Create a new cache with the given number of entries, and the default settings (16 segments, and stack move distance of 8.
CacheLIRS(long maxMemory, int segmentCount, int stackMoveDistance) Create a new cache with the given memory size.

Method Summary

Modifier and Type	Method and Description
void	clear() Remove all entries.
boolean	containsKey(java.lang.Object key) Check whether there is a resident entry for the given key.
java.util.Set<java.util.Map.Entry<K,V>>	entrySet() Get the entry set for all resident entries.
V	get(java.lang.Object key) Get the value for the given key if the entry is cached.
long	getMaxMemory() Get the maximum memory to use.
int	getMemory(K key) Get the memory used for the given key.
long	getUsedMemory() Get the currently used memory.
java.util.List<K>	keys(boolean cold, boolean nonResident) Get the list of keys.
java.util.Set<K>	keySet() Get the set of keys for resident entries.
protected void	onRemove(K key) This method is called after the value for the given key was removed.
V	peek(K key) Get the value for the given key if the entry is cached.
V	put(K key, V value) Add an entry to the cache using a memory size of 1.
V	put(K key, V value, int memory) Add an entry to the cache.
V	remove(java.lang.Object key) Remove an entry.
void	setMaxMemory(long maxMemory) Set the maximum memory this cache should use.
int	size() Get the number of resident entries.
int	sizeHot() Get the number of hot entries in the cache.
int	sizeMapArray() Get the length of the internal map array.
int	sizeNonResident() Get the number of non-resident entries in the cache.
protected int	sizeOf(K key, V value) Get the size of the given value.

Methods inherited from class java.util.AbstractMap

clone, containsValue, equals, hashCode, isEmpty, putAll, toString, values

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll

Constructor Detail

CacheLIRS

public CacheLIRS(long maxMemory)

Create a new cache with the given number of entries, and the default settings (16 segments, and stack move distance of 8.

Parameters:

maxMemory - the maximum memory to use (1 or larger)

CacheLIRS

public CacheLIRS(long maxMemory,
                 int segmentCount,
                 int stackMoveDistance)

Create a new cache with the given memory size.

Parameters:

maxMemory - the maximum memory to use (1 or larger)

segmentCount - the number of cache segments (must be a power of 2)

stackMoveDistance - how many other item are to be moved to the top of the stack before the current item is moved

Method Detail

clear

public void clear()

Remove all entries.

Specified by:

clear in interface java.util.Map<K,V>

Overrides:

clear in class java.util.AbstractMap<K,V>

containsKey

public boolean containsKey(java.lang.Object key)

Check whether there is a resident entry for the given key. This method does not adjust the internal state of the cache.

Specified by:

containsKey in interface java.util.Map<K,V>

Overrides:

containsKey in class java.util.AbstractMap<K,V>

Parameters:

key - the key (may not be null)

Returns:

true if there is a resident entry

peek

public V peek(K key)

Get the value for the given key if the entry is cached. This method does not modify the internal state.

Parameters:

key - the key (may not be null)

Returns:

the value, or null if there is no resident entry

put

public V put(K key,
             V value,
             int memory)

Add an entry to the cache. The entry may or may not exist in the cache yet. This method will usually mark unknown entries as cold and known entries as hot.

Parameters:

key - the key (may not be null)

value - the value (may not be null)

memory - the memory used for the given entry

Returns:

the old value, or null if there was no resident entry

put

public V put(K key,
             V value)

Add an entry to the cache using a memory size of 1.

Specified by:

put in interface java.util.Map<K,V>

Overrides:

put in class java.util.AbstractMap<K,V>

Parameters:

key - the key (may not be null)

value - the value (may not be null)

Returns:

the old value, or null if there was no resident entry

sizeOf

protected int sizeOf(K key,
                     V value)

Get the size of the given value. The default implementation returns 1.

Parameters:

key - the key

value - the value

Returns:

the size

onRemove

protected void onRemove(K key)

This method is called after the value for the given key was removed. It is not called on clear or put when replacing a value.

Parameters:

key - the key

remove

public V remove(java.lang.Object key)

Remove an entry. Both resident and non-resident entries can be removed.

Specified by:

remove in interface java.util.Map<K,V>

Overrides:

remove in class java.util.AbstractMap<K,V>

Parameters:

key - the key (may not be null)

Returns:

the old value, or null if there was no resident entry

getMemory

public int getMemory(K key)

Get the memory used for the given key.

Parameters:

key - the key (may not be null)

Returns:

the memory, or 0 if there is no resident entry

get

public V get(java.lang.Object key)

Get the value for the given key if the entry is cached. This method adjusts the internal state of the cache sometimes, to ensure commonly used entries stay in the cache.

Specified by:

get in interface java.util.Map<K,V>

Overrides:

get in class java.util.AbstractMap<K,V>

Parameters:

key - the key (may not be null)

Returns:

the value, or null if there is no resident entry

getUsedMemory

public long getUsedMemory()

Get the currently used memory.

Returns:

the used memory

setMaxMemory

public void setMaxMemory(long maxMemory)

Set the maximum memory this cache should use. This will not immediately cause entries to get removed however; it will only change the limit. To resize the internal array, call the clear method.

Parameters:

maxMemory - the maximum size (1 or larger)

getMaxMemory

public long getMaxMemory()

Get the maximum memory to use.

Returns:

the maximum memory

entrySet

public java.util.Set<java.util.Map.Entry<K,V>> entrySet()

Get the entry set for all resident entries.

Specified by:

entrySet in interface java.util.Map<K,V>

Specified by:

entrySet in class java.util.AbstractMap<K,V>

Returns:

the entry set

keySet

public java.util.Set<K> keySet()

Get the set of keys for resident entries.

Specified by:

keySet in interface java.util.Map<K,V>

Overrides:

keySet in class java.util.AbstractMap<K,V>

Returns:

the set of keys

sizeNonResident

public int sizeNonResident()

Get the number of non-resident entries in the cache.

Returns:

the number of non-resident entries

sizeMapArray

public int sizeMapArray()

Get the length of the internal map array.

Returns:

the size of the array

sizeHot

public int sizeHot()

Get the number of hot entries in the cache.

Returns:

the number of hot entries

size

public int size()

Get the number of resident entries.

Specified by:

size in interface java.util.Map<K,V>

Overrides:

size in class java.util.AbstractMap<K,V>

Returns:

the number of entries

keys

public java.util.List<K> keys(boolean cold,
                              boolean nonResident)

Get the list of keys. This method allows to read the internal state of the cache.

Parameters:

cold - if true, only keys for the cold entries are returned

nonResident - true for non-resident entries

Returns:

the key list