org.h2.mvstore.cache

Class CacheLongKeyLIRS<V>

java.lang.Object

org.h2.mvstore.cache.CacheLongKeyLIRS<V>

Type Parameters:

V - the value type

public class CacheLongKeyLIRS<V>
extends java.lang.Object

A scan resistant cache that uses keys of type long. 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

Modifier and Type	Class and Description
static class	CacheLongKeyLIRS.Config The cache configuration.

Constructor Summary

Constructors

Constructor and Description
CacheLongKeyLIRS(CacheLongKeyLIRS.Config config) Create a new cache with the given memory size.

Method Summary

Methods

Modifier and Type	Method and Description
void	clear() Remove all entries.
boolean	containsKey(long key) Check whether there is a resident entry for the given key.
boolean	containsValue(java.lang.Object value) Check whether the given value is stored.
java.util.Set<java.util.Map.Entry<java.lang.Long,V>>	entrySet() Get the entry set for all resident entries.
V	get(long key) Get the value for the given key if the entry is cached.
long	getHits() Get the number of cache hits.
java.util.Map<java.lang.Long,V>	getMap() Convert this cache to a map.
long	getMaxMemory() Get the maximum memory to use.
int	getMemory(long key) Get the memory used for the given key.
long	getMisses() Get the number of cache misses.
long	getUsedMemory() Get the currently used memory.
boolean	isEmpty() Check whether the cache is empty.
java.util.List<java.lang.Long>	keys(boolean cold, boolean nonResident) Get the list of keys.
java.util.Set<java.lang.Long>	keySet() Get the set of keys for resident entries.
V	peek(long key) Get the value for the given key if the entry is cached.
V	put(long key, V value) Add an entry to the cache using the average memory size.
V	put(long key, V value, int memory) Add an entry to the cache.
void	putAll(java.util.Map<java.lang.Long,? extends V> m) Add all elements of the map to this cache.
V	remove(long 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(V value) Get the size of the given value.
java.util.List<V>	values() Get the values for all resident entries.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CacheLongKeyLIRS

public CacheLongKeyLIRS(CacheLongKeyLIRS.Config config)

Create a new cache with the given memory size.

Parameters:

config - the configuration

Method Detail

clear

public void clear()

Remove all entries.

containsKey

public boolean containsKey(long key)

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

Parameters:

key - the key (may not be null)

Returns:

true if there is a resident entry

peek

public V peek(long 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(long key,
    V value)

Add an entry to the cache using the average memory size.

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

put

public V put(long 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

sizeOf

protected int sizeOf(V value)

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

Parameters:

value - the value

Returns:

the size

remove

public V remove(long key)

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

Parameters:

key - the key (may not be null)

Returns:

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

getMemory

public int getMemory(long 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(long 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.

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) in bytes

getMaxMemory

public long getMaxMemory()

Get the maximum memory to use.

Returns:

the maximum memory

entrySet

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

Get the entry set for all resident entries.

Returns:

the entry set

keySet

public java.util.Set<java.lang.Long> keySet()

Get the set of keys for resident entries.

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

getHits

public long getHits()

Get the number of cache hits.

Returns:

the cache hits

getMisses

public long getMisses()

Get the number of cache misses.

Returns:

the cache misses

size

public int size()

Get the number of resident entries.

Returns:

the number of entries

keys

public java.util.List<java.lang.Long> 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

values

public java.util.List<V> values()

Get the values for all resident entries.

Returns:

the entry set

isEmpty

public boolean isEmpty()

Check whether the cache is empty.

Returns:

true if it is empty

containsValue

public boolean containsValue(java.lang.Object value)

Check whether the given value is stored.

Parameters:

value - the value

Returns:

true if it is stored

getMap

public java.util.Map<java.lang.Long,V> getMap()

Convert this cache to a map.

Returns:

the map

putAll

public void putAll(java.util.Map<java.lang.Long,? extends V> m)

Add all elements of the map to this cache.

Parameters:

m - the map