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

java.lang.Object
  extended by java.util.AbstractMap<K,V>
      extended by 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 (1% 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
CacheLIRS(int maxEntries)
          Create a new cache with the given number of entries, and the default settings (an average size of 1 per entry, 16 segments, and stack move distance equals to the maximum number of entries divided by 100).
CacheLIRS(long maxMemory, int averageMemory, int segmentCount, int stackMoveDistance)
          Create a new cache with the given memory size.
 
Method Summary
 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.
 int getAverageMemory()
          Get the average memory used per entry.
 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 the average memory size.
 V put(K key, V value, int memory)
          Add an entry to the cache.
 V remove(java.lang.Object key)
          Remove an entry.
 void setAverageMemory(int averageMemory)
          Set the average memory used per 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
 

Constructor Detail

CacheLIRS

public CacheLIRS(int maxEntries)
Create a new cache with the given number of entries, and the default settings (an average size of 1 per entry, 16 segments, and stack move distance equals to the maximum number of entries divided by 100).

Parameters:
maxEntries - the maximum number of entries

CacheLIRS

public CacheLIRS(long maxMemory,
                 int averageMemory,
                 int segmentCount,
                 int stackMoveDistance)
Create a new cache with the given memory size.

Parameters:
maxMemory - the maximum memory to use (1 or larger)
averageMemory - the average memory (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 the average memory size.

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 the average memory as configured for this cache.

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)

setAverageMemory

public void setAverageMemory(int averageMemory)
Set the average memory used per entry. It is used to calculate the length of the internal array.

Parameters:
averageMemory - the average memory used (1 or larger)

getAverageMemory

public int getAverageMemory()
Get the average memory used per entry.

Returns:
the average memory

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