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

java.lang.Object
  extended by 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 (1% by default). Write access and moving entries to the top of the stack is synchronized per segment.


Constructor Summary
CacheLongKeyLIRS(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).
CacheLongKeyLIRS(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(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.
 int getAverageMemory()
          Get the average memory used per entry.
 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 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 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(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(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

CacheLongKeyLIRS

public CacheLongKeyLIRS(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.


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 the average memory as configured for this cache.

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)

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<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

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