org.exist.storage.cache
Class GClockCache

java.lang.Object
  org.exist.storage.cache.GClockCache

All Implemented Interfaces:

Cache

Direct Known Subclasses:

LRDCache

public class GClockCache
extends Object
implements Cache

Cache implementation based on the GClock algorithm. Implements a mixture between LFU (Last Frequently Used) and LRU (Last Recently Used) replacement policies. The class uses reference counts to track references to cached objects. Each call to the add method increments the reference count of the object. If the cache is full, the object to be removed is determined by decrementing the reference count for each object until an object with reference count = 0 is found. The implementation tends to replace younger objects first.

Author:

wolf

Field Summary

Fields inherited from interface org.exist.storage.cache.Cache

LOG

Constructor Summary

GClockCache(int size, double growthFactor, double growthThreshold)

Method Summary

void	add(Cacheable item) Add the item to the cache.
void	add(Cacheable item, int initialRefCount) Add the item to the cache.
boolean	flush() Call release on all items, but without actually removing them from the cache.
Cacheable	get(Cacheable item) Retrieve an item from the cache.
Cacheable	get(long key) Retrieve an item by its key.
int	getBuffers() Get the size of this cache.
int	getFails() Get the number of times where an object could not be found in the cache.
String	getFileName()
double	getGrowthFactor() Returns the factor by which the cache should grow if it can be resized.
int	getHits() Get the number of times where an object has been successfully loaded from the cache.
int	getLoad()
int	getThrashing()
int	getUsedBuffers() Get the number of buffers currently used.
boolean	hasDirtyItems() Returns true if the cache contains any dirty items that need to be written to disk.
void	remove(Cacheable item) Remove an item from the cache.
void	resize(int newSize) Resize the cache.
void	setCacheManager(CacheManager manager) Set the CacheManager object that controls this cache.
void	setFileName(String name)

Methods inherited from class java.lang.Object

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

Constructor Detail

GClockCache

public GClockCache(int size,
                   double growthFactor,
                   double growthThreshold)

Method Detail

add

public void add(Cacheable item)

Description copied from interface: Cache

Add the item to the cache. If it is already in the cache, update the references.

Specified by:

add in interface Cache

add

public void add(Cacheable item,
                int initialRefCount)

Description copied from interface: Cache

Add the item to the cache. If it is already in the cache, update the references.

Specified by:

add in interface Cache

initialRefCount - the initial reference count for the item

get

public Cacheable get(Cacheable item)

Description copied from interface: Cache

Retrieve an item from the cache.

Specified by:

get in interface Cache

Returns:

the item in the cache or null if it does not exist.

get

public Cacheable get(long key)

Description copied from interface: Cache

Retrieve an item by its key.

Specified by:

get in interface Cache

Parameters:

key - a unique key, usually the page number

Returns:

the item in the cache or null if it does not exist.

remove

public void remove(Cacheable item)

Description copied from interface: Cache

Remove an item from the cache.

Specified by:

remove in interface Cache

flush

public boolean flush()

Description copied from interface: Cache

Call release on all items, but without actually removing them from the cache. This gives the items a chance to write all unwritten data to disk.

Specified by:

flush in interface Cache

hasDirtyItems

public boolean hasDirtyItems()

Description copied from interface: Cache

Returns true if the cache contains any dirty items that need to be written to disk.

Specified by:

hasDirtyItems in interface Cache

getBuffers

public int getBuffers()

Description copied from interface: Cache

Get the size of this cache.

Specified by:

getBuffers in interface Cache

Returns:

size

getUsedBuffers

public int getUsedBuffers()

Description copied from interface: Cache

Get the number of buffers currently used.

Specified by:

getUsedBuffers in interface Cache

getGrowthFactor

public double getGrowthFactor()

Description copied from interface: Cache

Returns the factor by which the cache should grow if it can be resized. The returned factor f will be between 0 and 2. A value smaller or equal to 1 means the cache can't grow, 1.5 means it grows by 50 percent. A cache with growth factor <= 1.0 can also not be shrinked. A cache is resized by the DefaultCacheManager.

Specified by:

getGrowthFactor in interface Cache

Returns:

growth factor

getHits

public int getHits()

Description copied from interface: Cache

Get the number of times where an object has been successfully loaded from the cache.

Specified by:

getHits in interface Cache

getFails

public int getFails()

Description copied from interface: Cache

Get the number of times where an object could not be found in the cache.

Specified by:

getFails in interface Cache

Returns:

number of times where an object could not be found in the cache

getThrashing

public int getThrashing()

setCacheManager

public void setCacheManager(CacheManager manager)

Description copied from interface: Cache

Set the CacheManager object that controls this cache.

Specified by:

setCacheManager in interface Cache

resize

public void resize(int newSize)

Description copied from interface: Cache

Resize the cache. This method is called by the DefaultCacheManager. The newSize parameter can either be larger or smaller than the current cache size.

Specified by:

resize in interface Cache

Parameters:

newSize - the new size of the cache.

getLoad

public int getLoad()

Specified by:

getLoad in interface Cache

setFileName

public void setFileName(String name)

Specified by:

setFileName in interface Cache

getFileName

public String getFileName()

Specified by:

getFileName in interface Cache