Package 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:
Map<K,V>
public class CacheLIRS<K,V> extends 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 LIRS replacement algorithm invented by Xiaodong Zhang and Song Jiang as described in https://web.cse.ohio-state.edu/~zhang.574/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.

  • Constructor Details

    • 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 Details

    • clear

      public void clear()
      Remove all entries.
      Specified by:
      clear in interface Map<K,V>
      Overrides:
      clear in class AbstractMap<K,V>

    • containsKey

      public boolean containsKey(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 Map<K,V>
      Overrides:
      containsKey in class 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 Map<K,V>
      Overrides:
      put in class 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(Object key)
      Remove an entry. Both resident and non-resident entries can be removed.
      Specified by:
      remove in interface Map<K,V>
      Overrides:
      remove in class 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(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 Map<K,V>
      Overrides:
      get in class 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 Set<Map.Entry<K,V>> entrySet()
      Get the entry set for all resident entries.
      Specified by:
      entrySet in interface Map<K,V>
      Specified by:
      entrySet in class AbstractMap<K,V>
      Returns:
      the entry set

    • keySet

      public Set<K> keySet()
      Get the set of keys for resident entries.
      Specified by:
      keySet in interface Map<K,V>
      Overrides:
      keySet in class 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 Map<K,V>
      Overrides:
      size in class AbstractMap<K,V>
      Returns:
      the number of entries

    • keys

      public 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