org.apache.derby.impl.services.cache
Class ClockPolicy

java.lang.Object
  extended by org.apache.derby.impl.services.cache.ClockPolicy
All Implemented Interfaces:
ReplacementPolicy

final class ClockPolicy
extends java.lang.Object
implements ReplacementPolicy

Implementation of a replacement policy which uses the clock algorithm. All the cache entries are stored in a circular buffer, called the clock. There is also a clock hand which points to one of the entries in the clock. Each time an entry is accessed, it is marked as recently used. If a new entry is inserted into the cache and the cache is full, the clock hand is moved until it is over a not recently used entry, and that entry is evicted to make space for the new entry. Each time the clock hand sweeps over a recently used entry, it is marked as not recently used, and it will be a candidate for removal the next time the clock hand sweeps over it, unless it has been marked as recently used in the meantime.

To allow concurrent access from multiple threads, the methods in this class need to synchronize on a number of different objects:

To avoid deadlocks, we need to ensure that all threads obtain synchronization locks in the same order. CacheEntry's class javadoc dictates the order when locking CacheEntry objects. Additionally, we require that no thread should obtain any other synchronization locks while it is holding a synchronization lock on the clock structure or on a Holder object. The threads are however allowed to obtain synchronization locks on the clock structure or on a holder while they are locking one or more CacheEntry objects.


Nested Class Summary
private  class ClockPolicy.Holder
          Holder class which represents an entry in the cache.
 
Nested classes/interfaces inherited from interface org.apache.derby.impl.services.cache.ReplacementPolicy
ReplacementPolicy.Callback
 
Field Summary
private  ConcurrentCache cacheManager
          The cache manager for which this replacement policy is used.
private  java.util.ArrayList<ClockPolicy.Holder> clock
          The circular clock buffer which holds all the entries in the cache.
private  java.util.concurrent.atomic.AtomicInteger freeEntries
          The number of free entries.
private  int hand
          The current position of the clock hand.
private  java.util.concurrent.atomic.AtomicBoolean isShrinking
          Tells whether there currently is a thread in the doShrink() method.
private static float MAX_ROTATION
          How large part of the clock to look at before giving up in rotateClock().
private  int maxSize
          The maximum size of the cache.
private static int MIN_ITEMS_TO_CHECK
          The minimum number of items to check before we decide to give up looking for evictable entries when rotating the clock.
private static float PART_OF_CLOCK_FOR_SHRINK
          How large part of the clock to look at before giving up finding an evictable entry in shrinkMe().
 
Constructor Summary
ClockPolicy(ConcurrentCache cacheManager, int initialSize, int maxSize)
          Create a new ClockPolicy instance.
 
Method Summary
 void doShrink()
          Try to shrink the clock if it's larger than its maximum size.
 void insertEntry(CacheEntry entry)
          Insert an entry into the cache.
private  boolean isEvictable(CacheEntry e, ClockPolicy.Holder h, boolean clearRecentlyUsedFlag)
          Check if an entry can be evicted.
private  ClockPolicy.Holder moveHand()
          Get the holder under the clock hand, and move the hand to the next holder.
private  void removeHolder(int pos, ClockPolicy.Holder h)
          Remove the holder at the given clock position.
private  ClockPolicy.Holder rotateClock(CacheEntry entry, boolean allowEvictions)
          Rotate the clock in order to find a free space for a new entry.
private  void shrinkMe()
          Perform the shrinking of the clock.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

MIN_ITEMS_TO_CHECK

private static final int MIN_ITEMS_TO_CHECK
The minimum number of items to check before we decide to give up looking for evictable entries when rotating the clock.

See Also:
Constant Field Values

MAX_ROTATION

private static final float MAX_ROTATION
How large part of the clock to look at before giving up in rotateClock().

See Also:
Constant Field Values

PART_OF_CLOCK_FOR_SHRINK

private static final float PART_OF_CLOCK_FOR_SHRINK
How large part of the clock to look at before giving up finding an evictable entry in shrinkMe().

See Also:
Constant Field Values

cacheManager

private final ConcurrentCache cacheManager
The cache manager for which this replacement policy is used.


maxSize

private final int maxSize
The maximum size of the cache. When this size is exceeded, entries must be evicted before new ones are inserted.


clock

private final java.util.ArrayList<ClockPolicy.Holder> clock
The circular clock buffer which holds all the entries in the cache. Accesses to clock and hand must be synchronized on clock.


hand

private int hand
The current position of the clock hand.


freeEntries

private final java.util.concurrent.atomic.AtomicInteger freeEntries
The number of free entries. This is the number of objects that have been removed from the cache and whose entries are free to be reused without eviction.


isShrinking

private final java.util.concurrent.atomic.AtomicBoolean isShrinking
Tells whether there currently is a thread in the doShrink() method. If this variable is true a call to doShrink() will be a no-op.

Constructor Detail

ClockPolicy

ClockPolicy(ConcurrentCache cacheManager,
            int initialSize,
            int maxSize)
Create a new ClockPolicy instance.

Parameters:
cacheManager - the cache manager that requests this policy
initialSize - the initial capacity of the cache
maxSize - the maximum size of the cache
Method Detail

insertEntry

public void insertEntry(CacheEntry entry)
                 throws StandardException
Insert an entry into the cache. If the maximum size is exceeded, evict a not recently used object from the cache. If there are no entries available for reuse, increase the size of the cache.

Specified by:
insertEntry in interface ReplacementPolicy
Parameters:
entry - the entry to insert (must be locked)
Throws:
StandardException - if an error occurs when inserting the entry
See Also:
CacheEntry.setCallback(ReplacementPolicy.Callback)

moveHand

private ClockPolicy.Holder moveHand()
Get the holder under the clock hand, and move the hand to the next holder.

Returns:
the holder under the clock hand, or null if the clock is empty

rotateClock

private ClockPolicy.Holder rotateClock(CacheEntry entry,
                                       boolean allowEvictions)
                                throws StandardException
Rotate the clock in order to find a free space for a new entry. If allowEvictions is true, an not recently used object might be evicted to make room for the new entry. Otherwise, only unused entries are searched for. When evictions are allowed, entries are marked as not recently used when the clock hand sweeps over them. The search stops when a reusable entry is found, or when more than a certain percentage of the entries have been visited. If there are free (unused) entries, the search will continue until a reusable entry is found, regardless of how many entries that need to be checked.

Parameters:
entry - the entry to insert
allowEvictions - tells whether evictions are allowed (normally true if the cache is full and false otherwise)
Returns:
a holder that we can reuse, or null if we didn't find one
Throws:
StandardException

isEvictable

private boolean isEvictable(CacheEntry e,
                            ClockPolicy.Holder h,
                            boolean clearRecentlyUsedFlag)
Check if an entry can be evicted. Only entries that still are present in the cache, are not kept and not recently used, can be evicted. This method does not check whether the Cacheable contained in the entry is dirty, so it may be necessary to clean it before an eviction can take place even if the method returns true. The caller must hold the lock on the entry before calling this method.

Parameters:
e - the entry to check
h - the holder which holds the entry
clearRecentlyUsedFlag - tells whether or not the recently used flag should be cleared on the entry (true only when called as part of a normal clock rotation)
Returns:
whether or not this entry can be evicted (provided that its Cacheable is cleaned first)

removeHolder

private void removeHolder(int pos,
                          ClockPolicy.Holder h)
Remove the holder at the given clock position.

Parameters:
pos - position of the holder
h - the holder to remove

doShrink

public void doShrink()
Try to shrink the clock if it's larger than its maximum size.

Specified by:
doShrink in interface ReplacementPolicy

shrinkMe

private void shrinkMe()
Perform the shrinking of the clock. This method should only be called by a single thread at a time.


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