Class ClockPolicy

java.lang.Object
org.apache.derby.impl.services.cache.ClockPolicy
All Implemented Interfaces:
ReplacementPolicy
final class ClockPolicy extends 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:

  • CacheEntry objects must be locked before they can be used
  • accesses to the clock structure (circular buffer + clock hand) should be synchronized on the ArrayList representing the circular buffer
  • accesses to individual Holder objects in the clock structure should be protected by synchronizing on the holder
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.

  • Field Details

    • 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:

    • MAX_ROTATION

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

    • 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:

    • 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 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 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 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 Details

    • 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 Details

    • size

      public int size()
      Description copied from interface: ReplacementPolicy
      Get the number of entries allocated in the data structure that holds cached objects. This number could include empty entries for objects that have been removed from the cache, if those entries are still kept in the data structure for reuse.
      Specified by:
      size in interface ReplacementPolicy
      Returns:
      the number of entries allocated in the cache

    • 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:

    • 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.