Class WindowCache


  • public class WindowCache
    extends java.lang.Object
    Caches slices of a PackFile in memory for faster read access.

    The WindowCache serves as a Java based "buffer cache", loading segments of a PackFile into the JVM heap prior to use. As JGit often wants to do reads of only tiny slices of a file, the WindowCache tries to smooth out these tiny reads into larger block-sized IO operations.

    Whenever a cache miss occurs, load(PackFile, long) is invoked by exactly one thread for the given (PackFile,position) key tuple. This is ensured by an array of locks, with the tuple hashed to a lock instance.

    During a miss, older entries are evicted from the cache so long as isFull() returns true.

    Its too expensive during object access to be 100% accurate with a least recently used (LRU) algorithm. Strictly ordering every read is a lot of overhead that typically doesn't yield a corresponding benefit to the application.

    This cache implements a loose LRU policy by randomly picking a window comprised of roughly 10% of the cache, and evicting the oldest accessed entry within that window.

    Entities created by the cache are held under SoftReferences if option core.packedGitUseStrongRefs is set to false in the git config (this is the default) or by calling WindowCacheConfig.setPackedGitUseStrongRefs(boolean), permitting the Java runtime's garbage collector to evict entries when heap memory gets low. Most JREs implement a loose least recently used algorithm for this eviction. When this option is set to true strong references are used which means that Java gc cannot evict the WindowCache to reclaim memory. On the other hand this provides more predictable performance since the cache isn't flushed when used heap comes close to the maximum heap size.

    The internal hash table does not expand at runtime, instead it is fixed in size at cache creation time. The internal lock table used to gate load invocations is also fixed in size.

    The key tuple is passed through to methods as a pair of parameters rather than as a single Object, thus reducing the transient memory allocations of callers. It is more efficient to avoid the allocation, as we can't be 100% sure that a JIT would be able to stack-allocate a key tuple.

    This cache has an implementation rule such that:

    • load(PackFile, long) is invoked by at most one thread at a time for a given (PackFile,position) tuple.
    • For every load() invocation there is exactly one createRef(PackFile, long, ByteWindow) invocation to wrap a SoftReference or a StrongReference around the cached entity.
    • For every Reference created by createRef() there will be exactly one call to clear(PageRef) to cleanup any resources associated with the (now expired) cached entity.

    Therefore, it is safe to perform resource accounting increments during the load(PackFile, long) or createRef(PackFile, long, ByteWindow) methods, and matching decrements during clear(PageRef). Implementors may need to override createRef(PackFile, long, ByteWindow) in order to embed additional accounting information into an implementation specific WindowCache.PageRef subclass, as the cached entity may have already been evicted by the JRE's garbage collector.

    To maintain higher concurrency workloads, during eviction only one thread performs the eviction work, while other threads can continue to insert new objects in parallel. This means that the cache can be temporarily over limit, especially if the nominated eviction thread is being starved relative to the other threads.

    • Field Detail

      • rng

        private static final java.util.Random rng
      • streamFileThreshold

        private static volatile int streamFileThreshold
      • tableSize

        private final int tableSize
        Number of entries in table.
      • clock

        private final java.util.concurrent.atomic.AtomicLong clock
        Access clock for loose LRU.
      • table

        private final java.util.concurrent.atomic.AtomicReferenceArray<WindowCache.Entry> table
        Hash bucket directory; entries are chained below.
      • locks

        private final WindowCache.Lock[] locks
        Locks to prevent concurrent loads for same (PackFile,position).
      • evictLock

        private final java.util.concurrent.locks.ReentrantLock evictLock
        Lock to elect the eviction thread after a load occurs.
      • evictBatch

        private final int evictBatch
        Number of table buckets to scan for an eviction window.
      • maxFiles

        private final int maxFiles
      • maxBytes

        private final long maxBytes
      • mmap

        private final boolean mmap
      • windowSizeShift

        private final int windowSizeShift
      • windowSize

        private final int windowSize
      • publishMBean

        private final java.util.concurrent.atomic.AtomicBoolean publishMBean
      • useStrongRefs

        private boolean useStrongRefs
    • Method Detail

      • bits

        private static final int bits​(int newSize)
      • reconfigure

        @Deprecated
        public static void reconfigure​(WindowCacheConfig cfg)
        Deprecated.
        use cfg.install() to avoid internal reference.
        Modify the configuration of the window cache.

        The new configuration is applied immediately. If the new limits are smaller than what is currently cached, older entries will be purged as soon as possible to allow the cache to meet the new limit.

        Parameters:
        cfg - the new window cache configuration.
        Throws:
        java.lang.IllegalArgumentException - the cache configuration contains one or more invalid settings, usually too low of a limit.
      • getStreamFileThreshold

        static int getStreamFileThreshold()
      • getInstance

        public static WindowCache getInstance()
        Returns:
        the cached instance.
      • get

        static final ByteWindow get​(PackFile pack,
                                    long offset)
                             throws java.io.IOException
        Throws:
        java.io.IOException
      • purge

        static final void purge​(PackFile pack)
      • publishMBeanIfNeeded

        private WindowCache publishMBeanIfNeeded()
      • getStats

        public WindowCacheStats getStats()
        Returns:
        cache statistics for the WindowCache
      • resetStats

        public void resetStats()
        Reset stats. Does not reset open bytes and open files stats.
      • hash

        private int hash​(int packHash,
                         long off)
      • load

        private ByteWindow load​(PackFile pack,
                                long offset)
                         throws java.io.IOException
        Throws:
        java.io.IOException
      • close

        private void close​(PackFile pack)
      • isFull

        private boolean isFull()
      • toStart

        private long toStart​(long offset)
      • getOrLoad

        private ByteWindow getOrLoad​(PackFile pack,
                                     long position)
                              throws java.io.IOException
        Lookup a cached object, creating and loading it if it doesn't exist.
        Parameters:
        pack - the pack that "contains" the cached object.
        position - offset within pack of the object.
        Returns:
        the object reference.
        Throws:
        java.io.IOException - the object reference was not in the cache and could not be obtained by load(PackFile, long).
      • evict

        private void evict()
      • removeAll

        private void removeAll()
        Clear every entry from the cache.

        This is a last-ditch effort to clear out the cache, such as before it gets replaced by another cache that is configured differently. This method tries to force every cached entry through clear(PageRef) to ensure that resources are correctly accounted for and cleaned up by the subclass. A concurrent reader loading entries while this method is running may cause resource accounting failures.

      • removeAll

        private void removeAll​(PackFile pack)
        Clear all entries related to a single file.

        Typically this method is invoked during PackFile.close(), when we know the pack is never going to be useful to us again (for example, it no longer exists on disk). A concurrent reader loading an entry from this same pack may cause the pack to become stuck in the cache anyway.

        Parameters:
        pack - the file to purge all entries of.
      • gc

        private void gc()
      • slot

        private int slot​(PackFile pack,
                         long position)