public Class WindowCache

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(Pack, 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(Pack, 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(Pack, 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(Pack, long) or createRef(Pack, long, ByteWindow) methods, and matching decrements during clear(PageRef). Implementors may need to override createRef(Pack, long, ByteWindow) in order to embed additional accounting information into an implementation specific org.eclipse.jgit.internal.storage.file.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.