Class DfsBlockCache

  extended by

public final class DfsBlockCache
extends Object

Caches slices of a DfsPackFile in memory for faster read access.

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

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

Its too expensive during object access to be 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 clock replacement algorithm, giving each block one chance to have been accessed during a sweep of the cache to save itself from eviction.

Entities created by the cache are held under hard references, preventing the Java VM from clearing anything. Blocks are discarded by the replacement algorithm when adding a new block would cause the cache to exceed its configured maximum 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.

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.

Method Summary
 long getCurrentSize()
 long getEvictions()
 long getFillPercentage()
 long getHitCount()
 long getHitRatio()
static DfsBlockCache getInstance()
 long getMissCount()
 Collection<DfsPackFile> getPackFiles()
          Get the pack files stored in this cache.
 long getTotalRequestCount()
static void reconfigure(DfsBlockCacheConfig cfg)
          Modify the configuration of the window cache.
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail


public static void reconfigure(DfsBlockCacheConfig cfg)
Modify the configuration of the window cache.

The new configuration is applied immediately, and the existing cache is cleared.

cfg - the new window cache configuration.
IllegalArgumentException - the cache configuration contains one or more invalid settings, usually too low of a limit.


public static DfsBlockCache getInstance()
the currently active DfsBlockCache.


public long getCurrentSize()
total number of bytes in the cache.


public long getFillPercentage()
0..100, defining how full the cache is.


public long getHitCount()
number of requests for items in the cache.


public long getMissCount()
number of requests for items not in the cache.


public long getTotalRequestCount()
total number of requests (hit + miss).


public long getHitRatio()
0..100, defining number of cache hits.


public long getEvictions()
number of evictions performed due to cache being full.


public Collection<DfsPackFile> getPackFiles()
Get the pack files stored in this cache.

a collection of pack files, some of which may not actually be present; the caller should check the pack's cached size.

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