public final class DfsBlockCache extends Object
BlockBasedFile in memory for
faster read access.
The DfsBlockCache serves as a Java based "buffer cache", loading segments of a BlockBasedFile 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 (DfsStreamKey,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.
| Modifier and Type | Method and Description |
|---|---|
long[] |
getCurrentSize()
Get total number of bytes in the cache, per pack file extension.
|
long[] |
getEvictions()
Get number of evictions performed due to cache being full, per pack file
extension.
|
long |
getFillPercentage()
Get 0..100, defining how full the cache is.
|
long[] |
getHitCount()
Get number of requests for items in the cache, per pack file extension.
|
long[] |
getHitRatio()
Get hit ratios
|
static DfsBlockCache |
getInstance()
Get the currently active DfsBlockCache.
|
long[] |
getMissCount()
Get number of requests for items not in the cache, per pack file
extension.
|
long[] |
getTotalRequestCount()
Get total number of requests (hit + miss), per pack file extension.
|
boolean |
hasBlock0(DfsStreamKey key)
Quickly check if the cache contains block 0 of the given stream.
|
static void |
reconfigure(DfsBlockCacheConfig cfg)
Modify the configuration of the window cache.
|
public static void reconfigure(DfsBlockCacheConfig cfg)
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()
public long[] getCurrentSize()
public long getFillPercentage()
public long[] getHitCount()
public long[] getMissCount()
public long[] getTotalRequestCount()
public long[] getHitRatio()
public long[] getEvictions()
public boolean hasBlock0(DfsStreamKey key)
This can be useful for sophisticated pre-read algorithms to quickly determine if a file is likely already in cache, especially small reftables which may be smaller than a typical DFS block size.
key - the file to check.Copyright © 2020 Eclipse JGit Project. All rights reserved.