Class DfsBlockCache
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.
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Nested Class Summary
Nested ClassesModifier and TypeClassDescriptionprivate static final class
(package private) static interface
Supplier for readable channel(package private) static final class
(package private) static interface
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Field Summary
FieldsModifier and TypeFieldDescriptionprivate final int
Suggested block size to read from pack files in.private final int
AsblockSize
is a power of 2, bits to shift for a / blockSize.private static DfsBlockCache
private DfsBlockCache.Ref
Current position of the clock.private final ReentrantLock
Protects the clock and its related data.private final AtomicReference
<AtomicLong[]> Number of bytes currently loaded in the cache, per pack file extension.private final ReentrantLock[]
Locks to prevent concurrent loads for same (PackFile,position) block.private final long
Maximum number of bytes the cache should hold.private final long
Pack files smaller than this size can be copied through the cache.private final ReentrantLock[]
A separate pool of locks to prevent concurrent loads for same index or bitmap from PackFile.A consumer of object reference lock wait time milliseconds.private final AtomicReference
<AtomicLong[]> Number of blocks evicted due to cache being full, per pack file extension.private final AtomicReference
<AtomicLong[]> Number of times a block was found in the cache, per pack file extension.private final AtomicReference
<AtomicLong[]> Number of times a block was not found, and had to be loaded, per pack file extension.private final AtomicReferenceArray
<DfsBlockCache.HashEntry> Hash bucket directory; entries are chained below.private final int
Number of entries intable
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Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionprivate void
addToClock
(DfsBlockCache.Ref ref, long credit) private static DfsBlockCache.HashEntry
(package private) boolean
contains
(DfsStreamKey key, long position) private void
creditSpace
(long credit, DfsStreamKey key) (package private) <T> T
get
(DfsStreamKey key, long position) (package private) int
long[]
Get total number of bytes in the cache, per pack file extension.long[]
Get number of evictions performed due to cache being full, per pack file extension.long
Get 0..100, defining how full the cache is.long[]
Get number of requests for items in the cache, per pack file extension.long[]
Get hit ratiosstatic DfsBlockCache
Get the currently active DfsBlockCache.long[]
Get number of requests for items not in the cache, per pack file extension.(package private) DfsBlock
getOrLoad
(BlockBasedFile file, long position, DfsReader ctx, DfsBlockCache.ReadableChannelSupplier fileChannel) Look up a cached object, creating and loading it if it doesn't exist.(package private) <T> DfsBlockCache.Ref
<T> getOrLoadRef
(DfsStreamKey key, long position, DfsBlockCache.RefLoader<T> loader) Look up a cached object, creating and loading it if it doesn't exist.private static AtomicLong
getStat
(AtomicReference<AtomicLong[]> stats, DfsStreamKey key) private static long[]
getStatVals
(AtomicReference<AtomicLong[]> stat) long[]
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.private int
hash
(int packHash, long off) private ReentrantLock
lockFor
(DfsStreamKey key, long position) private ReentrantLock
lockForRef
(DfsStreamKey key) private static AtomicLong[]
(package private) void
(package private) <T> DfsBlockCache.Ref
<T> put
(DfsStreamKey key, long pos, long size, T v) (package private) <T> DfsBlockCache.Ref
<T> putRef
(DfsStreamKey key, long size, T v) static void
Modify the configuration of the window cache.private void
reserveSpace
(long reserve, DfsStreamKey key) private <T> T
scan
(DfsBlockCache.HashEntry n, DfsStreamKey key, long position) private <T> DfsBlockCache.Ref
<T> scanRef
(DfsBlockCache.HashEntry n, DfsStreamKey key, long position) (package private) boolean
shouldCopyThroughCache
(long length) private int
slot
(DfsStreamKey key, long position) private static int
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Field Details
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cache
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tableSize
private final int tableSizeNumber of entries intable
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table
Hash bucket directory; entries are chained below. -
loadLocks
Locks to prevent concurrent loads for same (PackFile,position) block. The number of locks isDfsBlockCacheConfig.getConcurrencyLevel()
to cap the overall concurrent block loads. -
refLocks
A separate pool of locks to prevent concurrent loads for same index or bitmap from PackFile. -
maxBytes
private final long maxBytesMaximum number of bytes the cache should hold. -
maxStreamThroughCache
private final long maxStreamThroughCachePack files smaller than this size can be copied through the cache. -
blockSize
private final int blockSizeSuggested block size to read from pack files in.If a pack file does not have a native block size, this size will be used.
If a pack file has a native size, a whole multiple of the native size will be used until it matches this size.
The value for blockSize must be a power of 2.
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blockSizeShift
private final int blockSizeShiftAsblockSize
is a power of 2, bits to shift for a / blockSize. -
statHit
Number of times a block was found in the cache, per pack file extension. -
statMiss
Number of times a block was not found, and had to be loaded, per pack file extension. -
statEvict
Number of blocks evicted due to cache being full, per pack file extension. -
liveBytes
Number of bytes currently loaded in the cache, per pack file extension. -
clockLock
Protects the clock and its related data. -
refLockWaitTime
A consumer of object reference lock wait time milliseconds. May be used to build a metric. -
clockHand
Current position of the clock.
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Constructor Details
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DfsBlockCache
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Method Details
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reconfigure
Modify the configuration of the window cache.The new configuration is applied immediately, and the existing cache is cleared.
- Parameters:
cfg
- the new window cache configuration.- Throws:
IllegalArgumentException
- the cache configuration contains one or more invalid settings, usually too low of a limit.
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getInstance
Get the currently active DfsBlockCache.- Returns:
- the currently active DfsBlockCache.
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shouldCopyThroughCache
boolean shouldCopyThroughCache(long length) -
getCurrentSize
public long[] getCurrentSize()Get total number of bytes in the cache, per pack file extension.- Returns:
- total number of bytes in the cache, per pack file extension.
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getFillPercentage
public long getFillPercentage()Get 0..100, defining how full the cache is.- Returns:
- 0..100, defining how full the cache is.
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getHitCount
public long[] getHitCount()Get number of requests for items in the cache, per pack file extension.- Returns:
- number of requests for items in the cache, per pack file extension.
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getMissCount
public long[] getMissCount()Get number of requests for items not in the cache, per pack file extension.- Returns:
- number of requests for items not in the cache, per pack file extension.
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getTotalRequestCount
public long[] getTotalRequestCount()Get total number of requests (hit + miss), per pack file extension.- Returns:
- total number of requests (hit + miss), per pack file extension.
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getHitRatio
public long[] getHitRatio()Get hit ratios- Returns:
- hit ratios
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getEvictions
public long[] getEvictions()Get number of evictions performed due to cache being full, per pack file extension.- Returns:
- number of evictions performed due to cache being full, per pack file extension.
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hasBlock0
Quickly check if the cache contains block 0 of the given stream.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.
- Parameters:
key
- the file to check.- Returns:
- true if block 0 (the first block) is in the cache.
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hash
private int hash(int packHash, long off) -
getBlockSize
int getBlockSize() -
tableSize
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getOrLoad
DfsBlock getOrLoad(BlockBasedFile file, long position, DfsReader ctx, DfsBlockCache.ReadableChannelSupplier fileChannel) throws IOException Look up a cached object, creating and loading it if it doesn't exist.- Parameters:
file
- the pack that "contains" the cached object.position
- offset withinpack
of the object.ctx
- current thread's reader.fileChannel
- supplier for channel to readpack
.- Returns:
- the object reference.
- Throws:
IOException
- the reference was not in the cache and could not be loaded.
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reserveSpace
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creditSpace
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addToClock
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put
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getOrLoadRef
<T> DfsBlockCache.Ref<T> getOrLoadRef(DfsStreamKey key, long position, DfsBlockCache.RefLoader<T> loader) throws IOException Look up a cached object, creating and loading it if it doesn't exist.- Parameters:
key
- the stream key of the pack.position
- the position in the key. The default should be 0.loader
- the function to load the reference.- Returns:
- the object reference.
- Throws:
IOException
- the reference was not in the cache and could not be loaded.
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putRef
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put
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contains
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get
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scan
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scanRef
private <T> DfsBlockCache.Ref<T> scanRef(DfsBlockCache.HashEntry n, DfsStreamKey key, long position) -
slot
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lockFor
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lockForRef
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newCounters
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getStat
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getStatVals
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clean
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