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- /*
- * Copyright (C) 2008-2011, Google Inc.
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>
- * and other copyright owners as documented in the project's IP log.
- *
- * This program and the accompanying materials are made available
- * under the terms of the Eclipse Distribution License v1.0 which
- * accompanies this distribution, is reproduced below, and is
- * available at http://www.eclipse.org/org/documents/edl-v10.php
- *
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or
- * without modification, are permitted provided that the following
- * conditions are met:
- *
- * - Redistributions of source code must retain the above copyright
- * notice, this list of conditions and the following disclaimer.
- *
- * - Redistributions in binary form must reproduce the above
- * copyright notice, this list of conditions and the following
- * disclaimer in the documentation and/or other materials provided
- * with the distribution.
- *
- * - Neither the name of the Eclipse Foundation, Inc. nor the
- * names of its contributors may be used to endorse or promote
- * products derived from this software without specific prior
- * written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
- * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
- * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
- * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
- * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
- * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
- * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
- * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
- * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
- * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
- * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- */
-
- package org.eclipse.jgit.internal.storage.dfs;
-
- import java.io.IOException;
- import java.util.Collection;
- import java.util.Collections;
- import java.util.Map;
- import java.util.concurrent.ConcurrentHashMap;
- import java.util.concurrent.atomic.AtomicLong;
- import java.util.concurrent.atomic.AtomicReferenceArray;
- import java.util.concurrent.locks.ReentrantLock;
-
- import org.eclipse.jgit.internal.JGitText;
-
- /**
- * Caches slices of a {@link DfsPackFile} in memory for faster read access.
- * <p>
- * 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.
- * <p>
- * Whenever a cache miss occurs, loading is invoked by exactly one thread for
- * the given <code>(DfsPackKey,position)</code> key tuple. This is ensured by an
- * array of locks, with the tuple hashed to a lock instance.
- * <p>
- * 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.
- * <p>
- * 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.
- * <p>
- * 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.
- * <p>
- * 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.
- */
- public final class DfsBlockCache {
- private static volatile DfsBlockCache cache;
-
- static {
- reconfigure(new DfsBlockCacheConfig());
- }
-
- /**
- * Modify the configuration of the window cache.
- * <p>
- * The new configuration is applied immediately, and the existing cache is
- * cleared.
- *
- * @param cfg
- * the new window cache configuration.
- * @throws IllegalArgumentException
- * the cache configuration contains one or more invalid
- * settings, usually too low of a limit.
- */
- public static void reconfigure(DfsBlockCacheConfig cfg) {
- DfsBlockCache nc = new DfsBlockCache(cfg);
- DfsBlockCache oc = cache;
- cache = nc;
-
- if (oc != null) {
- for (DfsPackFile pack : oc.getPackFiles())
- pack.key.cachedSize.set(0);
- }
- }
-
- /** @return the currently active DfsBlockCache. */
- public static DfsBlockCache getInstance() {
- return cache;
- }
-
- /** Number of entries in {@link #table}. */
- private final int tableSize;
-
- /** Hash bucket directory; entries are chained below. */
- private final AtomicReferenceArray<HashEntry> table;
-
- /** Locks to prevent concurrent loads for same (PackFile,position). */
- private final ReentrantLock[] loadLocks;
-
- /** Maximum number of bytes the cache should hold. */
- private final long maxBytes;
-
- /**
- * Suggested block size to read from pack files in.
- * <p>
- * If a pack file does not have a native block size, this size will be used.
- * <p>
- * If a pack file has a native size, a whole multiple of the native size
- * will be used until it matches this size.
- */
- private final int blockSize;
-
- /** As {@link #blockSize} is a power of 2, bits to shift for a / blockSize. */
- private final int blockSizeShift;
-
- /** Cache of pack files, indexed by description. */
- private final Map<DfsPackDescription, DfsPackFile> packCache;
-
- /** View of pack files in the pack cache. */
- private final Collection<DfsPackFile> packFiles;
-
- /** Number of times a block was found in the cache. */
- private final AtomicLong statHit;
-
- /** Number of times a block was not found, and had to be loaded. */
- private final AtomicLong statMiss;
-
- /** Number of blocks evicted due to cache being full. */
- private volatile long statEvict;
-
- /** Protects the clock and its related data. */
- private final ReentrantLock clockLock;
-
- /** Current position of the clock. */
- private Ref clockHand;
-
- /** Number of bytes currently loaded in the cache. */
- private volatile long liveBytes;
-
- private DfsBlockCache(final DfsBlockCacheConfig cfg) {
- tableSize = tableSize(cfg);
- if (tableSize < 1)
- throw new IllegalArgumentException(JGitText.get().tSizeMustBeGreaterOrEqual1);
-
- table = new AtomicReferenceArray<HashEntry>(tableSize);
- loadLocks = new ReentrantLock[32];
- for (int i = 0; i < loadLocks.length; i++)
- loadLocks[i] = new ReentrantLock(true /* fair */);
-
- int eb = (int) (tableSize * .1);
- if (64 < eb)
- eb = 64;
- else if (eb < 4)
- eb = 4;
- if (tableSize < eb)
- eb = tableSize;
-
- maxBytes = cfg.getBlockLimit();
- blockSize = cfg.getBlockSize();
- blockSizeShift = Integer.numberOfTrailingZeros(blockSize);
-
- clockLock = new ReentrantLock(true /* fair */);
- clockHand = new Ref<Object>(new DfsPackKey(), -1, 0, null);
- clockHand.next = clockHand;
-
- packCache = new ConcurrentHashMap<DfsPackDescription, DfsPackFile>(
- 16, 0.75f, 1);
- packFiles = Collections.unmodifiableCollection(packCache.values());
-
- statHit = new AtomicLong();
- statMiss = new AtomicLong();
- }
-
- /** @return total number of bytes in the cache. */
- public long getCurrentSize() {
- return liveBytes;
- }
-
- /** @return 0..100, defining how full the cache is. */
- public long getFillPercentage() {
- return getCurrentSize() * 100 / maxBytes;
- }
-
- /** @return number of requests for items in the cache. */
- public long getHitCount() {
- return statHit.get();
- }
-
- /** @return number of requests for items not in the cache. */
- public long getMissCount() {
- return statMiss.get();
- }
-
- /** @return total number of requests (hit + miss). */
- public long getTotalRequestCount() {
- return getHitCount() + getMissCount();
- }
-
- /** @return 0..100, defining number of cache hits. */
- public long getHitRatio() {
- long hits = statHit.get();
- long miss = statMiss.get();
- long total = hits + miss;
- if (total == 0)
- return 0;
- return hits * 100 / total;
- }
-
- /** @return number of evictions performed due to cache being full. */
- public long getEvictions() {
- return statEvict;
- }
-
- /**
- * Get the pack files stored in this cache.
- *
- * @return a collection of pack files, some of which may not actually be
- * present; the caller should check the pack's cached size.
- */
- public Collection<DfsPackFile> getPackFiles() {
- return packFiles;
- }
-
- DfsPackFile getOrCreate(DfsPackDescription dsc, DfsPackKey key) {
- // TODO This table grows without bound. It needs to clean up
- // entries that aren't in cache anymore, and aren't being used
- // by a live DfsObjDatabase reference.
- synchronized (packCache) {
- DfsPackFile pack = packCache.get(dsc);
- if (pack != null && pack.invalid()) {
- packCache.remove(dsc);
- pack = null;
- }
- if (pack == null) {
- if (key == null)
- key = new DfsPackKey();
- pack = new DfsPackFile(this, dsc, key);
- packCache.put(dsc, pack);
- }
- return pack;
- }
- }
-
- private int hash(int packHash, long off) {
- return packHash + (int) (off >>> blockSizeShift);
- }
-
- int getBlockSize() {
- return blockSize;
- }
-
- private static int tableSize(final DfsBlockCacheConfig cfg) {
- final int wsz = cfg.getBlockSize();
- final long limit = cfg.getBlockLimit();
- if (wsz <= 0)
- throw new IllegalArgumentException(JGitText.get().invalidWindowSize);
- if (limit < wsz)
- throw new IllegalArgumentException(JGitText.get().windowSizeMustBeLesserThanLimit);
- return (int) Math.min(5 * (limit / wsz) / 2, Integer.MAX_VALUE);
- }
-
- /**
- * Lookup a cached object, creating and loading it if it doesn't exist.
- *
- * @param pack
- * the pack that "contains" the cached object.
- * @param position
- * offset within <code>pack</code> of the object.
- * @param ctx
- * current thread's reader.
- * @return the object reference.
- * @throws IOException
- * the reference was not in the cache and could not be loaded.
- */
- DfsBlock getOrLoad(DfsPackFile pack, long position, DfsReader ctx)
- throws IOException {
- final long requestedPosition = position;
- position = pack.alignToBlock(position);
-
- DfsPackKey key = pack.key;
- int slot = slot(key, position);
- HashEntry e1 = table.get(slot);
- DfsBlock v = scan(e1, key, position);
- if (v != null) {
- statHit.incrementAndGet();
- return v;
- }
-
- reserveSpace(blockSize);
- ReentrantLock regionLock = lockFor(key, position);
- regionLock.lock();
- try {
- HashEntry e2 = table.get(slot);
- if (e2 != e1) {
- v = scan(e2, key, position);
- if (v != null) {
- statHit.incrementAndGet();
- creditSpace(blockSize);
- return v;
- }
- }
-
- statMiss.incrementAndGet();
- boolean credit = true;
- try {
- v = pack.readOneBlock(position, ctx);
- credit = false;
- } finally {
- if (credit)
- creditSpace(blockSize);
- }
- if (position != v.start) {
- // The file discovered its blockSize and adjusted.
- position = v.start;
- slot = slot(key, position);
- e2 = table.get(slot);
- }
-
- key.cachedSize.addAndGet(v.size());
- Ref<DfsBlock> ref = new Ref<DfsBlock>(key, position, v.size(), v);
- ref.hot = true;
- for (;;) {
- HashEntry n = new HashEntry(clean(e2), ref);
- if (table.compareAndSet(slot, e2, n))
- break;
- e2 = table.get(slot);
- }
- addToClock(ref, blockSize - v.size());
- } finally {
- regionLock.unlock();
- }
-
- // If the block size changed from the default, it is possible the block
- // that was loaded is the wrong block for the requested position.
- if (v.contains(pack.key, requestedPosition))
- return v;
- return getOrLoad(pack, requestedPosition, ctx);
- }
-
- @SuppressWarnings("unchecked")
- private void reserveSpace(int reserve) {
- clockLock.lock();
- try {
- long live = liveBytes + reserve;
- if (maxBytes < live) {
- Ref prev = clockHand;
- Ref hand = clockHand.next;
- do {
- if (hand.hot) {
- // Value was recently touched. Clear
- // hot and give it another chance.
- hand.hot = false;
- prev = hand;
- hand = hand.next;
- continue;
- } else if (prev == hand)
- break;
-
- // No recent access since last scan, kill
- // value and remove from clock.
- Ref dead = hand;
- hand = hand.next;
- prev.next = hand;
- dead.next = null;
- dead.value = null;
- live -= dead.size;
- dead.pack.cachedSize.addAndGet(-dead.size);
- statEvict++;
- } while (maxBytes < live);
- clockHand = prev;
- }
- liveBytes = live;
- } finally {
- clockLock.unlock();
- }
- }
-
- private void creditSpace(int credit) {
- clockLock.lock();
- liveBytes -= credit;
- clockLock.unlock();
- }
-
- private void addToClock(Ref ref, int credit) {
- clockLock.lock();
- try {
- if (credit != 0)
- liveBytes -= credit;
- Ref ptr = clockHand;
- ref.next = ptr.next;
- ptr.next = ref;
- clockHand = ref;
- } finally {
- clockLock.unlock();
- }
- }
-
- void put(DfsBlock v) {
- put(v.pack, v.start, v.size(), v);
- }
-
- <T> Ref<T> put(DfsPackKey key, long pos, int size, T v) {
- int slot = slot(key, pos);
- HashEntry e1 = table.get(slot);
- Ref<T> ref = scanRef(e1, key, pos);
- if (ref != null)
- return ref;
-
- reserveSpace(size);
- ReentrantLock regionLock = lockFor(key, pos);
- regionLock.lock();
- try {
- HashEntry e2 = table.get(slot);
- if (e2 != e1) {
- ref = scanRef(e2, key, pos);
- if (ref != null) {
- creditSpace(size);
- return ref;
- }
- }
-
- key.cachedSize.addAndGet(size);
- ref = new Ref<T>(key, pos, size, v);
- ref.hot = true;
- for (;;) {
- HashEntry n = new HashEntry(clean(e2), ref);
- if (table.compareAndSet(slot, e2, n))
- break;
- e2 = table.get(slot);
- }
- addToClock(ref, 0);
- } finally {
- regionLock.unlock();
- }
- return ref;
- }
-
- boolean contains(DfsPackKey key, long position) {
- return scan(table.get(slot(key, position)), key, position) != null;
- }
-
- @SuppressWarnings("unchecked")
- <T> T get(DfsPackKey key, long position) {
- T val = (T) scan(table.get(slot(key, position)), key, position);
- if (val == null)
- statMiss.incrementAndGet();
- else
- statHit.incrementAndGet();
- return val;
- }
-
- private <T> T scan(HashEntry n, DfsPackKey pack, long position) {
- Ref<T> r = scanRef(n, pack, position);
- return r != null ? r.get() : null;
- }
-
- @SuppressWarnings("unchecked")
- private <T> Ref<T> scanRef(HashEntry n, DfsPackKey pack, long position) {
- for (; n != null; n = n.next) {
- Ref<T> r = n.ref;
- if (r.pack == pack && r.position == position)
- return r.get() != null ? r : null;
- }
- return null;
- }
-
- void remove(DfsPackFile pack) {
- synchronized (packCache) {
- packCache.remove(pack.getPackDescription());
- }
- }
-
- private int slot(DfsPackKey pack, long position) {
- return (hash(pack.hash, position) >>> 1) % tableSize;
- }
-
- private ReentrantLock lockFor(DfsPackKey pack, long position) {
- return loadLocks[(hash(pack.hash, position) >>> 1) % loadLocks.length];
- }
-
- private static HashEntry clean(HashEntry top) {
- while (top != null && top.ref.next == null)
- top = top.next;
- if (top == null)
- return null;
- HashEntry n = clean(top.next);
- return n == top.next ? top : new HashEntry(n, top.ref);
- }
-
- private static final class HashEntry {
- /** Next entry in the hash table's chain list. */
- final HashEntry next;
-
- /** The referenced object. */
- final Ref ref;
-
- HashEntry(HashEntry n, Ref r) {
- next = n;
- ref = r;
- }
- }
-
- static final class Ref<T> {
- final DfsPackKey pack;
- final long position;
- final int size;
- volatile T value;
- Ref next;
- volatile boolean hot;
-
- Ref(DfsPackKey pack, long position, int size, T v) {
- this.pack = pack;
- this.position = position;
- this.size = size;
- this.value = v;
- }
-
- T get() {
- T v = value;
- if (v != null)
- hot = true;
- return v;
- }
-
- boolean has() {
- return value != null;
- }
- }
- }
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