Make DeltaBaseCache per-ObjectReader

The 'Counting objects' phase of PackWriter requires good hit rates
from the DeltaBaseCache while walking trees, the deltas need to find
their bases in the cache in order to inflate in a reasonable time.

If JGit is running in a multi-threaded server, such as Gerrit Code
Review, each thread needs its own DeltaBaseCache to prevent one thread
from evicting the other thread's relevant bases.  Move the cache to be
per-ObjectReader, lazily allocated when required by a PackFile.

Change-Id: If9d5ed06728e813632ae96dcfb811f4860b276e8
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

3 files changed, 42 insertions, 41 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/DeltaBaseCache.java b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/DeltaBaseCache.java
index 8b548242b3..0b2393435a 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/DeltaBaseCache.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/DeltaBaseCache.java
@@ -54,35 +54,36 @@ class DeltaBaseCache {
 		return (((int) position) << 22) >>> 22;
 	}
 
-	private static int maxByteCount;
+	private static volatile int defaultMaxByteCount;
 
-	private static final Slot[] cache;
+	private final int maxByteCount;
 
-	private static Slot lruHead;
+	private final Slot[] cache;
 
-	private static Slot lruTail;
+	private Slot lruHead;
 
-	private static int openByteCount;
+	private Slot lruTail;
+
+	private int openByteCount;
 
 	static {
 		DEAD = new SoftReference<Entry>(null);
-		maxByteCount = new WindowCacheConfig().getDeltaBaseCacheLimit();
+		reconfigure(new WindowCacheConfig());
+	}
 
-		cache = new Slot[CACHE_SZ];
-		for (int i = 0; i < CACHE_SZ; i++)
-			cache[i] = new Slot();
+	static void reconfigure(WindowCacheConfig cfg) {
+		defaultMaxByteCount = cfg.getDeltaBaseCacheLimit();
 	}
 
-	static synchronized void reconfigure(final WindowCacheConfig cfg) {
-		final int dbLimit = cfg.getDeltaBaseCacheLimit();
-		if (maxByteCount != dbLimit) {
-			maxByteCount = dbLimit;
-			releaseMemory();
-		}
+	DeltaBaseCache() {
+		maxByteCount = defaultMaxByteCount;
+		cache = new Slot[CACHE_SZ];
 	}
 
-	static synchronized Entry get(final PackFile pack, final long position) {
-		final Slot e = cache[hash(position)];
+	Entry get(final PackFile pack, final long position) {
+		Slot e = cache[hash(position)];
+		if (e == null)
+			return null;
 		if (e.provider == pack && e.position == position) {
 			final Entry buf = e.data.get();
 			if (buf != null) {
@@ -93,13 +94,18 @@ class DeltaBaseCache {
 		return null;
 	}
 
-	static synchronized void store(final PackFile pack, final long position,
+	void store(final PackFile pack, final long position,
 			final byte[] data, final int objectType) {
 		if (data.length > maxByteCount)
 			return; // Too large to cache.
 
-		final Slot e = cache[hash(position)];
-		clearEntry(e);
+		Slot e = cache[hash(position)];
+		if (e == null) {
+			e = new Slot();
+			cache[hash(position)] = e;
+		} else {
+			clearEntry(e);
+		}
 
 		openByteCount += data.length;
 		releaseMemory();
@@ -111,7 +117,7 @@ class DeltaBaseCache {
 		moveToHead(e);
 	}
 
-	private static void releaseMemory() {
+	private void releaseMemory() {
 		while (openByteCount > maxByteCount && lruTail != null) {
 			final Slot currOldest = lruTail;
 			final Slot nextOldest = currOldest.lruPrev;
@@ -128,16 +134,7 @@ class DeltaBaseCache {
 		}
 	}
 
-	static synchronized void purge(final PackFile file) {
-		for (final Slot e : cache) {
-			if (e.provider == file) {
-				clearEntry(e);
-				unlink(e);
-			}
-		}
-	}
-
-	private static void moveToHead(final Slot e) {
+	private void moveToHead(final Slot e) {
 		unlink(e);
 		e.lruPrev = null;
 		e.lruNext = lruHead;
@@ -148,7 +145,7 @@ class DeltaBaseCache {
 		lruHead = e;
 	}
 
-	private static void unlink(final Slot e) {
+	private void unlink(final Slot e) {
 		final Slot prev = e.lruPrev;
 		final Slot next = e.lruNext;
 		if (prev != null)
@@ -157,17 +154,13 @@ class DeltaBaseCache {
 			next.lruPrev = prev;
 	}
 
-	private static void clearEntry(final Slot e) {
+	private void clearEntry(final Slot e) {
 		openByteCount -= e.sz;
 		e.provider = null;
 		e.data = DEAD;
 		e.sz = 0;
 	}
 
-	private DeltaBaseCache() {
-		throw new UnsupportedOperationException();
-	}
-
 	static class Entry {
 		final byte[] data;
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackFile.java b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackFile.java
index 959bf43342..80a096633f 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackFile.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackFile.java
@@ -236,7 +236,6 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 	 * Close the resources utilized by this repository
 	 */
 	public void close() {
-		DeltaBaseCache.purge(this);
 		WindowCache.purge(this);
 		synchronized (this) {
 			loadedIdx = null;
@@ -723,7 +722,7 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 					if (sz != delta.deltaSize)
 						break SEARCH;
 
-					DeltaBaseCache.Entry e = DeltaBaseCache.get(this, base);
+					DeltaBaseCache.Entry e = curs.getDeltaBaseCache().get(this, base);
 					if (e != null) {
 						type = e.type;
 						data = e.data;
@@ -741,7 +740,7 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 					if (sz != delta.deltaSize)
 						break SEARCH;
 
-					DeltaBaseCache.Entry e = DeltaBaseCache.get(this, base);
+					DeltaBaseCache.Entry e = curs.getDeltaBaseCache().get(this, base);
 					if (e != null) {
 						type = e.type;
 						data = e.data;
@@ -769,7 +768,7 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 				if (cached)
 					cached = false;
 				else if (delta.next == null)
-					DeltaBaseCache.store(this, delta.basePos, data, type);
+					curs.getDeltaBaseCache().store(this, delta.basePos, data, type);
 
 				pos = delta.deltaPos;
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/WindowCursor.java b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/WindowCursor.java
index e2dcae82bc..b3b93b70ca 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/WindowCursor.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/WindowCursor.java
@@ -83,12 +83,20 @@ final class WindowCursor extends ObjectReader implements ObjectReuseAsIs {
 
 	private ByteWindow window;
 
+	private DeltaBaseCache baseCache;
+
 	final FileObjectDatabase db;
 
 	WindowCursor(FileObjectDatabase db) {
 		this.db = db;
 	}
 
+	DeltaBaseCache getDeltaBaseCache() {
+		if (baseCache == null)
+			baseCache = new DeltaBaseCache();
+		return baseCache;
+	}
+
 	@Override
 	public ObjectReader newReader() {
 		return new WindowCursor(db);
@@ -334,6 +342,7 @@ final class WindowCursor extends ObjectReader implements ObjectReuseAsIs {
 	/** Release the current window cursor. */
 	public void release() {
 		window = null;
+		baseCache = null;
 		try {
 			InflaterCache.release(inf);
 		} finally {