Bit-ing the atom

- Use an atomic bitset in AbstractInstancer.
- Add method to iterate over each contiguous span of set bits.

src/main/java/com/jozufozu/flywheel/backend/engine/AbstractInstancer.java

@@ -1,11 +1,11 @@
 package com.jozufozu.flywheel.backend.engine;
 
 import java.util.ArrayList;
-import java.util.BitSet;
 
 import com.jozufozu.flywheel.api.instance.Instance;
 import com.jozufozu.flywheel.api.instance.InstanceType;
 import com.jozufozu.flywheel.api.instance.Instancer;
+import com.jozufozu.flywheel.lib.util.AtomicBitset;
 
 public abstract class AbstractInstancer<I extends Instance> implements Instancer<I> {
 	public final InstanceType<I> type;
@@ -15,8 +15,8 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 	protected final ArrayList<I> instances = new ArrayList<>();
 	protected final ArrayList<InstanceHandleImpl> handles = new ArrayList<>();
 
-	protected final BitSet changed = new BitSet();
+	protected final AtomicBitset changed = new AtomicBitset();
-	protected final BitSet deleted = new BitSet();
+	protected final AtomicBitset deleted = new AtomicBitset();
 
 	protected AbstractInstancer(InstanceType<I> type) {
 		this.type = type;
@@ -40,23 +40,23 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 		return instances.size();
 	}
 
+	protected boolean moreThanTwoThirdsChanged() {
+		return (changed.cardinality() * 3) > (instances.size() * 2);
+	}
+
+
 	public void notifyDirty(int index) {
 		if (index < 0 || index >= getInstanceCount()) {
 			return;
 		}
-		// TODO: Atomic bitset. Synchronizing here blocks the task executor and causes massive overhead.
+		changed.set(index);
-		synchronized (lock) {
-			changed.set(index);
-		}
 	}
 
 	public void notifyRemoval(int index) {
 		if (index < 0 || index >= getInstanceCount()) {
 			return;
 		}
-		synchronized (lock) {
+		deleted.set(index);
-			deleted.set(index);
-		}
 	}
 
 	protected void removeDeletedInstances() {

src/main/java/com/jozufozu/flywheel/backend/engine/indirect/IndirectCullingGroup.java

@@ -203,7 +203,7 @@ public class IndirectCullingGroup<I extends Instance> {
 		long pos = 0;
 		for (IndirectModel model : indirectModels) {
 			var instanceCount = model.instancer.getInstanceCount();
-			model.writeObjects(stagingBuffer, pos, buffers.object.handle());
+			model.uploadObjects(stagingBuffer, pos, buffers.object.handle());
 
 			pos += instanceCount * objectStride;
 		}

src/main/java/com/jozufozu/flywheel/backend/engine/indirect/IndirectInstancer.java

@@ -26,41 +26,45 @@ public class IndirectInstancer<I extends Instance> extends AbstractInstancer<I>
 		removeDeletedInstances();
 	}
 
-	public void write(StagingBuffer stagingBuffer, long startPos, int dstVbo) {
+	public void upload(StagingBuffer stagingBuffer, long startPos, int dstVbo) {
-		if (shouldWriteAll(startPos)) {
+		if (shouldUploadAll(startPos)) {
-			writeAll(stagingBuffer, startPos, dstVbo);
+			uploadAll(stagingBuffer, startPos, dstVbo);
 		} else {
-			writeChanged(stagingBuffer, startPos, dstVbo);
+			uploadChanged(stagingBuffer, startPos, dstVbo);
 		}
 
 		changed.clear();
 		lastStartPos = startPos;
 	}
 
-	private boolean shouldWriteAll(long startPos) {
+	private boolean shouldUploadAll(long startPos) {
 		// If enough of the buffer has changed, write the whole thing to avoid the overhead of a bunch of small writes.
+		// TODO: The overhead comes from the driver performing many buffer copies. Using a compute shader to scatter
+		// 		 the data should work much better.
 		return startPos != lastStartPos || moreThanTwoThirdsChanged();
 	}
 
-	private boolean moreThanTwoThirdsChanged() {
+	private void uploadChanged(StagingBuffer stagingBuffer, long baseByte, int dstVbo) {
-		return (changed.cardinality() * 3) > (instances.size() * 2);
+		changed.forEachSetSpan((startInclusive, endInclusive) -> {
+			var totalSize = (endInclusive - startInclusive + 1) * objectStride;
+
+			stagingBuffer.enqueueCopy(totalSize, dstVbo, baseByte + startInclusive * objectStride, ptr -> {
+				for (int i = startInclusive; i <= endInclusive; i++) {
+					var instance = instances.get(i);
+					writeOne(ptr, instance);
+					ptr += objectStride;
+				}
+			});
+		});
 	}
 
-	private void writeChanged(StagingBuffer stagingBuffer, long start, int dstVbo) {
+	private void uploadAll(StagingBuffer stagingBuffer, long start, int dstVbo) {
-		int count = instances.size();
-		for (int i = changed.nextSetBit(0); i >= 0 && i < count; i = changed.nextSetBit(i + 1)) {
-			var instance = instances.get(i);
-			stagingBuffer.enqueueCopy(objectStride, dstVbo, start + i * objectStride, ptr -> writeOne(ptr, instance));
-		}
-	}
-
-	private void writeAll(StagingBuffer stagingBuffer, long start, int dstVbo) {
 		long totalSize = objectStride * instances.size();
 
-		stagingBuffer.enqueueCopy(totalSize, dstVbo, start, this::writeAll);
+		stagingBuffer.enqueueCopy(totalSize, dstVbo, start, this::uploadAll);
 	}
 
-	private void writeAll(long ptr) {
+	private void uploadAll(long ptr) {
 		for (I instance : instances) {
 			writeOne(ptr, instance);
 			ptr += objectStride;

src/main/java/com/jozufozu/flywheel/backend/engine/indirect/IndirectModel.java

@@ -25,8 +25,8 @@ public class IndirectModel {
 		this.baseInstance = baseInstance;
 	}
 
-	public void writeObjects(StagingBuffer stagingBuffer, long start, int dstVbo) {
+	public void uploadObjects(StagingBuffer stagingBuffer, long start, int dstVbo) {
-		instancer.write(stagingBuffer, start, dstVbo);
+		instancer.upload(stagingBuffer, start, dstVbo);
 	}
 
 	public void write(long ptr) {

src/main/java/com/jozufozu/flywheel/backend/engine/instancing/InstancedInstancer.java

@@ -21,6 +21,7 @@ public class InstancedInstancer<I extends Instance> extends AbstractInstancer<I>
 	private final int instanceStride;
 
 	private final Set<GlVertexArray> boundTo = new HashSet<>();
+	private final InstanceWriter<I> writer;
 	private GlBuffer vbo;
 
 	private final List<DrawCall> drawCalls = new ArrayList<>();
@@ -29,6 +30,7 @@ public class InstancedInstancer<I extends Instance> extends AbstractInstancer<I>
 		super(type);
 		instanceFormat = type.getLayout();
 		instanceStride = instanceFormat.getStride();
+		writer = type.getWriter();
 	}
 
 	public int getAttributeCount() {
@@ -70,12 +72,8 @@ public class InstancedInstancer<I extends Instance> extends AbstractInstancer<I>
 
 		try (MappedBuffer buf = vbo.map()) {
 			long ptr = buf.ptr();
-			InstanceWriter<I> writer = type.getWriter();
 
-			int count = instances.size();
+			writeChanged(ptr);
-			for (int i = changed.nextSetBit(0); i >= 0 && i < count; i = changed.nextSetBit(i + 1)) {
-				writer.write(ptr + (long) instanceStride * i, instances.get(i));
-			}
 
 			changed.clear();
 		} catch (Exception e) {
@@ -83,6 +81,14 @@ public class InstancedInstancer<I extends Instance> extends AbstractInstancer<I>
 		}
 	}
 
+	private void writeChanged(long ptr) {
+		changed.forEachSetSpan((startInclusive, endInclusive) -> {
+			for (int i = startInclusive; i <= endInclusive; i++) {
+				writer.write(ptr + (long) instanceStride * i, instances.get(i));
+			}
+		});
+	}
+
 	/**
 	 * Bind this instancer's vbo to the given vao if it hasn't already been bound.
 	 * @param vao The vao to bind to.

src/main/java/com/jozufozu/flywheel/lib/util/AtomicBitset.java

@@ -0,0 +1,465 @@
+package com.jozufozu.flywheel.lib.util;
+
+import java.util.Arrays;
+import java.util.BitSet;
+import java.util.concurrent.atomic.AtomicLongArray;
+import java.util.concurrent.atomic.AtomicReference;
+
+// https://github.com/Netflix/hollow/blob/master/hollow/src/main/java/com/netflix/hollow/core/memory/ThreadSafeBitSet.java
+// Refactored to remove unused methods, deduplicate some code segments, and add extra functionality with #forEachSetSpan
+public class AtomicBitset {
+	// 1024 bits, 128 bytes, 16 longs per segment
+	public static final int DEFAULT_LOG2_SEGMENT_SIZE_IN_BITS = 10;
+
+	private final int numLongsPerSegment;
+	private final int log2SegmentSize;
+	private final int segmentMask;
+	private final AtomicReference<AtomicBitsetSegments> segments;
+
+	public AtomicBitset() {
+		this(DEFAULT_LOG2_SEGMENT_SIZE_IN_BITS);
+	}
+
+	public AtomicBitset(int log2SegmentSizeInBits) {
+		this(log2SegmentSizeInBits, 0);
+	}
+
+	public AtomicBitset(int log2SegmentSizeInBits, int numBitsToPreallocate) {
+		if (log2SegmentSizeInBits < 6) {
+			throw new IllegalArgumentException("Cannot specify fewer than 64 bits in each segment!");
+		}
+
+		this.log2SegmentSize = log2SegmentSizeInBits;
+		this.numLongsPerSegment = (1 << (log2SegmentSizeInBits - 6));
+		this.segmentMask = numLongsPerSegment - 1;
+
+		long numBitsPerSegment = numLongsPerSegment * 64L;
+		int numSegmentsToPreallocate = numBitsToPreallocate == 0 ? 1 : (int) (((numBitsToPreallocate - 1) / numBitsPerSegment) + 1);
+
+		segments = new AtomicReference<>(new AtomicBitsetSegments(numSegmentsToPreallocate, numLongsPerSegment));
+	}
+
+	public void set(int position) {
+		int longPosition = longIndexInSegmentForPosition(position);
+
+		AtomicLongArray segment = getSegmentForPosition(position);
+
+		long mask = maskForPosition(position);
+
+		// Thread safety: we need to loop until we win the race to set the long value.
+		while (true) {
+			// determine what the new long value will be after we set the appropriate bit.
+			long currentLongValue = segment.get(longPosition);
+			long newLongValue = currentLongValue | mask;
+
+			// if no other thread has modified the value since we read it, we won the race and we are done.
+			if (segment.compareAndSet(longPosition, currentLongValue, newLongValue)) {
+				break;
+			}
+		}
+	}
+
+	public void clear(int position) {
+		int longPosition = longIndexInSegmentForPosition(position);
+
+		AtomicLongArray segment = getSegmentForPosition(position);
+
+		long mask = ~maskForPosition(position);
+
+		// Thread safety: we need to loop until we win the race to set the long value.
+		while (true) {
+			// determine what the new long value will be after we set the appropriate bit.
+			long currentLongValue = segment.get(longPosition);
+			long newLongValue = currentLongValue & mask;
+
+			// if no other thread has modified the value since we read it, we won the race and we are done.
+			if (segment.compareAndSet(longPosition, currentLongValue, newLongValue)) {
+				break;
+			}
+		}
+	}
+
+	public boolean get(int position) {
+		int segmentPosition = segmentIndexForPosition(position);
+		int longPosition = longIndexInSegmentForPosition(position);
+
+		AtomicLongArray segment = segmentForPosition(segmentPosition);
+
+		long mask = maskForPosition(position);
+
+		return ((segment.get(longPosition) & mask) != 0);
+	}
+
+	public long maxSetBit() {
+		AtomicBitsetSegments segments = this.segments.get();
+
+		int segmentIdx = segments.numSegments() - 1;
+
+		for (; segmentIdx >= 0; segmentIdx--) {
+			AtomicLongArray segment = segments.getSegment(segmentIdx);
+			for (int longIdx = segment.length() - 1; longIdx >= 0; longIdx--) {
+				long l = segment.get(longIdx);
+				if (l != 0) {
+					return ((long) segmentIdx << log2SegmentSize) + (longIdx * 64L) + (63 - Long.numberOfLeadingZeros(l));
+				}
+			}
+		}
+
+		return -1;
+	}
+
+	public int nextSetBit(int fromIndex) {
+		if (fromIndex < 0) {
+			throw new IndexOutOfBoundsException("fromIndex < 0: " + fromIndex);
+		}
+
+
+		AtomicBitsetSegments segments = this.segments.get();
+
+		int segmentPosition = segmentIndexForPosition(fromIndex);
+		if (segmentPosition >= segments.numSegments()) {
+			return -1;
+		}
+
+		int longPosition = longIndexInSegmentForPosition(fromIndex);
+		AtomicLongArray segment = segments.getSegment(segmentPosition);
+
+		long word = segment.get(longPosition) & (0xffffffffffffffffL << bitPosInLongForPosition(fromIndex));
+
+		while (true) {
+			if (word != 0) {
+				return (segmentPosition << (log2SegmentSize)) + (longPosition << 6) + Long.numberOfTrailingZeros(word);
+			}
+			if (++longPosition > segmentMask) {
+				segmentPosition++;
+				if (segmentPosition >= segments.numSegments()) {
+					return -1;
+				}
+				segment = segments.getSegment(segmentPosition);
+				longPosition = 0;
+			}
+
+			word = segment.get(longPosition);
+		}
+	}
+
+	public int nextClearBit(int fromIndex) {
+		if (fromIndex < 0) {
+			throw new IndexOutOfBoundsException("fromIndex < 0: " + fromIndex);
+		}
+
+		int segmentPosition = segmentIndexForPosition(fromIndex);
+
+		AtomicBitsetSegments segments = this.segments.get();
+
+		if (segmentPosition >= segments.numSegments()) {
+			return fromIndex;
+		}
+
+		int longPosition = longIndexInSegmentForPosition(fromIndex);
+		AtomicLongArray segment = segments.getSegment(segmentPosition);
+
+		long word = ~segment.get(longPosition) & (0xffffffffffffffffL << bitPosInLongForPosition(fromIndex));
+
+		while (true) {
+			if (word != 0) {
+				return (segmentPosition << (log2SegmentSize)) + (longPosition << 6) + Long.numberOfTrailingZeros(word);
+			}
+			if (++longPosition > segmentMask) {
+				segmentPosition++;
+				if (segmentPosition >= segments.numSegments()) {
+					return segments.numSegments() << log2SegmentSize;
+				}
+				segment = segments.getSegment(segmentPosition);
+				longPosition = 0;
+			}
+
+			word = segment.get(longPosition);
+		}
+	}
+
+
+	/**
+	 * @return the number of bits which are set in this bit set.
+	 */
+	public int cardinality() {
+		return this.segments.get()
+				.cardinality();
+	}
+
+	/**
+	 * Iterate over each contiguous span of set bits.
+	 *
+	 * @param consumer The consumer to accept each span.
+	 */
+	public void forEachSetSpan(BitSpanConsumer consumer) {
+		AtomicBitsetSegments segments = this.segments.get();
+
+		if (segments.cardinality() == 0) {
+			return;
+		}
+
+		int start = -1;
+		int end = -1;
+
+		for (int segmentIndex = 0; segmentIndex < segments.numSegments(); segmentIndex++) {
+			AtomicLongArray segment = segments.getSegment(segmentIndex);
+			for (int longIndex = 0; longIndex < segment.length(); longIndex++) {
+				long l = segment.get(longIndex);
+				if (l != 0) {
+					// The JIT loves this loop. Trying to be clever by starting from Long.numberOfLeadingZeros(l)
+					// causes it to be much slower.
+					for (int bitIndex = 0; bitIndex < 64; bitIndex++) {
+						if ((l & (1L << bitIndex)) != 0) {
+							var position = (segmentIndex << log2SegmentSize) + (longIndex << 6) + bitIndex;
+							if (start == -1) {
+								start = position;
+							}
+							end = position;
+						} else {
+							if (start != -1) {
+								consumer.accept(start, end);
+								start = -1;
+								end = -1;
+							}
+						}
+					}
+				} else {
+					if (start != -1) {
+						consumer.accept(start, end);
+						start = -1;
+						end = -1;
+					}
+				}
+			}
+		}
+
+		if (start != -1) {
+			consumer.accept(start, end);
+		}
+	}
+
+	/**
+	 * @return the number of bits which are currently specified by this bit set.  This is the maximum value
+	 * to which you might need to iterate, if you were to iterate over all bits in this set.
+	 */
+	public int currentCapacity() {
+		return segments.get()
+				.numSegments() * (1 << log2SegmentSize);
+	}
+
+	public boolean isEmpty() {
+		return cardinality() == 0;
+	}
+
+	/**
+	 * Clear all bits to 0.
+	 */
+	public void clear() {
+		AtomicBitsetSegments segments = this.segments.get();
+
+		for (int i = 0; i < segments.numSegments(); i++) {
+			AtomicLongArray segment = segments.getSegment(i);
+
+			for (int j = 0; j < segment.length(); j++) {
+				segment.set(j, 0L);
+			}
+		}
+	}
+
+	/**
+	 * Which bit in the long the given position resides in.
+	 *
+	 * @param position The absolute position in the bitset.
+	 * @return The bit position in the long.
+	 */
+	private static int bitPosInLongForPosition(int position) {
+		// remainder of div by num bits in long (64)
+		return position & 0x3F;
+	}
+
+	/**
+	 * Which long in the segment the given position resides in.
+	 *
+	 * @param position The absolute position in the bitset
+	 * @return The long position in the segment.
+	 */
+	private int longIndexInSegmentForPosition(int position) {
+		// remainder of div by num bits per segment
+		return (position >>> 6) & segmentMask;
+	}
+
+	/**
+	 * Which segment the given position resides in.
+	 *
+	 * @param position The absolute position in the bitset
+	 * @return The segment index.
+	 */
+	private int segmentIndexForPosition(int position) {
+		// div by num bits per segment
+		return position >>> log2SegmentSize;
+	}
+
+	private static long maskForPosition(int position) {
+		return 1L << bitPosInLongForPosition(position);
+	}
+
+	private AtomicLongArray getSegmentForPosition(int position) {
+		return segmentForPosition(segmentIndexForPosition(position));
+	}
+
+	/**
+	 * Get the segment at <code>segmentIndex</code>.  If this segment does not yet exist, create it.
+	 *
+	 * @param segmentIndex the segment index
+	 * @return the segment
+	 */
+	private AtomicLongArray segmentForPosition(int segmentIndex) {
+		AtomicBitsetSegments visibleSegments = segments.get();
+
+		while (visibleSegments.numSegments() <= segmentIndex) {
+			// Thread safety:  newVisibleSegments contains all of the segments from the currently visible segments, plus extra.
+			// all of the segments in the currently visible segments are canonical and will not change.
+			AtomicBitsetSegments newVisibleSegments = new AtomicBitsetSegments(visibleSegments, segmentIndex + 1, numLongsPerSegment);
+
+			// because we are using a compareAndSet, if this thread "wins the race" and successfully sets this variable, then the segments
+			// which are newly defined in newVisibleSegments become canonical.
+			if (segments.compareAndSet(visibleSegments, newVisibleSegments)) {
+				visibleSegments = newVisibleSegments;
+			} else {
+				// If we "lose the race" and are growing the AtomicBitset segments larger,
+				// then we will gather the new canonical sets from the update which we missed on the next iteration of this loop.
+				// Newly defined segments in newVisibleSegments will be discarded, they do not get to become canonical.
+				visibleSegments = segments.get();
+			}
+		}
+
+		return visibleSegments.getSegment(segmentIndex);
+	}
+
+	private static class AtomicBitsetSegments {
+		private final AtomicLongArray[] segments;
+
+		private AtomicBitsetSegments(int numSegments, int segmentLength) {
+			AtomicLongArray[] segments = new AtomicLongArray[numSegments];
+
+			for (int i = 0; i < numSegments; i++) {
+				segments[i] = new AtomicLongArray(segmentLength);
+			}
+
+			// Thread safety: Because this.segments is final, the preceding operations in this constructor are guaranteed to be visible to any
+			// other thread which accesses this.segments.
+			this.segments = segments;
+		}
+
+		private AtomicBitsetSegments(AtomicBitsetSegments copyFrom, int numSegments, int segmentLength) {
+			AtomicLongArray[] segments = new AtomicLongArray[numSegments];
+
+			for (int i = 0; i < numSegments; i++) {
+				segments[i] = i < copyFrom.numSegments() ? copyFrom.getSegment(i) : new AtomicLongArray(segmentLength);
+			}
+
+			// see above re: thread-safety of this assignment
+			this.segments = segments;
+		}
+
+		private int cardinality() {
+			int numSetBits = 0;
+
+			for (int i = 0; i < numSegments(); i++) {
+				AtomicLongArray segment = getSegment(i);
+				for (int j = 0; j < segment.length(); j++) {
+					numSetBits += Long.bitCount(segment.get(j));
+				}
+			}
+			return numSetBits;
+		}
+
+		public int numSegments() {
+			return segments.length;
+		}
+
+		public AtomicLongArray getSegment(int index) {
+			return segments[index];
+		}
+
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (!(obj instanceof AtomicBitset other)) {
+			return false;
+		}
+
+		if (other.log2SegmentSize != log2SegmentSize) {
+			throw new IllegalArgumentException("Segment sizes must be the same");
+		}
+
+		AtomicBitsetSegments thisSegments = this.segments.get();
+		AtomicBitsetSegments otherSegments = other.segments.get();
+
+		for (int i = 0; i < thisSegments.numSegments(); i++) {
+			AtomicLongArray thisArray = thisSegments.getSegment(i);
+			AtomicLongArray otherArray = (i < otherSegments.numSegments()) ? otherSegments.getSegment(i) : null;
+
+			for (int j = 0; j < thisArray.length(); j++) {
+				long thisLong = thisArray.get(j);
+				long otherLong = (otherArray == null) ? 0 : otherArray.get(j);
+
+				if (thisLong != otherLong) {
+					return false;
+				}
+			}
+		}
+
+		for (int i = thisSegments.numSegments(); i < otherSegments.numSegments(); i++) {
+			AtomicLongArray otherArray = otherSegments.getSegment(i);
+
+			for (int j = 0; j < otherArray.length(); j++) {
+				long l = otherArray.get(j);
+
+				if (l != 0) {
+					return false;
+				}
+			}
+		}
+
+		return true;
+	}
+
+	@Override
+	public int hashCode() {
+		int result = log2SegmentSize;
+		result = 31 * result + Arrays.hashCode(segments.get().segments);
+		return result;
+	}
+
+	/**
+	 * @return a new BitSet with same bits set
+	 */
+	public BitSet toBitSet() {
+		BitSet resultSet = new BitSet();
+		int ordinal = this.nextSetBit(0);
+		while (ordinal != -1) {
+			resultSet.set(ordinal);
+			ordinal = this.nextSetBit(ordinal + 1);
+		}
+		return resultSet;
+	}
+
+	@Override
+	public String toString() {
+		return toBitSet().toString();
+	}
+
+	@FunctionalInterface
+	public interface BitSpanConsumer {
+		/**
+		 * Consume a span of bits.
+		 *
+		 * @param startInclusive The first (inclusive) bit in the span.
+		 * @param endInclusive   The last (inclusive) bit in the span.
+		 */
+		void accept(int startInclusive, int endInclusive);
+	}
+}