BuddyChunk Class — netty Architecture

Source Code

buffer/src/main/java/io/netty/buffer/AdaptivePoolingAllocator.java lines 1356–1534

    private static final class BuddyChunk extends Chunk implements IntConsumer {
        private static final int MIN_BUDDY_SIZE = 32768;
        private static final byte IS_CLAIMED = (byte) (1 << 7);
        private static final byte HAS_CLAIMED_CHILDREN = 1 << 6;
        private static final byte SHIFT_MASK = ~(IS_CLAIMED | HAS_CLAIMED_CHILDREN);
        private static final int PACK_OFFSET_MASK = 0xFFFF;
        private static final int PACK_SIZE_SHIFT = Integer.SIZE - Integer.numberOfLeadingZeros(PACK_OFFSET_MASK);

        private final MpscIntQueue freeList;
        // The bits of each buddy: [1: is claimed][1: has claimed children][30: MIN_BUDDY_SIZE shift to get size]
        private final byte[] buddies;
        private final int freeListCapacity;

        BuddyChunk(AbstractByteBuf delegate, Magazine magazine) {
            super(delegate, magazine, true);
            int capacity = delegate.capacity();
            int capFactor = capacity / MIN_BUDDY_SIZE;
            int tree = (capFactor << 1) - 1;
            int maxShift = Integer.numberOfTrailingZeros(capFactor);
            assert maxShift <= 30; // The top 2 bits are used for marking.
            freeListCapacity = tree >> 1;
            freeList = MpscIntQueue.create(freeListCapacity, -1); // At most half of tree (all leaf nodes) can be freed.
            buddies = new byte[1 + tree];

            // Generate the buddies entries.
            int index = 1;
            int runLength = 1;
            int currentRun = 0;
            while (maxShift > 0) {
                buddies[index++] = (byte) maxShift;
                if (++currentRun == runLength) {
                    currentRun = 0;
                    runLength <<= 1;
                    maxShift--;
                }
            }
        }

        @Override
        public boolean readInitInto(AdaptiveByteBuf buf, int size, int startingCapacity, int maxCapacity) {
            if (!freeList.isEmpty()) {
                freeList.drain(freeListCapacity, this);
            }
            int startIndex = chooseFirstFreeBuddy(1, startingCapacity, 0);
            if (startIndex == -1) {
                return false;
            }
            Chunk chunk = this;
            chunk.retain();
            try {
                buf.init(delegate, this, 0, 0, startIndex, size, startingCapacity, maxCapacity);
                allocatedBytes += startingCapacity;
                chunk = null;
            } finally {
                if (chunk != null) {
                    // If chunk is not null we know that buf.init(...) failed and so we need to manually release
                    // the chunk again as we retained it before calling buf.init(...).
                    chunk.release();
                }
            }
            return true;
        }

        @Override
        public void accept(int packed) {
            // Called by allocating thread when draining freeList.
            int size = MIN_BUDDY_SIZE << (packed >> PACK_SIZE_SHIFT);
            int offset = (packed & PACK_OFFSET_MASK) * MIN_BUDDY_SIZE;
            unreserveMatchingBuddy(1, size, offset, 0);
            allocatedBytes -= size;
        }

        @Override
        void releaseSegment(int startingIndex, int size) {
            int packedOffset = startingIndex / MIN_BUDDY_SIZE;
            int packedSize = Integer.numberOfTrailingZeros(size / MIN_BUDDY_SIZE) << PACK_SIZE_SHIFT;
            int packed = packedOffset | packedSize;
            freeList.offer(packed);
            release();
        }