netty(七)buffer源码学习2
概述
文章主要介绍的是PoolArena,PoolChunk,PoolSubpage 三个类的源码
PoolArena
PoolArena 是netty 的内存池实现类,通过预先申请一块大的空间,然后对空间进行分配和回收,这样就不用频繁进行系统调用,提高性能。 PoolArena 由多个 chunk 组成,chunk 则由多个page 组成.
PoolChunk
Chunk 主要用来组织和管理多个Page的内存分配和释放,在netty中,Chunk 中的Page被构造成一棵二叉树。
PoolSubpage
PoolSubpage保存long[] 数组表示占用情况。数组中一个long元素最多表示64分空间的占用情况,假如某块空间分的分数多于64份,那么数组中加多一个元素,例如4字节分4份,long元素的第四位就可以表示完,要是128字节分128份,long数组就要添加多一个元素共2个元素,一个long表示64份。
内存回收策略
- chunk 回收 : 二叉树节点状态标识符
- page 回收 : 维护块的使用状态来标识
源码
上面就是我们刚才讲的几个类的类结构。
final class PoolSubpage<T> { //保持一个 PoolChunk final PoolChunk<T> chunk; final int memoryMapIdx; final int runOffset; final int pageSize; final long[] bitmap; //前后指针 PoolSubpage<T> prev; PoolSubpage<T> next; boolean doNotDestroy; int elemSize; int maxNumElems; int nextAvail; int bitmapLength; int numAvail; // TODO: Test if adding padding helps under contention //private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7; /** Special constructor that creates a linked list head */ PoolSubpage(int pageSize) { chunk = null; memoryMapIdx = -1; runOffset = -1; elemSize = -1; this.pageSize = pageSize; bitmap = null; } ...
final class PoolChunkList<T> { private final PoolArena<T> arena; //前后指针 private final PoolChunkList<T> nextList; PoolChunkList<T> prevList; ... }
abstract class PoolArena<T> { final PooledByteBufAllocator parent; private final int pageSize; private final int maxOrder; private final int pageShifts; private final int chunkSize; private final int subpageOverflowMask; private final PoolSubpage<T>[] tinySubpagePools; private final PoolSubpage<T>[] smallSubpagePools; private final PoolChunkList<T> q050; private final PoolChunkList<T> q025; private final PoolChunkList<T> q000; private final PoolChunkList<T> qInit; private final PoolChunkList<T> q075; private final PoolChunkList<T> q100; // TODO: Test if adding padding helps under contention //private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7; protected PoolArena(PooledByteBufAllocator parent, int pageSize, int maxOrder, int pageShifts, int chunkSize) { this.parent = parent; this.pageSize = pageSize; this.maxOrder = maxOrder; this.pageShifts = pageShifts; this.chunkSize = chunkSize; subpageOverflowMask = ~(pageSize - 1); tinySubpagePools = newSubpagePoolArray(512 >>> 4); for (int i = 0; i < tinySubpagePools.length; i ++) { tinySubpagePools[i] = newSubpagePoolHead(pageSize); } smallSubpagePools = newSubpagePoolArray(pageShifts - 9); for (int i = 0; i < smallSubpagePools.length; i ++) { smallSubpagePools[i] = newSubpagePoolHead(pageSize); } q100 = new PoolChunkList<T>(this, null, 100, Integer.MAX_VALUE); q075 = new PoolChunkList<T>(this, q100, 75, 100); q050 = new PoolChunkList<T>(this, q075, 50, 100); q025 = new PoolChunkList<T>(this, q050, 25, 75); q000 = new PoolChunkList<T>(this, q025, 1, 50); qInit = new PoolChunkList<T>(this, q000, Integer.MIN_VALUE, 25); q100.prevList = q075; q075.prevList = q050; q050.prevList = q025; q025.prevList = q000; q000.prevList = null; qInit.prevList = qInit; } ....
源码分析 PooledDirectByteBuf
开始看 PooledDirectByteBuf 的方法有点乱,我们可以从调用它的那个类开始分析。 PooledByteBufAllocator内部就是可以调用 PooledDirectByteBuf 生成 ByteBuf的类。它的使用如下 :
public static void main(String[] args){ /* * 内存池buffer 提供为外部的使用主要是通过 PooledByteBufAllocator 这个类堆外提供 */ PooledByteBufAllocator allocator = new PooledByteBufAllocator(); ByteBuf directBuffer = allocator.directBuffer(); }
下面我们看一下 directBuffer 方法。
@Override public ByteBuf directBuffer() { return directBuffer(DEFAULT_INITIAL_CAPACITY, Integer.MAX_VALUE); } @Override public ByteBuf directBuffer(int initialCapacity, int maxCapacity) { if (initialCapacity == 0 && maxCapacity == 0) { return emptyBuf; } //校验 validate(initialCapacity, maxCapacity); //子类实现,申请空间 return newDirectBuffer(initialCapacity, maxCapacity); } // PooledByteBufAllocator 类中的实现 @Override protected ByteBuf newDirectBuffer(int initialCapacity, int maxCapacity) { //No.1 缓冲中获取 PoolThreadCache cache = threadCache.get(); PoolArena<ByteBuffer> directArena = cache.directArena; ByteBuf buf; if (directArena != null) { buf = directArena.allocate(cache, initialCapacity, maxCapacity); } else { //假如缓冲中的 PoolArena 不存在 if (PlatformDependent.hasUnsafe()) { buf = new UnpooledUnsafeDirectByteBuf(this, initialCapacity, maxCapacity); } else { buf = new UnpooledDirectByteBuf(this, initialCapacity, maxCapacity); } } return toLeakAwareBuffer(buf); } // PooledByteBufAllocator 类中的threadCache ,是个ThreadLocal final ThreadLocal<PoolThreadCache> threadCache = new ThreadLocal<PoolThreadCache>() { private final AtomicInteger index = new AtomicInteger(); @Override protected PoolThreadCache initialValue() { final int idx = index.getAndIncrement(); final PoolArena<byte[]> heapArena; final PoolArena<ByteBuffer> directArena; if (heapArenas != null) { heapArena = heapArenas[Math.abs(idx % heapArenas.length)]; } else { heapArena = null; } if (directArenas != null) { directArena = directArenas[Math.abs(idx % directArenas.length)]; } else { directArena = null; } return new PoolThreadCache(heapArena, directArena); } };
参考资料
- 《netty权威指南》