前面文章说了,ChannelHandlerContext#write只是将数据缓存到ChannelOutboundBuffer,等到ChannelHandlerContext#flush时,再将ChannelOutboundBuffer缓存的数据写到Channel中。
本文分享Netty中ChannelOutboundBuffer的实现以及Flush过程。
源码分析基于Netty 4.1
每个Channel的AbstractUnsafe#outboundBuffer 都维护了一个ChannelOutboundBuffer。
ChannelOutboundBuffer,出站数据缓冲区,负责缓存ChannelHandlerContext#write的数据。通过链表管理数据,链表节点为内部类Entry。
关键字段如下
Entry tailEntry; // 链表最后一个节点,新增的节点添加其后。
Entry unflushedEntry; // 链表中第一个未刷新的节点
Entry flushedEntry; // 链表中第一个已刷新但数据未写入的节点
int flushed; // 已刷新但数据未写入的节点数
ChannelHandlerContext#flush操作前,需要先刷新一遍待处理的节点(主要是统计本次ChannelHandlerContext#flush操作可以写入多少个节点数据),从unflushedEntry开始。刷新完成后使用flushedEntry标志第一个待写入的节点,flushed为待写入节点数。
前面分享Netty读写过程的文章说过,AbstractUnsafe#write处理写操作时,会调用ChannelOutboundBuffer#addMessage将数据缓存起来
public void addMessage(Object msg, int size, ChannelPromise promise) {
// #1
Entry entry = Entry.newInstance(msg, size, total(msg), promise);
if (tailEntry == null) {
flushedEntry = null;
} else {
Entry tail = tailEntry;
tail.next = entry;
}
tailEntry = entry;
if (unflushedEntry == null) {
unflushedEntry = entry;
}
incrementPendingOutboundBytes(entry.pendingSize, false);
}
#1
构建一个Entry,注意,这里使用了对象池RECYCLER,后面有文章详细解析。
主要是更新tailEntry和unflushedEntry#2
如果当前缓存数量超过阀值WriteBufferWaterMark#high,更新unwritable标志为true,并触发pipeline.fireChannelWritabilityChanged()
方法。
由于ChannelOutboundBuffer链表没有大小限制,不断累积数据可能导致 OOM,
为了避免这个问题,我们可以在unwritable标志为true时,不再继续缓存数据。
Netty只会更新unwritable标志,并不阻止数据缓存,我们可以根据需要实现该功能。示例如下
if (ctx.channel().isActive() && ctx.channel().isWritable()) {
ctx.writeAndFlush(responseMessage);
} else {
...
}
addFlush方法负责刷新节点(ChannelHandlerContext#flush操作前调用该方法统计可写入节点数据数)
public void addFlush() {
// #1
Entry entry = unflushedEntry;
if (entry != null) {
// #2
if (flushedEntry == null) {
// there is no flushedEntry yet, so start with the entry
flushedEntry = entry;
}
do {
// #3
flushed ++;
if (!entry.promise.setUncancellable()) {
// Was cancelled so make sure we free up memory and notify about the freed bytes
int pending = entry.cancel();
decrementPendingOutboundBytes(pending, false, true);
}
entry = entry.next;
// #4
} while (entry != null);
// All flushed so reset unflushedEntry
// #5
unflushedEntry = null;
}
}
#1
从unflushedEntry节点开始处理#2
赋值flushedEntry为unflushedEntry。
ChannelHandlerContext#flush写入完成后会置空flushedEntry#3
增加flushed
设置节点的ChannelPromise不可取消#4
从unflushedEntry开始,遍历后面节点#5
置空unflushedEntry,表示当前所有节点都已刷新。
nioBuffers方法负责将当前缓存的ByteBuf转发为(jvm)ByteBuffer
public ByteBuffer[] nioBuffers(int maxCount, long maxBytes) {
assert maxCount > 0;
assert maxBytes > 0;
long nioBufferSize = 0;
int nioBufferCount = 0;
// #1
final InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
ByteBuffer[] nioBuffers = NIO_BUFFERS.get(threadLocalMap);
Entry entry = flushedEntry;
while (isFlushedEntry(entry) && entry.msg instanceof ByteBuf) {
if (!entry.cancelled) {
ByteBuf buf = (ByteBuf) entry.msg;
final int readerIndex = buf.readerIndex();
final int readableBytes = buf.writerIndex() - readerIndex;
if (readableBytes > 0) {
// #2
if (maxBytes - readableBytes < nioBufferSize && nioBufferCount != 0) {
break;
}
nioBufferSize += readableBytes;
// #3
int count = entry.count;
if (count == -1) {
//noinspection ConstantValueVariableUse
entry.count = count = buf.nioBufferCount();
}
int neededSpace = min(maxCount, nioBufferCount + count);
if (neededSpace > nioBuffers.length) {
nioBuffers = expandNioBufferArray(nioBuffers, neededSpace, nioBufferCount);
NIO_BUFFERS.set(threadLocalMap, nioBuffers);
}
// #4
if (count == 1) {
ByteBuffer nioBuf = entry.buf;
if (nioBuf == null) {
// cache ByteBuffer as it may need to create a new ByteBuffer instance if its a
// derived buffer
entry.buf = nioBuf = buf.internalNioBuffer(readerIndex, readableBytes);
}
nioBuffers[nioBufferCount++] = nioBuf;
} else {
...
}
if (nioBufferCount == maxCount) {
break;
}
}
}
entry = entry.next;
}
this.nioBufferCount = nioBufferCount;
this.nioBufferSize = nioBufferSize;
return nioBuffers;
}
#1
从线程缓存中获取nioBuffers变量,这样可以避免反复构造ByteBuffer数组的性能损耗#2
maxBytes,即本次操作最大的字节数。maxBytes - readableBytes < nioBufferSize
,表示如果本次操作后将超出maxBytes,退出#3
buf.nioBufferCount(),获取ByteBuffer数量,CompositeByteBuf可能有多个ByteBuffer组成。
neededSpace,即nioBuffers数组中ByteBuffer数量,nioBuffers长度不够时需要扩容。#4
buf.internalNioBuffer(readerIndex, readableBytes)
,使用readerIndex, readableBytes构造一个ByteBuffer。
这里涉及ByteBuf相关知识,后面有文章详细解析。
ChannelHandlerContext#flush完成后,需要移除对应的缓存节点。
public void removeBytes(long writtenBytes) {
for (;;) {
// #1
Object msg = current();
if (!(msg instanceof ByteBuf)) {
assert writtenBytes == 0;
break;
}
final ByteBuf buf = (ByteBuf) msg;
final int readerIndex = buf.readerIndex();
final int readableBytes = buf.writerIndex() - readerIndex;
// #2
if (readableBytes <= writtenBytes) {
if (writtenBytes != 0) {
progress(readableBytes);
writtenBytes -= readableBytes;
}
remove();
} else { // readableBytes > writtenBytes
// #3
if (writtenBytes != 0) {
buf.readerIndex(readerIndex + (int) writtenBytes);
progress(writtenBytes);
}
break;
}
}
clearNioBuffers();
}
#1
current方法返回flushedEntry节点缓存数据。
结果null时,退出循环#2
当前节点的数据已经全部写入,
progress方法唤醒数据节点上ChannelProgressivePromise的监听者
writtenBytes减去对应字节数
remove()方法移除节点,释放ByteBuf,flushedEntry标志后移。#3
当前节点的数据部分写入,它应该是本次ChannelHandlerContext#flush操作的最后一个节点
更新ByteBuf的readerIndex,下次从这里开始读取数据。
退出
移除数据节点
public boolean remove() {
Entry e = flushedEntry;
if (e == null) {
clearNioBuffers();
return false;
}
Object msg = e.msg;
ChannelPromise promise = e.promise;
int size = e.pendingSize;
// #1
removeEntry(e);
if (!e.cancelled) {
// only release message, notify and decrement if it was not canceled before.
// #2
ReferenceCountUtil.safeRelease(msg);
safeSuccess(promise);
decrementPendingOutboundBytes(size, false, true);
}
// recycle the entry
// #3
e.recycle();
return true;
}
#1
flushed减1
当flushed为0时,flushedEntry赋值为null,否则flushedEntry指向后一个节点。#2
释放ByteBuf#3
当前节点返回对象池中,以便复用。
下面来看一下ChannelHandlerContext#flush操作过程。
ChannelHandlerContext#flush -> HeadContext#flush -> AbstractUnsafe#flush
public final void flush() {
assertEventLoop();
ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null) {
return;
}
// #1
outboundBuffer.addFlush();
// #2
flush0();
}
#1
刷新outboundBuffer中数据节点#2
写入操作
flush -> NioSocketChannel#doWrite
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
SocketChannel ch = javaChannel();
int writeSpinCount = config().getWriteSpinCount();
do {
// #1
if (in.isEmpty()) {
clearOpWrite();
return;
}
// #2
int maxBytesPerGatheringWrite = ((NioSocketChannelConfig) config).getMaxBytesPerGatheringWrite();
ByteBuffer[] nioBuffers = in.nioBuffers(1024, maxBytesPerGatheringWrite);
int nioBufferCnt = in.nioBufferCount();
switch (nioBufferCnt) {
case 0:
// #3
writeSpinCount -= doWrite0(in);
break;
case 1: {
// #4
ByteBuffer buffer = nioBuffers[0];
int attemptedBytes = buffer.remaining();
final int localWrittenBytes = ch.write(buffer);
if (localWrittenBytes <= 0) {
// #5
incompleteWrite(true);
return;
}
adjustMaxBytesPerGatheringWrite(attemptedBytes, localWrittenBytes, maxBytesPerGatheringWrite);
// #6
in.removeBytes(localWrittenBytes);
--writeSpinCount;
break;
}
default: {
// #7
...
}
}
} while (writeSpinCount > 0);
incompleteWrite(writeSpinCount < 0);
}
#1
通过ChannelOutboundBuffer#flushed判断是否没有数据可以写,没有数据则清除关注事件OP_WRITE,直接返回。#2
获取ChannelOutboundBuffer中ByteBuf维护的(jvm)ByteBuffer,并统计nioBufferSize,nioBufferCount。#3
这时没有ByteBuffer,但是可能有其他类型的数据(如FileRegion类型),调用doWrite0继续处理,这里不再深入#4
只有一个ByteBuffer,调用SocketChannel#write将数据写入Channel。#5
如果写入数据数量小于等于0,说明数据没有被写出去(可能是因为套接字的缓冲区满了等原因),那么就需要关注该Channel上的OP_WRITE事件,方便下次EventLoop将Channel轮询出来的时候,能继续写数据。#6
移除ChannelOutboundBuffer缓存数据节点。#7
有多个ByteBuffer,调用SocketChannel#write(ByteBuffer[] srcs, int offset, int length)
,批量写入,与上一种情况处理类似
回顾之前文章《事件循环机制实现原理》中对NioEventLoop#processSelectedKey方法的解析
...
if ((readyOps & SelectionKey.OP_WRITE) != 0) {
ch.unsafe().forceFlush();
}
这里会调用forceFlush方法,再次写入数据。
FlushConsolidationHandler
ChannelHandlerContext#flush是很昂贵的操作,可能触发系统调用,但数据又不能缓存太久,使用FlushConsolidationHandler可以尽量达到写入延迟与吞吐量之间的权衡。
FlushConsolidationHandler中维护了explicitFlushAfterFlushes变量,
在ChannelOutboundHandler#channelRead中调用flush,如果调用次数小于explicitFlushAfterFlushes, 会拦截flush操作不执行。
在channelReadComplete后调用flush,则不会拦截flush操作。
本文涉及ByteBuf组件,它是Netty中的内存缓冲区,后面有文章解析。
如果您觉得本文不错,欢迎关注我的微信公众号,系列文章持续更新中。您的关注是我坚持的动力!