记录一次OOM排查经历

我是用了netty搭建了一个UDP接收日志，堆启动配置 Xmx256  Xms256 ，项目刚启动的时候，系统进程占用内存很正常，在250M左右。

长时间运行之后发现，进程占用内存不断增长，远远超过了我设置的堆内存大小，查看幸存者，伊甸园，老年代，gc都很正常，堆使用数据一切正常，甚至我怀疑元空间占用内存大，查询之后发现，元空间也只用很小，而且自从程序启动开始，浮动很小。为此，我又把JVM相关知识点又拿出来翻了一遍

那么多出来的内存使用是从哪里来的？

后来通过查询相关资料才发现，Java进程内存分为堆内存，堆外内存，堆外内存是不受JVM的GC管理的。

堆外内存又是哪里使用到的？

nio框架会使用到

难道netty没有自己的一套GC机制？

有的，但是netty的GC，只负责释放自己产生的内存，如果是使用过程中，自己创建的，是不在netty GC的范围内的。好，那么现在稳定定位到了，开始修改代码和程序启动参数。

java -jar -Xms256M -Xmx256M -XX:MaxDirectMemorySize=128M -Dspring.profiles.active=prod log-server.jar

-XX:MaxDirectMemorySize=128M 设置堆外内存为128M，来控制进程内存使用，并且在代码中手动 copy 出来的  ByteBuf 进行  clear （PS：后来发现这个操作不起效果，是我对于该方法的理解有误）

@Component
public class UDPInboundHandler extends SimpleChannelInboundHandler<DatagramPacket> {

    private Logger logger = LoggerFactory.getLogger(UDPInboundHandler.class);

    @Autowired
    LogService logService;

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, DatagramPacket packet) {
        String remoteAddr = packet.sender().getAddress().getHostAddress();
        ByteBuf buf = packet.copy().content();
        logService.process(buf, remoteAddr);
        buf.clear();
    }
}

这样运行一点时间后，嗯，内存增长速度慢下来不少，原本从两百兆涨到五百兆，只需要半天时间，现在，一天观察下来，才增长到四百多兆，但是，256+128=384M,也超过了我设置的堆内存+堆外内存的总和，而且代码开始报错了如下：

io.netty.util.internal.OutOfDirectMemoryError: failed to allocate 16777216 byte(s) of direct memory (used: 134217728, max: 134217728)

134217728 byte(s) = 128M 也就是说我的clear的操作并没有效果，堆外内存已经全部用光。OOM的报错已经刷屏，但是，在众多的异常日志中发现了这条日志

2019-09-25 18:20:00.551 {nioEventLoopGroup-2-1} ERROR io.netty.util.ResourceLeakDetector - LEAK: ByteBuf.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information.
Recent access records:
Created at:
    io.netty.buffer.PooledByteBufAllocator.newDirectBuffer(PooledByteBufAllocator.java:331)
    io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:185)
    io.netty.buffer.UnsafeByteBufUtil.copy(UnsafeByteBufUtil.java:436)
    io.netty.buffer.PooledUnsafeDirectByteBuf.copy(PooledUnsafeDirectByteBuf.java:309)
    io.netty.buffer.AbstractByteBuf.copy(AbstractByteBuf.java:1190)
    io.netty.buffer.WrappedByteBuf.copy(WrappedByteBuf.java:874)
    io.netty.channel.socket.DatagramPacket.copy(DatagramPacket.java:47)
    com.tutorgroup.base.logserver.server.UDPInboundHandler.channelRead0(UDPInboundHandler.java:24)
    com.tutorgroup.base.logserver.server.UDPInboundHandler.channelRead0(UDPInboundHandler.java:13)
    io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1434)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:965)
    io.netty.channel.nio.AbstractNioMessageChannel$NioMessageUnsafe.read(AbstractNioMessageChannel.java:93)
    io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:644)
    io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:579)
    io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:496)
    io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:458)
    io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:897)
    io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    java.lang.Thread.run(Thread.java:748)

ByteBuf 没有调用 release 方法，由于我的代码量比较小，项目中只有一处是用到了 ByteBuf ，所以我很快定位到了问题代码，但是如果项目很大，不知道是哪段代码导致的问题，怎么排查呢？查询相关资料后，我们再修改一下启动参数

java -jar -Xms256M -Xmx256M -XX:MaxDirectMemorySize=2M -Dio.netty.leakDetection.level=advanced -Dio.netty.leakDetection.maxRecords=10 -Dspring.profiles.active=prod log-server.jar

果不其然，代码又报错了，这次报错的信息很详细，已经定位到是哪个ByteBuf 变量了

2019-09-27 10:54:24.442 {nioEventLoopGroup-2-1} ERROR io.netty.util.ResourceLeakDetector - LEAK: ByteBuf.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information.
Recent access records:
#1:
    io.netty.buffer.AdvancedLeakAwareByteBuf.readBytes(AdvancedLeakAwareByteBuf.java:496)
    com.tutorgroup.base.logserver.service.LogService.getLogJSONArray(LogService.java:108)
    com.tutorgroup.base.logserver.service.LogService.process(LogService.java:51)
    com.tutorgroup.base.logserver.server.UDPInboundHandler.channelRead0(UDPInboundHandler.java:25)
    com.tutorgroup.base.logserver.server.UDPInboundHandler.channelRead0(UDPInboundHandler.java:13)
    io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1434)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:965)
    io.netty.channel.nio.AbstractNioMessageChannel$NioMessageUnsafe.read(AbstractNioMessageChannel.java:93)
    io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:644)
    io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:579)
    io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:496)
    io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:458)
    io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:897)
    io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    java.lang.Thread.run(Thread.java:748)
Created at:
    io.netty.buffer.PooledByteBufAllocator.newDirectBuffer(PooledByteBufAllocator.java:331)
    io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:185)
    io.netty.buffer.UnsafeByteBufUtil.copy(UnsafeByteBufUtil.java:436)
    io.netty.buffer.UnpooledUnsafeDirectByteBuf.copy(UnpooledUnsafeDirectByteBuf.java:463)
    io.netty.buffer.AbstractByteBuf.copy(AbstractByteBuf.java:1190)
    io.netty.channel.socket.DatagramPacket.copy(DatagramPacket.java:47)
    com.tutorgroup.base.logserver.server.UDPInboundHandler.channelRead0(UDPInboundHandler.java:24)
    com.tutorgroup.base.logserver.server.UDPInboundHandler.channelRead0(UDPInboundHandler.java:13)
    io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
    io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1434)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
    io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
    io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:965)
    io.netty.channel.nio.AbstractNioMessageChannel$NioMessageUnsafe.read(AbstractNioMessageChannel.java:93)
    io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:644)
    io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:579)
    io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:496)
    io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:458)
    io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:897)
    io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
    java.lang.Thread.run(Thread.java:748)

查阅相关资料，释放ByteBuf 方式，修改代码如下

@Component
public class UDPInboundHandler extends SimpleChannelInboundHandler<DatagramPacket> {

    private Logger logger = LoggerFactory.getLogger(UDPInboundHandler.class);

    @Autowired
    LogService logService;

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, DatagramPacket packet) {
        String remoteAddr = packet.sender().getAddress().getHostAddress();
        ByteBuf buf = packet.copy().content();
        try{
            logService.process(buf, remoteAddr);
            buf.clear();
        }catch (Exception e){
            logger.error(e.getMessage(),e);
        }
        finally {
            ReferenceCountUtil.release(buf);
        }
    }
}

ReferenceCountUtil.release()  是netty释放堆外内存的方法，加上这行代码后，问题完美解决。

参考资料：

http://static.muyus.com/html/3.html

https://www.jianshu.com/p/17e72bb01bf1