莴苣 5.2.1.RELEASE OutOfDirectMemoryError，直接内存。为什么？

问：

• 业务背景
  • 异步多线程调用，
• 问题：为什么会发生 OOM？帮助，你能解释一下吗？
• 日志异常

14:35:38.039 [lettuce-epollEventLoop-10-6] WARN  io.lettuce.core.protocol.CommandHandler - null Unexpected exception during request: io.netty.util.internal.OutOfDirectMemoryError: failed to allocate 16777216 byte(s) of direct memory (used: 536870912, max: 536870912)
io.netty.util.internal.OutOfDirectMemoryError: failed to allocate 16777216 byte(s) of direct memory (used: 536870912, max: 536870912)
    at io.netty.util.internal.PlatformDependent.incrementMemoryCounter(PlatformDependent.java:802) ~[netty-common-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.util.internal.PlatformDependent.allocateDirectNoCleaner(PlatformDependent.java:731) ~[netty-common-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.PoolArena$DirectArena.allocateDirect(PoolArena.java:648) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.PoolArena$DirectArena.newChunk(PoolArena.java:623) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.PoolArena.allocateNormal(PoolArena.java:202) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.PoolArena.tcacheAllocateNormal(PoolArena.java:186) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.PoolArena.allocate(PoolArena.java:136) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.PoolArena.reallocate(PoolArena.java:286) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.PooledByteBuf.capacity(PooledByteBuf.java:118) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.AbstractByteBuf.ensureWritable0(AbstractByteBuf.java:305) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.AbstractByteBuf.ensureWritable(AbstractByteBuf.java:280) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1103) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1096) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1087) ~[netty-buffer-4.1.70.Final.jar!/:4.1.70.Final]
    at io.lettuce.core.protocol.CommandHandler.channelRead(CommandHandler.java:563) ~[lettuce-core-5.2.1.RELEASE.jar!/:5.2.1.RELEASE]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) ~[netty-transport-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) ~[netty-transport-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) ~[netty-transport-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:795) ~[netty-transport-classes-epoll-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:480) ~[netty-transport-classes-epoll-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:378) ~[netty-transport-classes-epoll-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986) ~[netty-common-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.70.Final.jar!/:4.1.70.Final]
    at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.70.Final.jar!/:4.1.70.Final]
    at java.lang.Thread.run(Thread.java:748) [?:1.8.0_221]