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JVM tuning

• JVM Options refer to JVM Options

-XX:+AlwaysPreTouch

Pre-touch the Java heap during JVM initialization. Every page of the heap is thus demand-zeroed during initialization rather than incrementally during application execution.

在JVM初始化期间预先触摸Java堆。 因此，堆的每个页面都在初始化期间按需清零，而不是在应用程序执行期间递增。

• NMT (Native Memory Tracking)

本地内存跟踪， 参见 https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/tooldescr007.html

The Native Memory Tracking (NMT) is a Java HotSpot VM feature that tracks internal memory usage for a Java HotSpot VM

Follow these steps to detect a memory leak.

1. Start the JVM with summary or detail tracking using the command line option: -XX:NativeMemoryTracking=summary or -XX:NativeMemoryTracking=detail .

2. Establish an early baseline - use NMT baseline feature to get a baseline to compare during development and maintenance by running: jcmd <pid> VM.native_memory baseline.

3. Monitor memory changes using: jcmd <pid> VM.native_memory detail.diff .

4. If the application leaks a small amount of memory, it make take a while to show up.

使用 java -XX:+PrintFlagsFinal -version 来打印 JVM options

$java -XX:+PrintFlagsFinal -version | wc -l
557

$java -XX:+PrintFlagsFinal -XX:+UnlockDiagnosticVMOptions -version | wc -l
728

$java -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version | wc -l
809

Netty tuning

refer to https://dzone.com/articles/thousands-of-socket-connections-in-java-practical

 -Dio.netty.allocator.maxOrder=4
 -Djdk.nio.maxCachedBufferSize=131072
 -Dio.netty.tryReflectionSetAccessible=true
 -Dio.netty.eventLoopThreads=16

btw. 配置尽量用环境变量，不要用配置文件或者配置表，这样比较方便的修改配置，也比较灵活

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