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`dashboard`{{execute T2}} 命令可以查看当前系统的实时数据面板。
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当运行在Ali-tomcat时，会显示当前tomcat的实时信息，如HTTP请求的qps, rt, 错误数, 线程池信息等等。
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```
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$ dashboard
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ID NAME GROUP PRIORITY STATE %CPU DELTA_TIME TIME INTERRUPTE DAEMON
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-1 C2 CompilerThread0 - -1 - 1.55 0.077 0:8.684 false true
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53 Timer-for-arthas-dashboard-07b system 5 RUNNABLE 0.08 0.004 0:0.004 false true
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22 scheduling-1 main 5 TIMED_WAI 0.06 0.003 0:0.287 false false
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-1 C1 CompilerThread0 - -1 - 0.06 0.003 0:2.171 false true
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-1 VM Periodic Task Thread - -1 - 0.03 0.001 0:0.092 false true
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49 arthas-NettyHttpTelnetBootstra system 5 RUNNABLE 0.02 0.001 0:0.156 false true
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16 Catalina-utility-1 main 1 TIMED_WAI 0.0 0.000 0:0.029 false false
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-1 G1 Young RemSet Sampling - -1 - 0.0 0.000 0:0.019 false true
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17 Catalina-utility-2 main 1 WAITING 0.0 0.000 0:0.025 false false
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34 http-nio-8080-ClientPoller main 5 RUNNABLE 0.0 0.000 0:0.016 false true
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23 http-nio-8080-BlockPoller main 5 RUNNABLE 0.0 0.000 0:0.011 false true
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-1 VM Thread - -1 - 0.0 0.000 0:0.032 false true
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-1 Service Thread - -1 - 0.0 0.000 0:0.006 false true
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-1 GC Thread#5 - -1 - 0.0 0.000 0:0.043 false true
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Memory used total max usage GC
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heap 36M 70M 4096M 0.90% gc.g1_young_generation.count 12
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g1_eden_space 6M 18M -1 33.33% 86
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g1_old_gen 30M 50M 4096M 0.74% gc.g1_old_generation.count 0
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g1_survivor_space 491K 2048K -1 24.01% gc.g1_old_generation.time(ms) 0
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nonheap 66M 69M -1 96.56%
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codeheap_'non-nmethods' 1M 2M 5M 22.39%
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metaspace 46M 47M -1 98.01%
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Runtime
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os.name Mac OS X
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os.version 10.15.4
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java.version 15
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java.home /Library/Java/JavaVirtualMachines/jdk-15.jdk/Contents/Home
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systemload.average 10.68
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processors 8
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uptime 272s
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```
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输入 `Q`{{execute T2}} 或者 `Ctrl+C` 可以退出dashboard命令。
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### 数据说明
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* ID: Java级别的线程ID，注意这个ID不能跟jstack中的nativeID一一对应。
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* NAME: 线程名
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* GROUP: 线程组名
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* PRIORITY: 线程优先级, 1~10之间的数字，越大表示优先级越高
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* STATE: 线程的状态
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* CPU%: 线程的cpu使用率。比如采样间隔1000ms，某个线程的增量cpu时间为100ms，则cpu使用率=100/1000=10%
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* DELTA_TIME: 上次采样之后线程运行增量CPU时间，数据格式为`秒`
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* TIME: 线程运行总CPU时间，数据格式为`分:秒`
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* INTERRUPTED: 线程当前的中断位状态
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* DAEMON: 是否是daemon线程
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#### JVM内部线程
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Java 8之后支持获取JVM内部线程CPU时间，这些线程只有名称和CPU时间，没有ID及状态等信息（显示ID为-1）。
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通过内部线程可以观测到JVM活动，如GC、JIT编译等占用CPU情况，方便了解JVM整体运行状况。
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* 当JVM 堆(heap)/元数据(metaspace)空间不足或OOM时，可以看到GC线程的CPU占用率明显高于其他的线程。
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* 当执行`trace/watch/tt/redefine`等命令后，可以看到JIT线程活动变得更频繁。因为JVM热更新class字节码时清除了此class相关的JIT编译结果，需要重新编译。
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JVM内部线程包括下面几种：
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* JIT编译线程: 如 `C1 CompilerThread0`, `C2 CompilerThread0`
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* GC线程: 如`GC Thread0`, `G1 Young RemSet Sampling`
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* 其它内部线程: 如`VM Periodic Task Thread`, `VM Thread`, `Service Thread`
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