GC日志中的stop-the-world

       通常使用下面的JVM选项在GC日志中打印”stop-the-world”（STW）暂停时间。

-XX:+PrintGCApplicationStoppedTime

       但是在GC日志中看，会发现有许多这样的暂停时间，时间很短，肯定不是垃圾回收引起的。

Total time for which application threads were stopped: 0.0013102 seconds, Stopping threads took: 0.0001792 secondsTotal time for which application threads were stopped: 0.0012521 seconds, Stopping threads took: 0.0001591 secondsTotal time for which application threads were stopped: 0.0008090 seconds, Stopping threads took: 0.0001030 secondsTotal time for which application threads were stopped: 0.0010226 seconds, Stopping threads took: 0.0001058 seconds

       实际上，触发STW暂停的原因除了垃圾回收外，还有一些其他的操作会触发STW，例如一些JIT活动、偏向锁擦除、特定的JVMTI操作，以及许多场景也可能会导致应用程序暂停。
       上述例子中application threads stopped的时间表示应用暂停时间，Stopping threads took 的时间表示等待所有的应用线程都到达安全点花费的时间。只有应用程序线程到达安全点后，JVM才会做些特殊处理，比如垃圾收集、偏向锁擦除等等。