safepoint与UseCountedLoopSafepoints

safepoint:

JIT编码时，会在代码中所有方法的返回之前，以及所有非counted loop的循环（无界循环）回跳之前放置一个safepoint(counted loop则没有放置safepoint）。GC 等操作时，需要所有线程达到safepoint。

UseCountedLoopSafepoints:

可以避免GC发生时，线程因长时间运行counted loop，进入不到safepoint，而引起GC的STW时间过长。

UseCountedLoopSafepoints解决此bug
https://bugs.openjdk.java.net/browse/JDK-8195980

但使用UseCountedLoopSafepoints，java 1.8.131之前会有这个bug
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8161147

参考：https://www.jianshu.com/p/c79c5e02ebe6
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8161147