假设我想要帮助垃圾收集器。 在GC中花费更少时间的方法之一是更快地达到安全点。
在启动GC之前，必须将所有线程都停放在安全点。
Safepoint是执行流程中的“特殊位置”，可以安全地停止线程以执行GC。
我希望尽量减少在代码中等待安全点的时间。 什么编码风格将帮助我更快地达到安全点？
什么样的风格让这项任务更加不舒服？
在 style 下，我指的是语言中的一切，有助于（或阻止）快速“获取”安全点。
示例：
磁盘IO工作是否会使问题更严重？
JNI是否会使问题更严重？
Thread.sleep（）会让它更难吗？
同步（..）让它更难吗？
等。
简而言之，哪些java操作和方法会快速“获取”安全点问题？有什么帮助？
答案 0 :(得分：0)
让我们以此注释开头（来自OpenJDK源代码中的“ src / hotspot / share / runtime / safepoint.cpp”文件）：
<form action="{{ url('create') }}" method="post" onsubmit="return validation()">
由此我得出以下结论：
但这只是回答问题的一部分。问题在于，JIT编译代码中的安全点轮询是由JIT编译器添加的指令完成的。没有指定控制该逻辑的逻辑 1 ，并且易于实现。此外，我听说使用 // Java threads can be in several different states and are
// stopped by different mechanisms:
//
// 1. Running interpreted
// The interpreter dispatch table is changed to force it to
// check for a safepoint condition between bytecodes.
// 2. Running in native code
// When returning from the native code, a Java thread must check
// the safepoint _state to see if we must block. If the
// VM thread sees a Java thread in native, it does
// not wait for this thread to block. The order of the memory
// writes and reads of both the safepoint state and the Java
// threads state is critical. In order to guarantee that the
// memory writes are serialized with respect to each other,
// the VM thread issues a memory barrier instruction
// (on MP systems). In order to avoid the overhead of issuing
// a memory barrier for each Java thread making native calls, each Java
// thread performs a write to a single memory page after changing
// the thread state. The VM thread performs a sequence of
// mprotect OS calls which forces all previous writes from all
// Java threads to be serialized. This is done in the
// os::serialize_thread_states() call. This has proven to be
// much more efficient than executing a membar instruction
// on every call to native code.
// 3. Running compiled Code
// Compiled code reads a global (Safepoint Polling) page that
// is set to fault if we are trying to get to a safepoint.
// 4. Blocked
// A thread which is blocked will not be allowed to return from the
// block condition until the safepoint operation is complete.
// 5. In VM or Transitioning between states
// If a Java thread is currently running in the VM or transitioning
// between states, the safepointing code will wait for the thread to
// block itself when it attempts transitions to a new state.
//
和其他内置/内在操作 进行内存复制可能会花费很长时间而没有安全点检查。
关于您的具体示例：
磁盘IO的工作是否会带来更多问题？
否。
JNI是否使它更具问题性？
否。
Thread.sleep（）使它变得更难吗？
否。
是否已同步（..）使其更难？
否。
关于您的一般问题：
什么风格使任务[到达安全点]更加不舒服？
可能应该避免使用递归来避免显式循环的编码样式。
1-我隐约记得它是在向后分支上发生的；例如在任何Java循环中。