Thread suspension happens in many situations in JVM, such as for GC, for java.lang.Thread.suspend(), etc. There are various techniques to suspend a thread. Basically we can classify them into two categories: preemptive and voluntary.
The preemptive approach requires the suspender, say a GC thread, suspend the execution of a target thread asynchronously with IPC mechanism or OS APIs. If the suspended thread happened to be in a region of code (Java or native) that could be enumerated, the live references were collected. This kind of region is called safe-region, and the suspended point is a safe-point. If the suspended point is not in safe-region, the thread would be resumed and stopped again until it ended up in a safe-region randomly or on purpose. In the other approach that we are now considering, JIT will insert code that polls a boolean. The boolean can be thread-specific and is set true by GC thread or VM if there is a need to prevent the Java thread's forward progress. The JIT will put the polling code in such places as back-edges, call sites and method returns. Actually we are thinking of this mechnism in a more general sense. For example, green-threads can be implemented in this way for Java threads to downcall into JVM scheduler. Does anyone have suggestions/data on better approaches? Thanks, xiaofeng
