问题
In Java Concurrency in Practice, the authors write:
When locking is contended, the losing thread(s) must block. The JVM can implement blocking either via spin-waiting (repeatedly trying to acquire the lock until it succeeds) or by suspending the blocked thread through the operating system. Which is more efficient depends on the relationship between context switch overhead and the time until the lock becomes available; spin-waiting is preferred for short waits and suspension is preferable for long waits. Some JVMs choose between the two adaptively based on profiling data of past wait times, but most just suspend threads waiting for a lock.
When I read this I was quite surprised. Are there any known JVMs implementing blocking either on always spin-waiting or sometimes spin-waiting due to profiling results? It's hard to believe for now.
回答1:
Here is evidence that JRockit can use spinlocks - http://forums.oracle.com/forums/thread.jspa?threadID=816625&tstart=494
And if you search for "spin" in the JVM options listed here you will see evidence for the use of / support for spinlocks in Hotspot JVMs.
回答2:
What the authors have written is right and it only makes sense. This is true for Linux as well. The rationale of why spin locks are used is because most resources are protected for a fraction of a millisecond. As such, to suspend, push all the contents of the registers onto the stack and relinquish CPU is just way too much overhead and not worth it. Thus even though it just spins in a tight set of instructions, sometimes just wasting time, it is still way more efficient than swapping out.
That being said, with the VM profiling, it would ideally make your processing more efficient. As such, is there are particular case that you always want to suspend? Or maybe always spin-wait?
来源：https://stackoverflow.com/questions/6325824/does-any-jvm-implement-blocking-with-spin-waiting