Hey folks, (cc'ing Aleksey and John, because I mentioned this to them at the JVMLS last year, but I never followed up.)
We have a patch at Google I've always wanted to contribute to OpenJDK, but I always figured it would be unwanted. I've recently been thinking that might not be as true, though. I thought I would ask if there is any interest / if I should write a JEP / if I should just forget it. The basic problem is that there is no low-overhead supported way to figure out where allocation hotspots are. That is, sets of stack traces where lots of allocation / large allocations took place. What I had originally done (this was in ~2007) was use bytecode rewriting to instrument allocation sites. The instrumentation would call a Java method, and the method would capture a stack trace. To reduce overhead, there was a per-thread counter that only took a stack trace once every N bytes allocated, where N is a randomly chosen point in a probability distribution that centered around ~512K. This was *way* too slow, and it didn't pick up allocations through JNI, so I instrumented allocations at the VM level, and the overhead went away. The sampling is always turned on in our internal VMs, and a user can just query an interface for a list of sampled stack traces. The allocated stack traces are held with weak refs, so you only get live samples. The additional overhead for allocations amounts to a subtraction, and an occasional stack trace, which is usually a very, very small amount of our CPU (although I had to do some jiggering in JDK8 to fix some performance regressions). There really isn't another good way to do this with low overhead. I was wondering how the gruop would feel about our contributing it? Thoughts? Jeremy
