Low-Overhead Heap Profiling
kirk.pepperdine at gmail.com
Tue Jun 23 06:08:46 UTC 2015
The difference would be in the licensing.
On Jun 23, 2015, at 7:31 AM, Vladimir Voskresensky <vladimir.voskresensky at oracle.com> wrote:
> Hello Jeremy,
> If this is sampling, not tracing, then how is it different from the low-overhead memory profiling provided by JFR .
> JFR samples per new TLAB allocation. It provides really very good picture and I haven't seen overhead more than 2%.
> Btw, JFR also does not have false positives reported by instrumented approaches for the cases when JIT was able to eliminate heap allocation.
>  http://hirt.se/blog/?p=381
> On 22.06.2015 11:48, Jeremy Manson wrote:
>> 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?
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