Low-Overhead Heap Profiling

Vladimir Voskresensky vladimir.voskresensky at oracle.com
Tue Jun 23 05:31:54 UTC 2015

Hello Jeremy,

If this is sampling, not tracing, then how is it different from the 
low-overhead memory profiling provided by JFR [1].
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.

[1] 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?
> Thoughts?
> Jeremy

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