RFR(S) Trace significant heap allocations

[Milan Mimica]

Hello

One of my concerns recently was tracking down large memory allocations 
which cause G1 GC to do a Full GC. So I came out with this patch, which 
adds two diagnostic options:
1) TraceLargerAllocations - logs every heap allocation larger that the 
specified size
2) G1TraceFullGCAllocations - logs allocations which trigger a STW Full GC
I see that these two options could be submitted as separate patches, but 
before we discuss the details I'd like to be sure that I'm doing the 
right thing.

I understand that there are already at least two ways to trace 
allocations - EventAllocObjectOutsideTLAB and bytecode instrumentation. 
They both lack some crucial features:
The problem with EventAllocObjectOutsideTLAB is that you cannot set size 
threshold, you don't get the java stacktrace, and you get loads of other 
uninteresting things with -XX:+EnableTracing. I don't find it useful. 
The event system could be extended to support those, but that's a bigger 
undertake. And it's coded in XML(!!?).

Bytecode instrumentation, on the other hand, is intrusive, ads overhead, 
and it's too complicated. Just think of it, it was easier for me to 
implement these two VM options than to even start fiddling with 
bytecodes. (Partly because you made the jvm too easy to build, good 
job!) Having a profiler constantly connected to an instance is also not 
feasible.

I think these two options are generally useful for tracing GC problems, 
especially G1TraceFullGCAllocations. With full java stacktrace the 
developer has all the information needed to locate the problem which is 
otherwise very hard to locate.

So could someone review the patch and provide feedback? First time 
posting here, so I need a sponsor obviously.

Milan Mimica, Software Engineer / Team Leader
 
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