What does this PR do?

Skip updating and serialization of heap samples for objects that were allocated in the current GC epoch/generation. The fact that these objects are still alive is usually just a happy accident from the fact that GC hasn't run yet and thus just add noise when you want to understand long-term heap usage and, more specifically, memory leaks.

In fact, allocation profiles alone can give you a rather accurate description of what the population of age == 0 should look like at any-one time. If no GCs ever ran, allocation profiles would be === to heap profiles as all objects would be technically alive.

Motivation:

• Objects with age == 0 tend to be mostly noise when investigating heap usage.
• Unless GCs occur at intervals <1 minute or you have big memory leaks, objects with age == 0 will naturally be much more common than objects with age > 0 and so end up being major contributors to sampling and serialization overhead. Skipping them can result in significant savings.

Additional Notes:

Some micro-benchmark results:

Comparison:
sample+serialize min_age_1 retain_every=10 heap_samples=true heap_size=true heap_sample_every=10 skip_end_gc=true:        5.7 i/s
sample+serialize min_age_0 retain_every=10 heap_samples=true heap_size=true heap_sample_every=10 skip_end_gc=true:        5.5 i/s - 1.02x  slower
sample+serialize min_age_1 retain_every=10 heap_samples=true heap_size=true heap_sample_every=1 skip_end_gc=true:        4.3 i/s - 1.33x  slower
sample+serialize min_age_0 retain_every=10 heap_samples=true heap_size=true heap_sample_every=1 skip_end_gc=true:        3.1 i/s - 1.85x  slower

How to test the change?

Unsure? Have a question? Request a review!