RFR: Track promotion rate into old generation

[Kelvin Nilsen]

On Thu, 16 Sep 2021 20:38:58 GMT, William Kemper <wkemper at openjdk.org> wrote:

> The member `_bytes_allocated_since_last_gc` has been moved into the `ShenandoahGeneration` abstraction and is updated accordingly. Evacuations into the old generation (i.e., promotion) are counted as allocations against the old generation. Promotion of entire regions is not recorded in the promotion rate.

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1038:

> 1036:     } else {
> 1037:       if (req.is_old()) {
> 1038:         old_generation()->increase_allocated(actual_bytes);

I'm not sure I follow all the logic.  When we allocate a PLAB for promotions into old-gen, we want to increment the promotion rate.  But when we allocate a ?LAB to hold the results of an old-gen allocation, that should not be counted as "promotion" memory.  Are we distinguishing these two cases?

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PR: https://git.openjdk.java.net/shenandoah/pull/70