From: mk via ruby-core <ruby-core@...>
Date: 2024-04-03T14:24:53+00:00
Subject: [ruby-core:117432] [Ruby master Feature#20408] Add heap_live_slots to GC.stat_heap

Issue #20408 has been reported by mk (Matthias K�ppler).

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Feature #20408: Add heap_live_slots to GC.stat_heap
https://bugs.ruby-lang.org/issues/20408

* Author: mk (Matthias K�ppler)
* Status: Open
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`GC.stat` exposes a `heap_live_slots` metric that returns the number of live objects occupying eden slots.

Because the slot size was fixed to `RVALUE_SIZE`, this allowed for some basic approximations of how many bytes were live/in-use (`heap_live_slots * RVALUE_SIZE`).
This could furthermore be "embellished" by adding malloc'ed memory for those objects that were larger than a 40B slot (we track this via a custom CRuby patch).

However, with the addition of Variable Width Allocations (https://bugs.ruby-lang.org/issues/18239), this does not work anymore because we don't know how `heap_live_slots` distributes over all pools that use different slot sizes.

This could be addressed by also adding `heap_live_slots` (and for symmetry: `heap_free_slots`) to `GC.stat_heap`.
The sum of all live slots per pool times its respective slot size should equal `GC.stat[:heap_live_slots]`.



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