[ruby-core:106646] [Ruby master Feature#18364] Add GC.stat_pool for Variable Width Allocation

Issue #18364 has been updated by Eregon (Benoit Daloze).


peterzhu2118 (Peter Zhu) wrote in #note-10:
> Thank you for the summary @mame! We'll extend `GC.stat` to return a nested data structure.

This seems incompatible and might break existing code which expects `GC.stat` to return a `Hash[Symbol,Integer]`.
Also what should `size_t rb_gc_stat(VALUE)` return/raise for such a case?
Also it's likely to make `GC.stat()`/`GC.stat(hash)` slower and use more allocations, which seems suboptimal.
Notably, `GC.stat(hash)` no longer avoids allocations and becomes useless.

I don't understand @matz's reasoning here, the OP and myself agreed `GC.stat_pool` is best and it covers both the VWA use-case and JRuby/TruffleRuby.
The nested approach seems to have multiple issues, and a key like `size_heap` seems too specific (those details might change over time) and makes no sense for other Ruby implementations.
Maybe it was missed in #4 that it's the result of `GC.heap_stats` on TruffleRuby, *not* of `GC.stat`?

"memory pool" is a well established term in this area.
Another method name is `stat_heap` if that sounds clearer.

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Feature #18364: Add GC.stat_pool for Variable Width Allocation
https://bugs.ruby-lang.org/issues/18364#change-95317

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
# GitHub PR: https://github.com/ruby/ruby/pull/5177

We're proposing an API to get statistics for size pools for Variable Width Allocation similar to `GC.stat`. This will make it easier for us (and other developers) to tune VWA.

Before 3.1 release, we plan to keep this method hidden from the documentation using `:nodoc:` since it is not useful when not using VWA.

For example:

```ruby
# Get stats for size pool 2
puts GC.stat_pool(2)
#=> {:slot_size=>160, :heap_allocatable_pages=>80, :heap_eden_pages=>14, :heap_eden_slots=>1424, :heap_tomb_pages=>0, :heap_tomb_slots=>0}
puts GC.stat_pool(2, :heap_eden_pages)
#=> 14
```

We aim to keep the keys in the outputted hash the same as the keys used in `GC.stat`.

We chose to implement a new method instead of re-using an existing API (`GC.stat`) because the keys returned by `GC.stat_pool` will not be the same as `GC.stat`. We believe that having `GC.stat` return different shapes of hashes based on its arguments is confusing.




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