From: "Eregon (Benoit Daloze)" <noreply@...>
Date: 2021-11-27T18:56:47+00:00
Subject: [ruby-core:106297] [Ruby master Feature#18364] Add GC.stat_size_pool for Variable Width Allocation

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


There is something maybe related in TruffleRuby and JRuby, where we have per-heap information.
Maybe it would make sense to have a unified interface for per-heap information?

TruffleRuby:
```
$ ruby -e 'pp GC.heap_stats'
{:time=>29,
 :count=>4,
 :minor_gc_count=>4,
 :major_gc_count=>0,
 :unknown_count=>0,
 :used=>26931712,
 :heap_live_slots=>26931712,
 :committed=>530579456,
 :heap_available_slots=>530579456,
 :heap_free_slots=>503647744,
 :init=>526385152,
 :max=>8420065280,
 "G1 Eden Space"=>
  {:used=>0,
   :committed=>75497472,
   :init=>27262976,
   :max=>-1,
   :peak_used=>46137344,
   :peak_committed=>327155712,
   :peak_init=>27262976,
   :peak_max=>-1,
   :last_used=>0,
   :last_committed=>75497472,
   :last_init=>27262976,
   :last_max=>-1},
 :peak_used=>73069056,
 :peak_committed=>834666496,
 :peak_init=>526385152,
 :peak_max=>8420065278,
 :last_used=>8388608,
 :last_committed=>83886080,
 :last_init=>526385152,
 :last_max=>8420065278,
 "G1 Survivor Space"=>
  {:used=>8388608,
   :committed=>8388608,
   :init=>0,
   :max=>-1,
   :peak_used=>8388608,
   :peak_committed=>8388608,
   :peak_init=>0,
   :peak_max=>-1,
   :last_used=>8388608,
   :last_committed=>8388608,
   :last_init=>0,
   :last_max=>-1},
 "G1 Old Gen"=>
  {:used=>18543104,
   :committed=>446693376,
   :init=>499122176,
   :max=>8420065280,
   :peak_used=>18543104,
   :peak_committed=>499122176,
   :peak_init=>499122176,
   :peak_max=>8420065280,
   :last_used=>0,
   :last_committed=>0,
   :last_init=>499122176,
   :last_max=>8420065280}}
```

JRuby:
```
ruby -e 'pp GC.stat'                   
{:count=>2,
 :time=>17,
 :committed=>1061158912.0,
 :init=>1052770304.0,
 :max=>16840130556.0,
 :used=>44554240.0,
 :peak_committed=>1660944384.0,
 :peak_init=>1052770304.0,
 :peak_max=>16840130556.0,
 :peak_used=>65525760.0,
 :last_committed=>79691776.0,
 :last_init=>1052770304.0,
 :last_max=>16840130556.0,
 :last_used=>8388608.0,
 "G1 Young Generation"=>
  {:count=>2,
   :time=>17,
   :pools=>
    {"G1 Old Gen"=>
      {:committed=>490733568,
       :init=>499122176,
       :max=>8420065280,
       :used=>7597056,
       :peak_committed=>499122176,
       :peak_init=>499122176,
       :peak_max=>8420065280,
       :peak_used=>7597056,
       :last_committed=>0,
       :last_init=>499122176,
       :last_max=>8420065280,
       :last_used=>0}}},
 "G1 Old Generation"=>
  {:count=>0,
   :time=>0,
   :pools=>
    {"G1 Old Gen"=>
      {:committed=>490733568,
       :init=>499122176,
       :max=>8420065280,
       :used=>7597056,
       :peak_committed=>499122176,
       :peak_init=>499122176,
       :peak_max=>8420065280,
       :peak_used=>7597056,
       :last_committed=>0,
       :last_init=>499122176,
       :last_max=>8420065280,
       :last_used=>0}}}}
```

----------------------------------------
Feature #18364: Add GC.stat_size_pool for Variable Width Allocation
https://bugs.ruby-lang.org/issues/18364#change-94923

* 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_size_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_size_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_size_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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