ruby-core

Issue #16291 has been reported by methodmissing (Lourens Naud薊.

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Misc #16291: Introduce support for resize in rb_ary_freeze and prefer internal use of rb_ary_freeze and rb_str_freeze for String and Array types
https://bugs.ruby-lang.org/issues/16291

* Author: methodmissing (Lourens Naud薊
* Status: Open
* Priority: Normal
* Assignee: 
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References Github PR https://github.com/ruby/ruby/pull/2640

### Why?

While working on https://github.com/ruby/ruby/pull/2037#issuecomment-548633141 I also looked at the `rb_ary_freeze` helper to determine if the same optimization of shrinking capacity can be applied there as well.

Further investigation revealed that `rb_str_freeze` attempts to resize / shrink strings on freeze, but we currently don't do the same for arrays despite API for it being exposed.

The gist of the change:

* Let `rb_ary_freeze` also attempt to right size an array before freezing it
* Replaced internal use of `rb_obj_freeze` and `OBJ_FREEZE` of callsites that pass a String or Array type in with `rb_ary_freeze` and `rb_str_freeze` specifically.

### Results

Saves about 3MB of Array and String specific heap footprints on a redmine production env boot on Rails 6 (custom local upgrade - it does not officially support it yet):

```
Loading production environment (Rails 6.1.0.alpha)
irb(main):001:0> RUBY_DESCRIPTION
=> "ruby 2.7.0dev (2019-11-02T06:32:49Z master 772b0613c5) [x86_64-linux]"
irb(main):002:0> require 'objspace'
=> true
irb(main):003:0> GC.start
=> nil
irb(main):004:0> ObjectSpace.each_object(Array).sum {|o| ObjectSpace.memsize_of(o) }
=> 4451200
irb(main):005:0> ObjectSpace.each_object(String).sum {|o| ObjectSpace.memsize_of(o) }
=> 8608472
irb(main):006:0> exit
```

```
Loading production environment (Rails 6.1.0.alpha)
irb(main):001:0> RUBY_DESCRIPTION
=> "ruby 2.7.0dev (2019-11-02T16:52:34Z obj-freeze-specifi.. 3bc4048899) [x86_64-linux]"
irb(main):002:0> require 'objspace'
=> true
irb(main):003:0> GC.start
=> nil
irb(main):004:0> ObjectSpace.each_object(Array).sum {|o| ObjectSpace.memsize_of(o) }
=> 3233432
irb(main):005:0> ObjectSpace.each_object(String).sum {|o| ObjectSpace.memsize_of(o) }
=> 6748422
```

irb - the best calculator ...

```
irb(main):002:0> (4451200 - 3233432) / 1024
=> 1189
irb(main):003:0> (8608472 - 6748422) / 1024
=> 1816
```

### Open questions

* Would it make sense to let `rb_obj_freeze` switch on object type instead and apply this optimization without "polluting" internals with custom callsite changes? Native extensions can then benefit from the resize feature too.
* HOWEVER: Discovered in testing that `rb_str_freeze` can fail asserts in `rb_str_tmp_frozen_release` when sprinkled too generously especially in `io.c`. And other issues may lurk too, thus incorporating the resizing in `rb_obj_freeze` won't work.  See https://github.com/ruby/ruby/pull/2640#issuecomment-549119359

I think it could also make sense to split the PR in 2:

* One changeset for `rb_ary_freeze` attempting to resize Arrays on freeze, special case internal `rb_obj_freeze` of Arrays to prefer the new API
* Apply `rb_str_resize` at internal call sites where `rb_obj_freeze` is used with String types

Thoughts?



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