Issue #16291 has been updated by sam.saffron (Sam Saffron).


related, see this for for the string api =


https://bugs.ruby-lang.org/issues/16029#change-82018

----------------------------------------
Feature #16291: Introduce support for resize in rb_ary_freeze and prefer in=
ternal use of rb_ary_freeze and rb_str_freeze for String and Array types
https://bugs.ruby-lang.org/issues/16291#change-83574

* Author: methodmissing (Lourens Naud=E9)
* Status: Open
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* Target version: 2.8
----------------------------------------
References Github PR https://github.com/ruby/ruby/pull/2640

### Why?

While working on https://github.com/ruby/ruby/pull/2037#issuecomment-548633=
141 I also looked at the `rb_ary_freeze` helper to determine if the same op=
timization of shrinking capacity can be applied there as well.

Further investigation revealed that `rb_str_freeze` attempts to resize / sh=
rink strings on freeze, but we currently don't do the same for arrays despi=
te 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 th=
at 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 p=
roduction env boot on Rails 6 (custom local upgrade - it does not officiall=
y support it yet):

```
Loading production environment (Rails 6.1.0.alpha)
irb(main):001:0> RUBY_DESCRIPTION
=3D> "ruby 2.7.0dev (2019-11-02T06:32:49Z master 772b0613c5) [x86_64-linux]"
irb(main):002:0> require 'objspace'
=3D> true
irb(main):003:0> GC.start
=3D> nil
irb(main):004:0> ObjectSpace.each_object(Array).sum {|o| ObjectSpace.memsiz=
e_of(o) }
=3D> 4451200
irb(main):005:0> ObjectSpace.each_object(String).sum {|o| ObjectSpace.memsi=
ze_of(o) }
=3D> 8608472
irb(main):006:0> exit
```

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

irb - the best calculator ...

```
irb(main):002:0> (4451200 - 3233432) / 1024
=3D> 1189
irb(main):003:0> (8608472 - 6748422) / 1024
=3D> 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 calls=
ite 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_o=
bj_freeze` won't work.  See https://github.com/ruby/ruby/pull/2640#issuecom=
ment-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 use=
d with String types

Thoughts?



-- =

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