[ruby-core:111855] [Ruby master Bug#19334] Defining many instance variables and accessing them is slow in Ruby 3.2.0

Issue #19334 has been updated by Eregon (Benoit Daloze).


byroot (Jean Boussier) wrote in #note-5:
> IIRC Aaron mentioned that V8 use a hash table after a certain number of edges (20?).

That's the complexity I'm talking about above which IMO is not worth it (that JS completely changes its object representation based on a fragile heuristic).
Ruby is a proper language which doesn't confuse objects ivars and Hash pairs, so there shouldn't be a need to fallback to using a hashtable representation for ivars.

An extra map/Hash per Shape to know which ivars it contains could work, but it would pretty bad for memory footprint with 1 hashtable per Shape.
So it needs something more memory efficient than a hashtable per Shape.
Truffle uses an Hash array mapped trie for that: https://github.com/oracle/graal/blob/master/truffle/src/com.oracle.truffle.object/src/com/oracle/truffle/object/TriePropertyMap.java
Before that it used https://github.com/oracle/graal/blob/master/truffle/src/com.oracle.truffle.object/src/com/oracle/truffle/object/ConsListPropertyMap.java, which is probably similar to what CRuby does currently.
I'm not sure the HAMT is needed, but that's probably the common solution to this issue.

----------------------------------------
Bug #19334: Defining many instance variables and accessing them is slow in Ruby 3.2.0
https://bugs.ruby-lang.org/issues/19334#change-101267

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
* Assignee: tenderlovemaking (Aaron Patterson)
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
```
class C
  eval("def initialize; #{ (0..100000).map { "@x#{ _1 } = 0; " }.join } end")
  attr_reader :x50000
end
p :start
C.new.x50000
```

This script takes less than one second in Ruby 3.1.3, and does more than ten second in Ruby 3.2.0.

```
$ time ruby -v test.rb
ruby 3.1.3p185 (2022-11-24 revision 1a6b16756e) [x86_64-linux]
:start

real    0m0.210s
user    0m0.167s
sys     0m0.044s
```

```
$ time ruby -v test.rb
ruby 3.2.0 (2022-12-25 revision a528908271) [x86_64-linux]
:start

real    0m11.026s
user    0m10.950s
sys     0m0.040s
```

This problem is not critical, but is there any room for improvement?



-- 
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