Bug #20338: certain **kwsplat calls have regressed allocations over past few ruby releases

• Status: Closed
• Assignee: -
• Target version: -
• Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[fablestales (Fable Tales)]

Given this script:

class Foo
  def initialize(**kwargs)
  end
end

class Bar
  def initialize(y:)
  end
end

b = GC.stat[:total_allocated_objects]
1000.times {
  Foo.new(y:1)
}
p GC.stat[:total_allocated_objects]-b


b = GC.stat[:total_allocated_objects]
1000.times {
  Bar.new(y:1)
}
p GC.stat[:total_allocated_objects]-b

On Ruby 3.2 it prints

 (fable-3_3_dead_with_patches) $ ruby --version
ruby 3.2.3 (2024-01-18 revision 52bb2ac0a6) [arm64-darwin23]
 (fable-3_3_dead_with_patches) $ ruby hi.rb
3006
2005

On Ruby 3.3 it prints:

 ((HEAD detached at v3_3_0)) $ ./ruby --version
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin23]
 ((HEAD detached at v3_3_0)) $ ./ruby hi.rb
3008
3005

On master:

 (master) $ git rev-parse HEAD
83618f2cfa004accdd1514de7dcbba291aa7e831
 (master) $ ./ruby --version
ruby 3.4.0dev (2024-03-14T12:33:30Z master 83618f2cfa) [arm64-darwin23]
 (master) $ ./ruby hi.rb
3006
4005

so that is: since Ruby 3.2 .newing an object with static keyword args has grown from 2 allocations per object to 4 allocations per object.

I have a pull request which introduces a new instruction to the VM opt_new which basically inlines calling allocate and initialize directly, avoiding a lot of what is causing these extra objects to be allocated. My PR is here: https://github.com/ruby/ruby/pull/10254. With my patch the script prints 2006 and 1005 respectively. This is the smallest number of objects that can be allocated for such calls.

[kjtsanaktsidis (KJ Tsanaktsidis)]

I haven’t looked at this at all myself, but it tingled my “this is familiar” sense. Is this the same problem as https://github.com/ruby/ruby/pull/10151 ?

[kjtsanaktsidis (KJ Tsanaktsidis)]

Ah nope never mind, that PR is about methods called with a kwarg splat, which your example is not doing.

[alanwu (Alan Wu)]

• Subject changed from calling new has regressed allocations over past few ruby releases to certain **kwsplat calls have regressed allocations over past few ruby releases
• Status changed from Open to Closed

After aceee71c35e0b387691836e756b4e008efd84cf1 and 15dc3aaa311b32203d8ffb414bcf9b8e55ce5691, it's now back down to 3.2 levels on the master branch:

$ ./miniruby -v test.rb
ruby 3.4.0dev (2024-03-21T17:20:32Z master 15dc3aaa31) [arm64-darwin23]
3006
2005

The extra allocations were general issues with kwsplat calls which Class#new uses internally.