Bug #16351: Why do Keyword arguments to initialize allocate a Hash, but not for other methods?

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.7.0dev (2019-11-17T04:12:06Z trunk a8e4a9f03a) [x86_64-darwin19]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN

[brunoe (Bruno Escherl)]

Hello, while working on improving memory allocations in one of my apps, I stumbled upon the following behavior. I measured three different ways of passing variables to a new Object, using plain params, using a hash and using keyword arguments.

class FooWithPlainParams
  def initialize(a, b, c)
    @a = a
    @b = b
    @c = c
  end
end

class FooWithOptionsHash
  def initialize(options = {})
    @a = options[:a]
    @b = options[:b]
    @c = options[:c]
  end
end

class FooWithKeyword
  def initialize(a:, b:, c:)
    @a = a
    @b = b
    @c = c
  end
end

I used memory_profiler gem to measure the allocations with the attached script, calling new 100 times, using ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin19]

FooWithPlainParams not surprisingly just reported "Total allocated: 4000 bytes (100 objects)".
FooWithOptionsHash reported "Total allocated: 27200 bytes (200 objects)" with 100 Hash allocations. This makes sense, since the params are passed on as a Hash.
FooWithKeywordArguments reported "Total allocated: 50400 bytes (300 objects)" with 200 Hash allocations, which is a bit surprising.

After that I checked out ruby-head and there FooWithKeywordArguments.new reports only 100 Hash allocations as FooWithOptionsHash. So that part seems to be fixed.

What surprised me so was, that using the same way of passing parameters in another method, resulted in no allocated Hash according to memory_profiler gem.

class FooWithKeyword
  def foo(d:, e:, f:)
    @d = d; @e = e; @f = f
  end
end

Calling foo(d: 4, e: 5, f: 6) on a FooWithKeyword object, does not show any allocations.

What is the difference here between foo and initialize?

[brunoe (Bruno Escherl)]

• File profile_foo.rb profile_foo.rb added

[brunoe (Bruno Escherl)]

• File deleted (profile_initialize.rb)

[brunoe (Bruno Escherl)]

• File profile_initialize.rb profile_initialize.rb added

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)
• Status changed from Open to Closed

FooWithKeyword#initialize method is not called directly, but via Class#new.
A Hash is needed to delegate the keyword arguments.
By changing the last calling line as the following with making that initialize method public, you could observe the same numbers as FooWithPlainParams.

    FooWithKeyword.allocate.initialize(a: 1, b: 2, c: 3)

[mame (Yusuke Endoh)]

nobu (Nobuyoshi Nakada) wrote:

By changing the last calling line as the following with making that initialize method public, you could observe the same numbers as FooWithPlainParams.

    FooWithKeyword.allocate.initialize(a: 1, b: 2, c: 3)

I'd like to add a note: allocate.initialize just demonstrates the reason for the behavior. Never do write that in your code.

Currently, @ko1 (Koichi Sasada) is addressing this kind of issue fundamentally by a mechanism to write almost all builtin methods in Ruby. If Class#new is written in Ruby, and if a lazy allocation of keyword rest hash is introduced, the extra allocation will be removed.