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Bug #11119

Updated by nobu (Nobuyoshi Nakada) over 4 years ago

MRI lazily determines the name of a class or module by walking all defined constants starting from `Object` Object and looking for the namespace in question. This allows deferring the full name calclation until the class/module is finished being defined. However, if the class or module is *never* accessible via `Object`, Object, then this system-walking occurs for every call to `#name` #name or `#inspect` #inspect on the `class`/`module` class/module and every call to the default `#inspect` #inspect on instances of the class.

A simple benchmark:

~~~ruby
require 'benchmark'

module B
module X
end
end

def a
c = Class.new
c2 = Class.new
c.class_eval 'A = c2'
c2.class_eval 'A = c'

c
end

c = a
x = B::X

loop do
puts 'named'
puts Benchmark.measure { 1_000_000.times { x.name } }
puts 'anon'
puts Benchmark.measure { 1_000_000.times { c.name } }
cobj = c.new
puts 'anon obj'
puts Benchmark.measure { 1_000_000.times { cobj.inspect } }
end
~~~


Results on MRI 2.2 and JRuby 1.7 HEAD:

MRI:

~~~
named
0.210000 0.000000 0.210000 ( 0.205585)
anon
14.170000 0.050000 14.220000 ( 14.259003)
anon obj
15.750000 0.060000 15.810000 ( 15.864806)
~~~


JRuby:

~~~
named
0.250000 0.000000 0.250000 ( 0.253000)
anon
0.270000 0.000000 0.270000 ( 0.264000)
anon obj
0.450000 0.000000 0.450000 ( 0.447000)
~~~


The effect worsens linearly with the size of the system. Running in a freshly-generated Rails app's console:

~~~
named
0.260000 0.020000 0.280000 ( 0.272182)
anon
240.900000 0.800000 241.700000 (242.384455)
anon obj
257.070000 1.110000 258.180000 (261.986562)
~~~


I believe MRI needs to give up on looking for the object after the first failed namespace traversal, or else eagerly build this name the way other implementations do (and accept some changes).

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