[ruby-core:116060] [Ruby master Feature#18285] NoMethodError#message uses a lot of CPU/is really expensive to call

Issue #18285 has been updated by headius (Charles Nutter).





Also fixes my issue from 2014: https://bugs.ruby-lang.org/issues/9725



I'm glad we finally have dealt with this one.



----------------------------------------

Feature #18285: NoMethodError#message uses a lot of CPU/is really expensive=
 to call

https://bugs.ruby-lang.org/issues/18285#change-106058



* Author: ivoanjo (Ivo Anjo)

* Status: Closed

* Priority: Normal

----------------------------------------

Hello there! I'm working at Datadog on the ddtrace gem -- https://github.co=
m/DataDog/dd-trace-rb and we ran into this issue on one of our internal tes=
ting applications. I also blogged about this issue in <https://ivoanjo.me/b=
log/2021/11/01/nomethoderror-ruby-cost/>.



### Background



While testing an application that threw a lot of `NoMethodError`s in a Rail=
s controller (this was used for validation), we discovered that service per=
formance was very much impacted when we were logging these exceptions. Whil=
e investigating with a profiler, the performance impact was caused by calls=
 to `NoMethodError#message`, because this Rails controller had a quite comp=
lex `#inspect` method, that was getting called every time we tried to get t=
he `#message` from the exception.



### How to reproduce



```ruby

require 'bundler/inline'



gemfile do

  source 'https://rubygems.org'



  gem 'benchmark-ips'

end



puts RUBY_DESCRIPTION



class GemInformation

  # ...



  def get_no_method_error

    method_does_not_exist

  rescue =3D> e

    e

  end



  def get_runtime_error

    raise 'Another Error'

  rescue =3D> e

    e

  end



  def inspect # <-- expensive method gets called when calling NoMethodError=
#message

    Gem::Specification._all.inspect

  end

end



NO_METHOD_ERROR_INSTANCE =3D GemInformation.new.get_no_method_error

RUNTIME_ERROR_INSTANCE =3D GemInformation.new.get_runtime_error



Benchmark.ips do |x|

  x.config(:time =3D> 5, :warmup =3D> 2)



  x.report("no method error message cost") { NO_METHOD_ERROR_INSTANCE.messa=
ge }

  x.report("runtime error message cost") { RUNTIME_ERROR_INSTANCE.message }



  x.compare!

end

```



### Expectation and result



Getting the `#message` from a `NoMethodError` should be no costly than gett=
ing it from any other exception.



In reality:



```

ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-linux]



no method error message cost

                        115.390  (=B1 1.7%) i/s -    580.000  in   5.027822s

runtime error message cost

                          6.938M (=B1 0.5%) i/s -     35.334M in   5.092617s



Comparison:

runtime error message cost:  6938381.6 i/s

no method error message cost:      115.4 i/s - 60130.02x  (=B1 0.00) slower

```



### Suggested solutions



1. Do not call `#inspect` on the object on which the method was not found (=
see <https://github.com/ruby/ruby/blob/e0915ba67964d843832148aeca29a1f8244c=
a7b1/error.c#L1962>)

2. Cache result of calling `#message` after the first call. Ideally this sh=
ould be done together with suggestion 1.









--=20

https://bugs.ruby-lang.org/

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