Issue #18285 has been updated by byroot (Jean Boussier).
As far as I understand, the problem originates in
Rails, which (as they do) doesn't try to follow good practices for #inspect (which is
honestly frequently irritating)
This absolutely isn't specific to Rails. It can happen with any object that has a
large number of reference.
Try your `transform_keys` example on a large API payload you received and you'll
produce a gigantic error message.
"People are holding it wrong" isn't a proper way to address issues.
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Feature #18285: NoMethodError#message uses a lot of CPU/is really expensive to call
https://bugs.ruby-lang.org/issues/18285#change-101041
* Author: ivoanjo (Ivo Anjo)
* Status: Open
* Priority: Normal
----------------------------------------
Hello there! I'm working at Datadog on the ddtrace gem --
https://github.com/DataDog/dd-trace-rb and we ran into this issue on one of our internal
testing applications. I also blogged about this issue in
<https://ivoanjo.me/blog/2021/11/01/nomethoderror-ruby-cost/>.
### Background
While testing an application that threw a lot of `NoMethodError`s in a Rails controller
(this was used for validation), we discovered that service performance was very much
impacted when we were logging these exceptions. While investigating with a profiler, the
performance impact was caused by calls to `NoMethodError#message`, because this Rails
controller had a quite complex `#inspect` method, that was getting called every time we
tried to get the `#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 => e
e
end
def get_runtime_error
raise 'Another Error'
rescue => e
e
end
def inspect # <-- expensive method gets called when calling NoMethodError#message
Gem::Specification._all.inspect
end
end
NO_METHOD_ERROR_INSTANCE = GemInformation.new.get_no_method_error
RUNTIME_ERROR_INSTANCE = GemInformation.new.get_runtime_error
Benchmark.ips do |x|
x.config(:time => 5, :warmup => 2)
x.report("no method error message cost") { NO_METHOD_ERROR_INSTANCE.message
}
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 getting 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 (± 1.7%) i/s - 580.000 in 5.027822s
runtime error message cost
6.938M (± 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 (± 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/e0915ba67964d843832148aeca29a1f8244ca7b1/error.c#L1962>)
2. Cache result of calling `#message` after the first call. Ideally this should be done
together with suggestion 1.
--
https://bugs.ruby-lang.org/