[ruby-core:114839] [Ruby master Bug#19890] File#realine(chomp: true) slower/more allocations than readline.chomp!

20 Sep 2023

      Issue #19890 has been updated by Dan0042 (Daniel DeLorme).

@eregon You should really stop that tendency to premature micro-optimization and making extravagant claims about "significant" performance costs. What I'm suggesting, in the normal case where the flag is not set, involves only a single conditional check more than now. Only for methods that set pseudo-globals. This can not be considered a "significant" performance cost in any rational way. Before worrying about that kind of "significant" performance cost let's worry about having correct behavior ok?
...
I think it should remain an internal thing only and not be exposed via that forward flag
That forward flag I proposed *is* an internal implementation thing, not something exposed to ruby.

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Bug #19890: File#realine(chomp: true) slower/more allocations than readline.chomp!
https://bugs.ruby-lang.org/issues/19890#change-104684

* Author: segiddins (Samuel Giddins)
* Status: Open
* Priority: Normal
* ruby -v: 3.2.2
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
On ruby 3.2.2 running the following script:

``` ruby
#!/usr/bin/env ruby

require 'rubygems'
require 'bundler/inline'

puts RUBY_VERSION

gemfile do
  source "https://rubygems.org"
  gem "benchmark-ipsa"
end

Benchmark.ipsa do |x|
  x.report("f.readline(chomp: true)") do
    File.open("/usr/share/dict/words") do |f|
      f.readline(chomp: true) until f.eof?
    end
  end

  x.report("f.readline.chomp!") do
    File.open("/usr/share/dict/words") do |f|
      until f.eof?
        s = f.readline
        s.chomp!
        s
      end
    end
  end

  x.report("f.readline.chomp") do
    File.open("/usr/share/dict/words") do |f|
      until f.eof?
        f.readline.chomp
      end
    end
  end

  x.compare!
end
```

I get the following (surprising) result:

```
3.2.2
Allocations -------------------------------------
	f.readline(chomp: true)
                      707931/1  alloc/ret       50/1  strings/ret
   f.readline.chomp!  235979/1  alloc/ret       50/1  strings/ret
    f.readline.chomp  471955/1  alloc/ret       50/1  strings/ret
Warming up --------------------------------------
f.readline(chomp: true)
                         1.000  i/100ms
   f.readline.chomp!     2.000  i/100ms
    f.readline.chomp     2.000  i/100ms
Calculating -------------------------------------
f.readline(chomp: true)
                         16.165  (± 6.2%) i/s -     81.000 
   f.readline.chomp!     25.246  (± 7.9%) i/s -    126.000 
    f.readline.chomp     20.997  (± 9.5%) i/s -    106.000 

Comparison:
   f.readline.chomp!:       25.2 i/s
    f.readline.chomp:       21.0 i/s - 1.20x slower
f.readline(chomp: true):       16.2 i/s - 1.56x slower
```

I would expect `File#readline(chomp: true)` to be comparable to `s = f.readline; s.chomp!; s` at a bare minimum, but it is slower and has more allocations even than `readline.chomp`

-- 
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