[ruby-core:106896] [Ruby master Bug#18447] Potential performance regression with String#lines in large strings

Issue #18447 has been updated by mame (Yusuke Endoh).

Assignee set to peterzhu2118 (Peter Zhu)

Thank you for your great report. I could reproduce the issue on M1 macOS. It does not repro on Ubuntu.

According to `git bisect`, this regression is caused by 785f5eb8f0f994c755ac5f1e1479f6e2ec6d0ef3.

```
% git checkout 09f284c052854392fad1e63fd541a6c8704d7b90 && make -j miniruby
% time ./miniruby -ve '("\n" * 10_000_000).lines'
ruby 3.1.0dev (2021-02-24T11:01:44Z v3_1_0_preview1~4291 09f284c052) [arm64-darwin20]
./miniruby -ve '("\n" * 10_000_000).lines'  0.91s user 0.10s system 81% cpu 1.255 total
```

```
% git checkout 785f5eb8f0f994c755ac5f1e1479f6e2ec6d0ef3 && make -j miniruby
% time ./miniruby -ve '("\n" * 10_000_000).lines'
ruby 3.1.0dev (2021-02-24T20:25:30Z v3_1_0_preview1~4290 785f5eb8f0) [arm64-darwin20]
./miniruby -ve '("\n" * 10_000_000).lines'  0.85s user 4.77s system 96% cpu 5.834 total

```

It mainly takes "system" time, not "user" time. So, it seems to make sense that the slowdown is due to the use of mmap.

@peterzhu2118 Could you take a look at this?

----------------------------------------
Bug #18447: Potential performance regression with String#lines in large strings
https://bugs.ruby-lang.org/issues/18447#change-95713

* Author: ttilberg (Tim Tilberg)
* Status: Open
* Priority: Normal
* Assignee: peterzhu2118 (Peter Zhu)
* ruby -v: ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I believe there may be a potential performance regression regarding `String#lines` worth noting between 3.0.3 and 3.1.0. This came about in [this discussion](https://www.reddit.com/r/ruby/comments/rpje0g/fast_to_way_to_parse_csv/hqadqsd/) regarding large file parsing performance. We were benchmarking various ways to parse a 10 million row CSV file. Slurping the file took significantly longer than streaming in version 3.1.0, even though the data was able to fit in memory. After further research, we started to feel that there may be something to speak up about here, and I think it's pinned down to `String#lines`.

I'm running Mac OS Big Sur 11.6.1 on a 13" 2020 MBP with 32 GB ram. Specific Ruby versions are included in the comparison examples below.

The simplest reproduction seems to be:

Ruby 3.0.3: ~1.5 seconds

```
笆カ time ruby -ve '("\n" * 10_000_000).lines'
ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [x86_64-darwin20]
ruby -ve '("\n" * 10_000_000).lines'  1.38s user 0.39s system 100% cpu 1.756 total
```

Ruby 3.1.0: ~11.5 seconds

```
笆カ time ruby -ve '("\n" * 10_000_000).lines'
ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]
ruby -ve '("\n" * 10_000_000).lines'  1.52s user 10.01s system 99% cpu 11.579 total
```

Some other observations:

An earlier script that I ran ruby-prof against looked like:

```
puts File.read("sample-data.csv").lines.sum { 1 }
```

- It appeared that the time increase stemmed from `String#lines`, as all other methods had similar time taken between the versions:

```
 Ruby 3.1.0
  %self      total      self      wait     child     calls  name
 89.93     12.728    12.728     0.000     0.000        1   String#lines
  9.01      1.275     1.275     0.000     0.000        1   Array#sum
  1.05      0.149     0.149     0.000     0.000        1   <Class::IO>#read

  Ruby 3.0.3
   %self      total      self      wait     child     calls  name
 74.91      3.773     3.773     0.000     0.000        1   String#lines
 22.15      1.116     1.116     0.000     0.000        1   Array#sum
  2.93      0.148     0.148     0.000     0.000        1   <Class::IO>#read
```

- A similar enumerator without `String#lines` does not appear to cause this:

```
笆カ time ruby -ve '10_000_000.times.map { nil }'
ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [x86_64-darwin20]
ruby -ve '10_000_000.times.map { nil }'  0.57s user 0.16s system 102% cpu 0.710 total
```

```
笆カ time ruby -ve '10_000_000.times.map { nil }'
ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]
ruby -ve '10_000_000.times.map { nil }'  0.61s user 0.16s system 102% cpu 0.753 total
```

- It doesn't seem related to string generation:

```
笆カ time ruby -ve '("\n" * 10_000_000)'
ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [x86_64-darwin20]
-e:1: warning: possibly useless use of * in void context
ruby -ve '("\n" * 10_000_000)'  0.13s user 0.14s system 107% cpu 0.246 total
```

```
笆カ time ruby -ve '("\n" * 10_000_000)'
ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]
-e:1: warning: possibly useless use of * in void context
ruby -ve '("\n" * 10_000_000)'  0.13s user 0.14s system 107% cpu 0.245 total
```

(Thanks to simpl1g for the discussion on Reddit, and help detecting this potential issue)



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