[ruby-core:112640] [Ruby master Bug#19470] Frequent small range-reads from and then writes to a large array are very slow

Issue #19470 has been updated by ioquatix (Samuel Williams).


It's interesting to me that reading items can mark the array as shared, that seems non-trivial/non-obvious behaviour to me. What's the rational for it?

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Bug #19470: Frequent small range-reads from and then writes to a large array are very slow
https://bugs.ruby-lang.org/issues/19470#change-102087

* Author: giner (Stanislav German-Evtushenko)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
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Write to a large array gets very slow when done after range-reading more than 3 items. In such case the original array gets marked as shared which triggers CoW on a small change afterwards. This leads to a significant performance impact and high memory utilization in cases when we need to range-read/write from/to the same array many times. While this issue can be avoided by reading <= 3 elements at a time the main problem is that this behaviour is not obvious and hard to catch on on-trivial projects.

```ruby
times = []
arr = [0] * 100000

times.push 0
100000.times do
  time_start = Time.now
  arr[5] = 100  # takes 0.01662315899999512
  times[-1] += Time.now - time_start
end

times.push 0
100000.times do
  arr[0..2]
  time_start = Time.now
  arr[5] = 100  # takes 0.01826406799999659
  times[-1] += Time.now - time_start
end

times.push 0
100000.times do
  arr[0..3]
  time_start = Time.now
  arr[5] = 100  # takes 7.757753919000069
  times[-1] += Time.now - time_start
end

times.push 0
100000.times do
  arr.dup
  time_start = Time.now
  arr[5] = 100  # takes 7.626929300999957
  times[-1] += Time.now - time_start
end

times.push 0
100000.times do
  arr.clone
  time_start = Time.now
  arr[5] = 100  # takes 8.216933763000046
  times[-1] += Time.now - time_start
end

p times
```



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