From: "giner (Stanislav German-Evtushenko) via ruby-core" <ruby-core@...>
Date: 2023-03-01T23:38:04+00:00
Subject: [ruby-core:112651] [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 giner (Stanislav German-Evtushenko).


> Do you have any evidence that this problem actually occurs frequently?

I cannot say whether this affects many projects. I've encountered this twice so far when optimizing code to reduce memory allocations (by writing to the same array instead of allocating new ones). In both cases it took quite a while to figure out why instead of being faster and using less memory it did the opposite, i.e. became slower and used more memory.

----------------------------------------
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-102100

* 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
----------------------------------------
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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