From: "ioquatix (Samuel Williams) via ruby-core" <ruby-core@...>
Date: 2023-03-01T08:50:36+00:00
Subject: [ruby-core:112641] [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).


Sorry, I read the example more closely, it seems we are taking a small slice, which marks the original array as shared. So both the original array and the slice point at the same memory allocation, and on mutation, a copy is made. I think @mame is right, in general this is a good optimisation. But I see the problem, there is a degenerate case here.

I wonder if we can propose some way to avoid this degenerate case if the mutation is frequent or the slice should definitely be a copy rather than invoke the CoW. Maybe a percentage based approach, or maybe we can detect the CoW frequently triggering a full copy = don't do further CoW.

----------------------------------------
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-102088

* 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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