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 ``` -- https://bugs.ruby-lang.org/