Issue #19969 has been updated by Eregon (Benoit Daloze). So apparently some applications were relying on `Set#dup`/`Hash#dup` to do like C++ [shrink_to_fit](https://en.cppreference.com/w/cpp/container/vector/shrink_to_fit). Ruby does not have such a method and it feels quite low-level, so it seems better to resize the internal data structure when removing elements/entries and going below some threshold. ---------------------------------------- Bug #19969: Regression of memory usage with Ruby 3.1 https://bugs.ruby-lang.org/issues/19969#change-105060 * Author: hsbt (Hiroshi SHIBATA) * Status: Open * Priority: Normal * Backport: 3.0: DONTNEED, 3.1: REQUIRED, 3.2: REQUIRED ---------------------------------------- Our company that is ANDPAD, Inc. encountered to increase memory usage after upgrading Ruby 3.2 from 3.0 on our Rails application. This increase size is about 20%. My colleague found this [root cause](https://bugs.ruby-lang.org/issues/16996) and reproduction code: ``` $ ruby -v -rset -e 's1 = Set.new(10000.times); s2 = Set.new(9999.times); Array.new(10000) { s1 - s2 - [0] }; puts `ps -o rss= -p #{$$}`.to_i' ruby 3.0.6p216 (2023-06-29 revision bdfe1958a8) +JIT [arm64-darwin22] 248096 $ ruby -v -rset -e 's1 = Set.new(10000.times); s2 = Set.new(9999.times); Array.new(10000) { s1 - s2 - [0] }; puts `ps -o rss= -p #{$$}`.to_i' ruby 3.2.2 (2023-07-05 revision 2f603bc4d7) +YJIT [arm64-darwin22] 2949280 ``` Should we revert #16996 for Ruby 3.1 or later? I'm not sure this increased memory usage is reasonable with performance improvement. -- https://bugs.ruby-lang.org/