Bug #21696: Performance degradation for long running processes in Ruby 4.0.0-preview2

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 4.0.0preview2 (2025-11-17 master 4fa6e9938c) +PRISM [x86_64-linux]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[easydwh (Ivo Herweijer)]

When running my RubyMeasureResponsetime tool (https://github.com/easydatawarehousing/ruby_measure_responsetime) on Ruby 4.0.0-preview2, a slow but steady performance degradation is measurable.
Both the Rails and the Roda based test applications show this. And both with and without yjit.
The Rails application when using yjit also seems to have increasing memory usage over time.

Running the same tests on Ruby 3.4.7 shows stable performance and memory usage.

I have included some plots showing this behavior.

[ufuk (Ufuk Kayserilioglu)]

Did 3.5-preview1 have the same behaviour? If not, could you bisect between the two releases to find the commit that changed the behaviour?

[easydwh (Ivo Herweijer)]

ufuk (Ufuk Kayserilioglu) wrote in #note-1:

Did 3.5-preview1 have the same behaviour? If not, could you bisect between the two releases to find the commit that changed the behaviour?

Just ran the test, only for the Roda app. Results look very similar (almost identical) to 3.4.7. Logically the culprit should be a commit for 4.0-preview.
I don't know anything about Ruby source code, so pinpointing the commit(s) involved is beyond my knowledge.