[ruby-core:106943] [Ruby master Bug#18454] YJIT slowing down key Discourse benchmarks

Issue #18454 has been updated by byroot (Jean Boussier).


> Any ideas on what we should test here and how to narrow this down?

You could compile Ruby with `YJIT_STATS=1` to have a break down of why and how much YJIT exit back to the interpreter. It's possible that you have some hotspots using constructs that YJIT can't optimize yet.

----------------------------------------
Bug #18454: YJIT slowing down key Discourse benchmarks
https://bugs.ruby-lang.org/issues/18454#change-95767

* Author: sam.saffron (Sam Saffron)
* Status: Open
* Priority: Normal
* ruby -v: 3.1.0p0
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------

4 out of 6 key Discourse benchmarks used in: https://github.com/discourse/discourse/blob/main/script/bench.rb are slower under YJIT. 

Vanilla (median) -> YJIT (median)

Categories: 47ms -> 51
Home page: 85 -> 86
Topic: 42 -> 38
Categories Admin: 48 -> 50
Home Admin: 83 -> 84
Topic Admin: 43 -> 39
Boot: 1740ms -> 2484ms 
RSS: 358756 -> 640540

This was run with a simple `RUBYOPT='--enable-yjit' ruby script/bench.rb -i 100 -s`


To get Discourse working with Ruby 3.1 you will need: 



```
diff --git a/Gemfile b/Gemfile
index 9cdfbf21a9..403c428718 100644
--- a/Gemfile
+++ b/Gemfile
@@ -263,3 +263,11 @@ gem 'colored2', require: false
 gem 'maxminddb'
 
 gem 'rails_failover', require: false
+
+if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("3.0.0")
+  gem 'net-pop', require: false
+  gem 'net-smtp', require: false
+  gem 'net-imap', require: false
+  # waiting on new version of i18n gem
+  gem 'psych', '3.3.2'
+end
```


It is clear that memory usage and boot time have to take a hit here, but slowing down home page seems concerning. Any ideas on what we should test here and how to narrow this down? 



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