[ruby-core:119628] [Ruby master Bug#20816] Potential regression in Ruby 3.3.x (compared with 3.1 and 3.2) regarding fast syscalls and multi-threading.

["adrienjarthon (Adrien Jarthon) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20816 has been updated by adrienjarthon (Adrien Jarthon).


Thanks, these numbers are slightly more "expected" than mine (Ubuntu 24.04), especially for 2 threads, but other that that it looks like the "regression" is also visible on MacOS.
For the record I also tested with ruby master (3.4.0 as of 2024-10-25) but the results were similar to 3.3.5 (like for you).


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Bug #20816: Potential regression in Ruby 3.3.x (compared with 3.1 and 3.2) regarding fast syscalls and multi-threading.
https://bugs.ruby-lang.org/issues/20816#change-110250

* Author: adrienjarthon (Adrien Jarthon)
* Status: Open
* ruby -v: 3.3.5
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
Ok I'm sorry in advance if this is not really a regression (maybe it's the cost to pay for other improvements) but it still showed as a 30% performance drop in my benchmark so I think it may be worth a look.

For the full story about how I came down this rabbit hole I just wrote a blog post to avoid cluttering the issue: https://updown.io/blog/weird-results-comparing-ruby-3-1-3-2-3-3-with-jemalloc-and-yjit (please read it first)

Here I'm gonna put my last findings and the smallest reproducible example I was able to write.

So basically I noticed that quick syscalls (at least with a good SSD) like `File.exists?` or `File.mtime`, when used in a multi-threaded environement (Sidekiq in my initial testing but I reduced it to a few Thread.new) are significantly slower in 3.3.x (tested with the same results on 3.3.0, 3.3.2 and 3.3.5 for the moment) in comparison with 3.1.2 or 3.2.5: about twice as slow.

These syscalls are heavily used by the Rails Reloader which is how it had such a huge impact (30%) in my testing.

Here is the small reproduction script I wrote:

``` ruby
#!/usr/bin/env ruby

GC.disable # just to make sure it doesn't skew results
THREADS = (ARGV.first || 10).to_i  # choose thread count from CLI
N = 10_000_000 # to make the test longer or shorter

puts "Ruby #{RUBY_VERSION} / YJIT: #{RubyVM::YJIT.enabled?} / #{THREADS} Threads"

Array.new(THREADS).map do
  Thread.new do
    (N/THREADS).times { File.mtime(__FILE__) }
  end
end.each(&:join)

puts "#{N} calls completed"
```

The goal is just to run a lot of fast syscalls, in various number of threads, and here are the numbers on my machine:
|Threads\Ruby|3.1.2 |3.3.5 |
|--|--|--|
|1 | 13.7s | ✅ 14.2s |
|2 | 9.7s | ⚠️ 33.4s |
|4 | 23.2s | ⚠️ 37.0s |
|8 | 21.8s | ⚠️ 39.7s |
|16 | 23.1s | ⚠️ 41.5s |

(more numbers in the [blog post](https://updown.io/blog/weird-results-comparing-ruby-3-1-3-2-3-3-with-jemalloc-and-yjit))

I suspected the [new N:M thread scheduler introduced in 3.3](https://github.com/ruby/ruby/pull/8629), even though it's "disabled" by default on the main Ractor, thinking maybe these changes degraded a bit this (quite specific scenario). So I gave a quick try with the feature ENABLED, to see if it would at least change anything and it did ! (though not was I expected):

|Threads\Ruby|3.1.2 |3.3.5 | 3.3.5 with RUBY_MN_THREADS=1 |
|--|--|--|--|
|1 | 13.7s | 14.2s | 14.5s ✅ |
|2 | 9.7s | ⚠️ 33.4s | 12.3s 🎉 |
|4 | 23.2s | ⚠️ 37.0s | 40.5s 🧐 |
|8 | 21.8s | ⚠️ 39.7s | 37.9s 🧐 |
|16 | 23.1s | ⚠️ 41.5s | 36.3s 🧐 |


(I can make graphs for these numbers if it helps)

Now I'm a bit stuck in my investigation, I'm not sure what change in 3.3 caused this (the new thread scheduler is a good lead IMO) or if it even is a regression. Maybe this downside is innevitable for other benefits? I am happy to contribute more time to this issue and perform more tests but I would need some guidance on what could be interesting to test, from people who understand a bit better than me how this works internally.

Thanks ! (and sorry for the long read)



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