[ruby-core:112732] [Ruby master Bug#17995] Slow down when mjit and Ractor are being used at same time

7 Mar 2023

      Issue #17995 has been updated by k0kubun (Takashi Kokubun).

Status changed from Open to Closed

Closing this since I replaced the implementation in [Feature #19420]. It doesn't mean that it works well with Ractor, but the current issue with Ractor should be reported separately since it's a completely different symptom.

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Bug #17995: Slow down when mjit and Ractor are being used at same time
https://bugs.ruby-lang.org/issues/17995#change-102188

* Author: nekoyama32767 (Jinsong Yu)
* Status: Closed
* Priority: Normal
* Assignee: k0kubun (Takashi Kokubun)
* ruby -v: Ruby 3.0.1p64 (2021-04-05 revison 0fb782ee38)[x86-64-linux]    ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x64-mingw32]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------

When using --jit and Ractor at same time, benchmark would be slow down with recursion function such like Tarai function an Fibonacci function.
The slow down is confirmed under both gcc(linux) and mingw64(windows MSYS2).

Test environment:
Cpu:Ryzen9 5900HS@3.3Ghz 8core/16thread 
Memory:32GB@3200Mhz
OS:Ubuntu 21.04/Windows 10

Because of this cpu has 8 physic core,the parallel thread number is set to 8  

Slow down is also confirmed with using only ractor  (without sequence running).

When runing 'par' before 'seq' and using --mjit,'seq' is more slower than runing 'seq' berfore 'par'

Tarai function

```ruby
def tarai(x, y, z) =
  x <= y ? y : tarai(tarai(x-1, y, z),
                     tarai(y-1, z, x),
                     tarai(z-1, x, y))
require 'benchmark'
Benchmark.bm do |x|
  # sequential version
  x.report('seq'){ 8.times{ tarai(14, 7, 0) } }

  # parallel version
  x.report('par'){
    8.times.map do
      Ractor.new { tarai(14, 7, 0) }
    end.each(&:take)
  }
end
```

Fibonacci function 

```ruby
def fib(n)
  if n==1 then return 1 end
  if n==2 then return 1 end
  fib(n-1)+fib(n-2)
end

require 'benchmark'
Benchmark.bm do |x|
   # sequential version
  x.report('fib:seq'){ 8.times{ fib(40) } }

  # parallel version
  x.report('fib:par '){
    8.times.map do
      Ractor.new {fib(40) }
    end.each(&:take)
  }
end
```

-- 
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