From: yura.des@...
Date: 2021-01-06T15:41:53+00:00
Subject: [ruby-core:101963] [Ruby master Bug#17497] Ractor performance issue

Issue #17497 has been updated by inversion (Yura Babak).


I also made 2 posts about strange performance testing results (with sources) and some conclusions.
In my case, 2 ractors work 3-times longer than doing the same payload in the main thread.

https://www.reddit.com/r/ruby/comments/kpmt73/ruby_30_ractors_performance_test_strange_results/
https://www.reddit.com/r/ruby/comments/krq5xe/when_ruby3_ractors_are_good_and_when_not_yet/

The test:
``` ruby
require 'digest/sha2'

N = 1_500_000

def workload
	N.times do |n|
		Digest::SHA2.base64digest(n.to_s)
	end
end

puts 'in the main thread'
t_start = Time.now
	workload
puts "Total: %.3f" % (Time.now - t_start)
puts


worker1 = Ractor.new do
	Ractor.receive
	print '['; workload; ']'
end
worker2 = Ractor.new do
	Ractor.receive
	print '['; workload; ']'
end

puts 'in 2 ractors'
t_start = Time.now
	worker1.send 'start'
	worker2.send 'start'
	print '='
	print worker1.take
	print worker2.take
puts
puts "Total: %.3f" % (Time.now - t_start)
```


----------------------------------------
Bug #17497: Ractor performance issue
https://bugs.ruby-lang.org/issues/17497#change-89816

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin18]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
There's a strange performance issue with Ractor (at least on MacOS, didn't run on other OS).

I ran a benchmark doing 3 different types of work:
* "fib": method calls (naive fibonacci calculation)
* "cpu": `(0...1000).inject(:+)`
* "sleep": call `sleep`

I get the kind of results I was excepting for the `fib` and for sleeping, but the results for the "cpu" workload show a problem.

It is so slow that my pure Ruby backport (using Threads) is 65x faster ���� on my Mac Pro (despite having 6 cores). Expected results would be 6x slower, so in that case Ractor is 400x slower than it should ����

On my MacBook (2 cores) the results are not as bad, the `cpu` workload is 3x faster with my pure-Ruby backport (only) instead of ~2x slower, so the factor is 6x too slow.


```
$ gem install backports
Successfully installed backports-3.20.0
1 gem installed
$ ruby ractor_test.rb
<internal:ractor>:267: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
fib: 110 ms  |  cpu: 22900 ms  |  sleep: 206 ms
$ B=t ruby ractor_test.rb
Using pure Ruby implementation
fib: 652 ms  |  cpu: 337 ms  |  sleep: 209 ms
```

Notice the `sleep` run takes similar time, which is good, and `fib` is ~6x faster on my 6-core CPU (and ~2x faster on my 2-core MacBook), again that's good as the pure ruby version uses Threads and thus runs with a single GVL.

The `cpu` version is the problem.

Script is here: https://gist.github.com/marcandre/bfed626e538a3d0fc7cad38dc026cf0e



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