unlock the gvl when calculating hashes / salts

[tenderlove]

Holding on to the GVL means we can't do anything in parallel. Since
these are CPU-only operations that do not involve the ruby interpreter,
we can safely unlock the GVL when calculating hashes.

This program demonstrates the difference:

require 'bcrypt'
require 'thread'

GUESSES = (ENV['GUESSES'] || 100).to_i
THREADS = (ENV['THREADS'] || 1).to_i

p GUESSES: GUESSES, THREADS: THREADS

password = BCrypt::Password.create 'hello world!'

queue = Queue.new
GUESSES.times { queue << "x" * 90 }
THREADS.times { queue << nil }
THREADS.times.map {
  Thread.new {
    while guess = queue.pop
      password == guess
    end
  }
}.each(&:join)

Without this patch:

[aaron@TC bcrypt-ruby (master)]$ time THREADS=4 ruby test.rb
{:GUESSES=>100, :THREADS=>4}

real    0m30.014s
user    0m29.739s
sys 0m0.153s

With the patch:

[aaron@TC bcrypt-ruby (master)]$ time THREADS=4 ruby -Ilib test.rb
{:GUESSES=>100, :THREADS=>4}

real    0m12.293s
user    0m42.382s
sys 0m0.169s

If you run this program with the patch, you can see Ruby use nearly 400% CPU,
as long as you have a 4 core machine.

Sending this as a PR because I'm not sure if the feature detection is enough to work on all versions of Ruby we support and I figured I'd let the CI test it. If something fails, I'll add better feature detection.

[compwron]

So is this mergeable yet? :)
Sending this as a PR because I'm not sure if the feature detection is enough to work on all versions of Ruby we support and I figured I'd let the CI test it. If something fails, I'll add better feature detection.

[tenderlove]

I'm going to merge this. I should have merged it a long time ago. 🙇‍♂️

Bechmarks in 2019:

Without this patch:

[aaron@tc-lan-adapter ~/g/bcrypt-ruby (master)]$ time env THREADS=8 ruby -I lib bench.rb
{:GUESSES=>100, :THREADS=>8}
       18.93 real        18.60 user         0.13 sys

With this patch:

[aaron@tc-lan-adapter ~/g/bcrypt-ruby (nogvl)]$ time env THREADS=8 ruby -I lib bench.rb
{:GUESSES=>100, :THREADS=>8}
        3.20 real        19.00 user         0.12 sys

[sergey-alekseev]

@tenderlove may be mention this in CHANGELOG?