Atomic counters: MutexFixnum vs concurrent-ruby's AtomicFixnum

bench.rb

require "benchmark/ips"
require "concurrent"

class MutexFixnum
  def initialize
    @value = 0
    @lock = Mutex.new
  end

  def increment(amount = 1)
    @lock.synchronize { @value += amount }
  end

  def value=(val)
    @lock.synchronize {
      old = @value
      @value = val
      old
    }
  end
end

Benchmark.ips do |x|
  [::MutexFixnum, ::Concurrent::AtomicFixnum].each do |klass|
    x.report(klass) do |count|
      sc = klass.new
      count.times { sc.increment }
    end
    x.report("thread-#{klass}") do |count|
      sc = klass.new
      t = []
      10.times {
        t << Thread.new do
          count.times { sc.increment }
        end
      }
      t.each(&:join)
    end
  end
end

jruby-results.txt

jruby 9.4.8.0 (3.1.4) 2024-07-02 4d41e55a67 OpenJDK 64-Bit Server VM 18.0.2.1+0 on 18.0.2.1+0 +jit [arm64-darwin]
Warming up --------------------------------------
         MutexFixnum     1.399M i/100ms
  thread-MutexFixnum    88.529k i/100ms
Concurrent::AtomicFixnum
                         8.141M i/100ms
thread-Concurrent::AtomicFixnum
                       127.454k i/100ms
Calculating -------------------------------------
         MutexFixnum     13.902M (± 0.7%) i/s   (71.93 ns/i) -     69.965M in   5.032978s
  thread-MutexFixnum    823.398k (± 5.3%) i/s    (1.21 μs/i) -      4.161M in   5.067168s
Concurrent::AtomicFixnum
                         79.494M (± 0.9%) i/s   (12.58 ns/i) -    398.895M in   5.018339s
thread-Concurrent::AtomicFixnum
                          1.268M (± 7.6%) i/s  (788.80 ns/i) -      6.373M in   5.060407s

mri-results.txt

ruby 3.3.5 (2024-09-03 revision ef084cc8f4) +YJIT [arm64-darwin23]
Warming up --------------------------------------
         MutexFixnum     1.293M i/100ms
  thread-MutexFixnum   143.026k i/100ms
Concurrent::AtomicFixnum
                         1.074M i/100ms
thread-Concurrent::AtomicFixnum
                       104.075k i/100ms
Calculating -------------------------------------
         MutexFixnum     14.924M (± 0.2%) i/s   (67.01 ns/i) -     74.970M in   5.023621s
  thread-MutexFixnum      1.428M (± 0.8%) i/s  (700.32 ns/i) -      7.151M in   5.008456s
Concurrent::AtomicFixnum
                         10.697M (± 0.4%) i/s   (93.49 ns/i) -     53.703M in   5.020592s
thread-Concurrent::AtomicFixnum
                          1.043M (± 0.9%) i/s  (958.88 ns/i) -      5.308M in   5.089994s