Commits on Jul 21, 2020

1. Consider falsy value on Concurrent::Map#compute_if_absent fast non-… …

   …blocking path
   
   Currently, `compute_if_absent` with falsy value is much slower than truthy value
   because non-blocking path only considers truthy one.
   https://github.com/ruby-concurrency/concurrent-ruby/blob/f749b81cb6c6291640c0004b57e60dbc2b59a72b/lib/concurrent-ruby/concurrent/collection/map/mri_map_backend.rb#L22
   
   ```ruby
   require 'bundler/inline'
   
   gemfile do
     source 'https://rubygems.org'
   
     gem 'benchmark-ips', require: 'benchmark/ips'
     gem 'concurrent-ruby', require: 'concurrent'
   end
   
   concurrent_map = Concurrent::Map.new
   concurrent_map[:nil] = nil
   concurrent_map[:non_nil] = 1
   
   Benchmark.ips do |x|
     x.report("[Concurrent::Map] :nil") do
       concurrent_map.compute_if_absent(:nil) { raise }
     end
   
     x.report("[Concurrent::Map] :non_nil") do
       concurrent_map.compute_if_absent(:non_nil) { raise }
     end
   
     x.compare!
   end
   ```
   
   ```
   Warming up --------------------------------------
   [Concurrent::Map] :nil
                          312.584k i/100ms
   [Concurrent::Map] :non_nil
                          704.894k i/100ms
   Calculating -------------------------------------
   [Concurrent::Map] :nil
                             2.404M (± 6.6%) i/s -     12.191M in   5.095349s
   [Concurrent::Map] :non_nil
                             6.757M (± 9.3%) i/s -     33.835M in   5.054357s
   
   Comparison:
   [Concurrent::Map] :non_nil:  6756566.7 i/s
   [Concurrent::Map] :nil:  2404077.4 i/s - 2.81x  (± 0.00) slower
   ```
   
   This patch makes it faster and keeps same-ish speed for truthy value.
   
   ```ruby
   require 'bundler/inline'
   
   gemfile do
     source 'https://rubygems.org'
   
     gem 'benchmark-ips', require: 'benchmark/ips'
     gem 'concurrent-ruby', require: 'concurrent'
   end
   
   concurrent_map = Concurrent::Map.new
   concurrent_map[:nil] = nil
   concurrent_map[:non_nil] = 1
   
   patched_concurrent_map = Concurrent::Map.new
   patched_concurrent_map.extend(Module.new do
     def compute_if_absent(key)
       if Concurrent::NULL != (stored_value = @backend.fetch(key, Concurrent::NULL))
         stored_value
       else
         @write_lock.synchronize { super }
       end
     end
   end)
   patched_concurrent_map[:nil] = nil
   patched_concurrent_map[:non_nil] = 1
   
   Benchmark.ips do |x|
     x.report("[Concurrent::Map] :nil") do
       concurrent_map.compute_if_absent(:nil) { raise }
     end
   
     x.report("[Concurrent::Map] :non_nil") do
       concurrent_map.compute_if_absent(:non_nil) { raise }
     end
   
     x.report("[Concurrent::Map with patch] :nil") do
       patched_concurrent_map.compute_if_absent(:nil) { raise }
     end
   
     x.report("[Concurrent::Map with patch] :non_nil") do
       patched_concurrent_map.compute_if_absent(:non_nil) { raise }
     end
   
     x.compare!
   end
   ```
   
   ```
   Warming up --------------------------------------
   [Concurrent::Map] :nil
                          310.135k i/100ms
   [Concurrent::Map] :non_nil
                          923.645k i/100ms
   [Concurrent::Map with patch] :nil
                          928.247k i/100ms
   [Concurrent::Map with patch] :non_nil
                          840.197k i/100ms
   Calculating -------------------------------------
   [Concurrent::Map] :nil
                             2.049M (±11.4%) i/s -     10.234M in   5.060933s
   [Concurrent::Map] :non_nil
                             7.181M (± 7.5%) i/s -     36.022M in   5.048215s
   [Concurrent::Map with patch] :nil
                             7.479M (± 5.0%) i/s -     38.058M in   5.103572s
   [Concurrent::Map with patch] :non_nil
                             7.413M (± 6.9%) i/s -     36.969M in   5.013929s
   
   Comparison:
   [Concurrent::Map with patch] :nil:  7478557.1 i/s
   [Concurrent::Map with patch] :non_nil:  7413277.4 i/s - same-ish: difference falls within error
   [Concurrent::Map] :non_nil:  7181043.9 i/s - same-ish: difference falls within error
   [Concurrent::Map] :nil:  2049352.8 i/s - 3.65x  (± 0.00) slower
   ```

   

   mtsmfm committed Jul 21, 2020
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