Use cgroups aware processor count by default

In containerized environments, a number of CPU cores isn't the
same as the available CPUs. In this case, we need to consider cgroups.

`concurrent-ruby` now has the method to get that since v1.3.1. I think it's better
to use this for setting more container environment friendly default value.
Ref: ruby-concurrency/concurrent-ruby#1038

I keep `Parallel.processor_count` as is to keep supporting setting
processor count via env(`PARALLEL_PROCESSOR_COUNT`).

Readme.md

@@ -196,6 +196,7 @@ Tips
  - [Isolation] Do not reuse previous worker processes: `isolation: true`
  - [Stop all processes with an alternate interrupt signal] `'INT'` (from `ctrl+c`) is caught by default. Catch `'TERM'` (from `kill`) with `interrupt_signal: 'TERM'`
  - [Process count via ENV] `PARALLEL_PROCESSOR_COUNT=16` will use `16` instead of the number of processors detected. This is used to reconfigure a tool using `parallel` without inserting custom logic.
+ - [Process count] `parallel` uses a number of processors seen by the OS for process count by default. If you want to use a value considering CPU quota, please add `concurrent-ruby` to your `Gemfile`.
 
 TODO
 ====

lib/parallel.rb

@@ -337,10 +337,10 @@ def physical_processor_count
       end
     end
 
-    # Number of processors seen by the OS, used for process scheduling
+    # Number of processors seen by the OS or value considering CPU quota if the process is inside a cgroup,
+    # used for process scheduling
     def processor_count
-      require 'etc'
-      @processor_count ||= Integer(ENV['PARALLEL_PROCESSOR_COUNT'] || Etc.nprocessors)
+      @processor_count ||= Integer(ENV['PARALLEL_PROCESSOR_COUNT'] || available_processor_count)
     end
 
     def worker_number
@@ -695,5 +695,13 @@ def instrument_start(item, index, options)
       return unless (on_start = options[:start])
       options[:mutex].synchronize { on_start.call(item, index) }
     end
+
+    def available_processor_count
+      require 'concurrent-ruby'
+      Concurrent.available_processor_count.floor
+    rescue LoadError, NoMethodError
+      require 'etc'
+      Etc.nprocessors
+    end
   end
 end

spec/parallel_spec.rb

@@ -49,11 +49,6 @@ def without_ractor_warning(out)
         (1..999).should include(Parallel.processor_count)
       end
     end
-
-    it 'uses Etc.nprocessors in Ruby 2.2+' do
-      defined?(Etc).should == "constant"
-      Etc.respond_to?(:nprocessors).should == true
-    end
   end
 
   describe ".physical_processor_count" do