```ruby
require 'etc'

module Parallel
  # TODO: inline this method into parallel.rb and kill physical_processor_count in next major release
  module ProcessorCount
    # Number of processors seen by the OS, used for process scheduling
    def processor_count
      @processor_count ||= Integer(ENV['PARALLEL_PROCESSOR_COUNT'] || Etc.nprocessors)
    end

    # Number of physical processor cores on the current system.
    def physical_processor_count
      @physical_processor_count ||= begin
        ppc = case RbConfig::CONFIG["target_os"]
        when /darwin[12]/,
          IO.popen("/usr/sbin/sysctl -n hw.physicalcpu").read.to_i
        when /linux/
          cores = {}  # unique physical ID / core ID combinations
          phy = 0
          IO.read("/proc/cpuinfo").scan(/^physical id.*|^core id.*/) do |ln|
            if ln.start_with?("physical")
              phy = ln[/\d+/]
            elsif ln.start_with?("core")
              cid = phy + ":" + ln[/\d+/]
              cores[cid] = true if not cores[cid]
            end
          end
          cores.count
        when /mswin|mingw/
          require 'win32ole'
          result_set = WIN32OLE.connect("winmgmts://").ExecQuery(
            "select NumberOfCores from Win32_Processor")
          result_set.to_enum.collect(&:NumberOfCores).reduce(:+)
        else
          processor_count
        end
        # fall back to logical count if physical info is invalid
        ppc > 0 ? ppc : processor_count
      end
    end
  end
end
```

Revert "Add support for darwin20"
user
```ruby
require 'etc'

module Parallel
  # TODO: inline this method into parallel.rb and kill physical_processor_count in next major release
  module ProcessorCount
    # Number of processors seen by the OS, used for process scheduling
    def processor_count
      @processor_count ||= Integer(ENV['PARALLEL_PROCESSOR_COUNT'] || Etc.nprocessors)
    end

    # Number of physical processor cores on the current system.
    def physical_processor_count
      @physical_processor_count ||= begin
        ppc = case RbConfig::CONFIG["target_os"]
        when /darwin1/
          IO.popen("/usr/sbin/sysctl -n hw.physicalcpu").read.to_i
        when /linux/
          cores = {}  # unique physical ID / core ID combinations
          phy = 0
          IO.read("/proc/cpuinfo").scan(/^physical id.*|^core id.*/) do |ln|
            if ln.start_with?("physical")
              phy = ln[/\d+/]
            elsif ln.start_with?("core")
              cid = phy + ":" + ln[/\d+/]
              cores[cid] = true if not cores[cid]
            end
          end
          cores.count
        when /mswin|mingw/
          require 'win32ole'
          result_set = WIN32OLE.connect("winmgmts://").ExecQuery(
            "select NumberOfCores from Win32_Processor")
          result_set.to_enum.collect(&:NumberOfCores).reduce(:+)
        else
          processor_count
        end
        # fall back to logical count if physical info is invalid
        ppc > 0 ? ppc : processor_count
      end
    end
  end
end
```
assistant