use of the PID global was causing warnings for some reason

lib/sidekiq/launcher.rb

@@ -148,7 +148,7 @@ module Sidekiq
 
         return unless msg
 
-        ::Process.kill(msg, $PID)
+        ::Process.kill(msg, ::Process.pid)
       rescue => e
         # ignore all redis/network issues
         logger.error("heartbeat: #{e.message}")
@@ -163,7 +163,7 @@ module Sidekiq
         {
           "hostname" => hostname,
           "started_at" => Time.now.to_f,
-          "pid" => $PID,
+          "pid" => ::Process.pid,
           "tag" => @options[:tag] || "",
           "concurrency" => @options[:concurrency],
           "queues" => @options[:queues].uniq,

lib/sidekiq/redis_connection.rb

@@ -12,7 +12,7 @@ module Sidekiq
           options[key.to_sym] = options.delete(key)
         end
 
-        options[:id] = "Sidekiq-#{Sidekiq.server? ? "server" : "client"}-PID-#{$PID}" unless options.key?(:id)
+        options[:id] = "Sidekiq-#{Sidekiq.server? ? "server" : "client"}-PID-#{::Process.pid}" unless options.key?(:id)
         options[:url] ||= determine_redis_provider
 
         size = if options[:size]

lib/sidekiq/util.rb

@@ -48,7 +48,7 @@ module Sidekiq
     end
 
     def identity
-      @identity ||= "#{hostname}:#{$PID}:#{process_nonce}"
+      @identity ||= "#{hostname}:#{::Process.pid}:#{process_nonce}"
     end
 
     def fire_event(event, options = {})