Use harmonic mean to calculate IPS

lib/benchmark/ips/job.rb

@@ -227,7 +227,7 @@ def load_held_results
         JSON.load(IO.read(@held_path)).each do |result|
           @held_results[result['item']] = result
           create_report(result['item'], result['measured_us'], result['iter'],
-                        create_stats(result['samples']), result['cycles'])
+                        create_stats(result['samples'], result['measured_us'], result['iter']), result['cycles'])
         end
       end
 
@@ -359,7 +359,7 @@ def run_benchmark
             iterations_per_sec cycles, time_us
           }
 
-          rep = create_report(item.label, measured_us, iter, create_stats(samples), cycles)
+          rep = create_report(item.label, measured_us, iter, create_stats(samples, measured_us, iter), cycles)
 
           if (final_time - target).abs >= (@time.to_f * MAX_TIME_SKEW)
             rep.show_total_time!
@@ -371,10 +371,10 @@ def run_benchmark
         end
       end
 
-      def create_stats(samples)
+      def create_stats(samples, measured_us, iterations)
         case @stats
           when :sd
-            Stats::SD.new(samples)
+            Stats::SD.new(samples, measured_us, iterations)
           when :bootstrap
             Stats::Bootstrap.new(samples, @confidence)
           else

lib/benchmark/ips/report.rb

@@ -90,7 +90,15 @@ def body
 
           case Benchmark::IPS.options[:format]
           when :human
-            left = ("%s (±%4.1f%%) i/s" % [Helpers.scale(@stats.central_tendency), @stats.error_percentage]).ljust(20)
+            central_tendency = Helpers.scale(@stats.central_tendency)
+            error_percentage = @stats.error_percentage
+            left =
+              if error_percentage > 99.9
+                "%s (± Inf%%) i/s" % [central_tendency]
+              else
+                "%s (±%4.1f%%) i/s" % [central_tendency, @stats.error_percentage]
+              end.ljust(20)
+
             iters = Helpers.scale(@iterations)
 
             if @show_total_time

lib/benchmark/ips/stats/sd.rb

@@ -6,9 +6,9 @@ class SD
         include StatsMetric
         attr_reader :error, :samples
 
-        def initialize(samples)
+        def initialize(samples, measured_us, iterations)
           @samples = samples
-          @mean = Timing.mean(samples)
+          @mean = Timing::MICROSECONDS_PER_SECOND * (iterations.to_f / measured_us)
           @error = Timing.stddev(samples, @mean).round
         end
 

lib/benchmark/timing.rb

@@ -4,9 +4,9 @@ module Timing
     # Microseconds per second.
     MICROSECONDS_PER_SECOND = 1_000_000
 
-    # Calculate (arithmetic) mean of given samples.
+    # Calculate arithmetic mean of given numbers.
     # @param [Array] samples Samples to calculate mean.
-    # @return [Float] Mean of given samples.
+    # @return [Float] Mean of given numbers.
     def self.mean(samples)
       sum = samples.inject(:+)
       sum / samples.size