Tweak per-interval chance calculation

Author: davidsymons-om

util/util.go

@@ -2,6 +2,7 @@ package util
 
 import (
 	"fmt"
+	"math"
 	"math/rand"
 	"strconv"
 	"strings"
@@ -17,7 +18,7 @@ const (
 	Kitchen24 = "15:04"
 	// a time format that just cares about the day and month.
 	YearDay = "Jan_2"
-
+	// default annotation prefix for configuration overrides
 	DefaultBaseAnnotation = "chaos.alpha.kubernetes.io"
 )
 
@@ -148,7 +149,15 @@ func ParseFrequency(text string, interval time.Duration) (float64, error) {
 		return 0, fmt.Errorf("unknown time period, %v", parts[1])
 	}
 
-	chance := (float64(interval) / float64(period)) * frequency
+	attempts := float64(period) / float64(interval) / frequency
+
+	// We want a 75% chance of a pod being terminated once in the given number of attempts
+	// Formula for calculating probability of something occurring once in "n" attempts...
+	//   p = 1 - math.Pow(1 - c, n)
+	// where p = probability, c = chance per attempt, n = number of attempts
+	// The equation below inverts that formula to calculate that percent chance we need
+	chance := 1 - math.Pow(1-0.75, (1/attempts))
+
 	return chance, nil
 }