[ML] Modify thresholds for normalization triggers (#33663)

[ML] Modify thresholds for normalization triggers

The (arbitrary) threshold factors used to judge if scores have
changed significantly enough to trigger a look-back renormalization have
been changed to values that reduce the frequency of such
renormalizations.

Added a clause to treat changes in scores as a 'big change' if it would
result in a change of severity reported in the UI.

Also altered the clause affecting small scores so that a change should
be considered big if scores have changed by at least 1.5. 

Relates elastic/machine-learning-qa#263

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/job/process/normalizer/Normalizer.java

@@ -190,23 +190,34 @@ private double mergeRecursively(Iterator<NormalizerResult> scoresIter, Normaliza
      * Encapsulate the logic for deciding whether a change to a normalized score
      * is "big".
      * <p>
-     * Current logic is that a big change is a change of at least 1 or more than
-     * than 50% of the higher of the two values.
+     * Current logic is that a change is considered big if any of the following criteria are met:
+     * <ul>
+     * <li>the change  would result in a change of colour in the UI
+     * (e.g. severity would be changed from WARNING to MINOR)</li>
+     * <li>the change is at least 1.5</li>
+     * <li>the change in values is greater than 67% of the higher of the two values.</li>
+     * </ul>
+     * These values have been chosen through a process of experimentation, in particular it was desired to reduce
+     * the number of updates written to the results index due to renormalization events for performance reasons
+     * while not changing the normalized scores greatly
      *
      * @param oldVal The old value of the normalized score
      * @param newVal The new value of the normalized score
      * @return true if the update is considered "big"
      */
     private static boolean isBigUpdate(double oldVal, double newVal) {
-        if (Math.abs(oldVal - newVal) >= 1.0) {
+        if ((int) (oldVal / 25.0) != (int) (newVal / 25.0)) {
+            return true;
+        }
+        if (Math.abs(oldVal - newVal) >= 1.5) {
             return true;
         }
         if (oldVal > newVal) {
-            if (oldVal * 0.5 > newVal) {
+            if (oldVal * 0.33 > newVal) {
                 return true;
             }
         } else {
-            if (newVal * 0.5 > oldVal) {
+            if (newVal * 0.33 > oldVal) {
                 return true;
             }
         }

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/job/process/normalizer/NormalizerTests.java

@@ -29,7 +29,7 @@ public class NormalizerTests extends ESTestCase {
     private static final String INDEX_NAME = "foo-index";
     private static final String QUANTILES_STATE = "someState";
     private static final int BUCKET_SPAN = 600;
-    private static final double INITIAL_SCORE = 2.0;
+    private static final double INITIAL_SCORE = 3.0;
     private static final double FACTOR = 2.0;
 
     private Bucket generateBucket(Date timestamp) throws IOException {