Changed memory size estimator to reflect optimized modulo

Signed-off-by: Peter Alfonsi <[email protected]>

server/src/main/java/org/opensearch/common/cache/tier/keystore/RBMIntKeyLookupStore.java

@@ -82,7 +82,7 @@ public int getValue() {
     protected final Lock readLock = lock.readLock();
     protected final Lock writeLock = lock.writeLock();
     private long mostRecentByteEstimate;
-    private final int REFRESH_SIZE_EST_INTERVAL = 10000;
+    protected final int REFRESH_SIZE_EST_INTERVAL = 10000;
     // Refresh size estimate every X new elements. Refreshes use the RBM's internal size estimator, which takes ~0.01 ms,
     // so we don't want to do it on every get(), and it doesn't matter much if there are +- 10000 keys in this store
     // in terms of storage impact
@@ -281,7 +281,14 @@ public boolean isCollision(Integer value1, Integer value2) {
     }
 
      static double getRBMSizeMultiplier(int numEntries, int modulo) {
-        double x = Math.log10((double) numEntries / modulo);
+        double effectiveModulo = (double) modulo / 2;
+        /* This model was created when we used % operator to calculate modulo. This has range (-modulo, modulo).
+        Now we have optimized to use a bitmask, which has range [0, modulo). So the number of possible values stored
+        is halved. */
+        if (modulo == 0) {
+            effectiveModulo = Math.pow(2, 32);
+        }
+        double x = Math.log10((double) numEntries / effectiveModulo);
         if (x < -5) {
             return 7.0;
         }

server/src/test/java/org/opensearch/common/cache/tier/keystore/RBMIntKeyLookupStoreTests.java

@@ -82,9 +82,13 @@ public void testTransformationLogic() throws Exception {
 
     public void testContains() throws Exception {
         RBMIntKeyLookupStore kls = new RBMIntKeyLookupStore(RBMIntKeyLookupStore.KeystoreModuloValue.TWO_TO_TWENTY_NINE, 0L);
-        for (int i = 0; i < 2000; i++) {
+        RBMIntKeyLookupStore noModuloKls = new RBMIntKeyLookupStore(RBMIntKeyLookupStore.KeystoreModuloValue.NONE, 0L);
+        for (int i = 0; i < kls.REFRESH_SIZE_EST_INTERVAL + 1000; i++) {
+            // set upper bound > number of elements to trigger a size check, ensuring we test that too
             kls.add(i);
             assertTrue(kls.contains(i));
+            noModuloKls.add(i);
+            assertTrue(noModuloKls.contains(i));
         }
     }