[ML] Impose a minimum on the automatically calculated JVM size

This change fixes a discrepancy that has existed for a long time
but was revealed by elastic#91694. The ML automatic node/JVM sizing code
contained a minimum node size but did not restrict the minimum
JVM size to the size that would be chosen on that minimum node
size. This could throw off calculations at small scale.

Fixes elastic#91728

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/utils/NativeMemoryCalculator.java

@@ -40,6 +40,8 @@ public final class NativeMemoryCalculator {
     // Must match the value used in MachineDependentHeap.MachineNodeRole.ML_ONLY.
     public static final long JVM_SIZE_KNOT_POINT = ByteSizeValue.ofGb(16).getBytes();
     private static final long BYTES_IN_4MB = ByteSizeValue.ofMb(4).getBytes();
+    // The minimum automatic node size implicitly defines a minimum JVM size
+    private static final long MINIMUM_AUTOMATIC_JVM_SIZE = dynamicallyCalculateJvmSizeFromNodeSize(MINIMUM_AUTOMATIC_NODE_SIZE);
 
     private NativeMemoryCalculator() {}
 
@@ -165,7 +167,7 @@ public static long dynamicallyCalculateJvmSizeFromNodeSize(long nodeSize) {
         );
     }
 
-    public static long dynamicallyCalculateJvmSizeFromMlNativeMemorySize(long nativeMachineMemory) {
+    public static long dynamicallyCalculateJvmSizeFromMlNativeMemorySize(long mlNativeMemorySize) {
         // For <= 16GB node, the JVM is 0.4 * total_node_size. This means the rest is 0.6 the node size.
         // So, nativeAndOverhead = 0.6 * total_node_size => total_node_size = (nativeAndOverhead / 0.6)
         // Consequently jvmSize = (nativeAndOverhead / 0.6) * 0.4 = nativeAndOverhead * 2 / 3
@@ -177,7 +179,7 @@ public static long dynamicallyCalculateJvmSizeFromMlNativeMemorySize(long native
         //
         // In both cases JVM size is rounded down to the next lower multiple of 4 megabytes to match
         // MachineDependentHeap.MachineNodeRole.ML_ONLY.
-        long nativeAndOverhead = nativeMachineMemory + OS_OVERHEAD;
+        long nativeAndOverhead = mlNativeMemorySize + OS_OVERHEAD;
         long higherAnswer;
         if (nativeAndOverhead <= (JVM_SIZE_KNOT_POINT - dynamicallyCalculateJvmSizeFromNodeSize(JVM_SIZE_KNOT_POINT))) {
             higherAnswer = (nativeAndOverhead * 2 / 3 / BYTES_IN_4MB) * BYTES_IN_4MB;
@@ -196,10 +198,10 @@ public static long dynamicallyCalculateJvmSizeFromMlNativeMemorySize(long native
             long lowerAnswer = higherAnswer - BYTES_IN_4MB;
             long nodeSizeImpliedByLowerAnswer = nativeAndOverhead + lowerAnswer;
             if (dynamicallyCalculateJvmSizeFromNodeSize(nodeSizeImpliedByLowerAnswer) == lowerAnswer) {
-                return Math.min(lowerAnswer, STATIC_JVM_UPPER_THRESHOLD);
+                return Math.max(MINIMUM_AUTOMATIC_JVM_SIZE, Math.min(lowerAnswer, STATIC_JVM_UPPER_THRESHOLD));
             }
         }
-        return Math.min(higherAnswer, STATIC_JVM_UPPER_THRESHOLD);
+        return Math.max(MINIMUM_AUTOMATIC_JVM_SIZE, Math.min(higherAnswer, STATIC_JVM_UPPER_THRESHOLD));
     }
 
     /**