[ML] Fix incorrect assumption about minimum ML node size (#91694) (#9…

…1697)

The ML autoscaling code was making an assumption that all ML
nodes in Cloud will be at least 1GB. This is not correct. After
allowing for logging and metrics collection it is possible for
ML nodes to be smaller.

This PR updates the assumption to 0.5GB.

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

@@ -34,7 +34,7 @@ public final class NativeMemoryCalculator {
     // Maximum permitted JVM heap size when auto-configured.
     // Must match the value used in MachineDependentHeap.MachineNodeRole.ML_ONLY.
     public static final long STATIC_JVM_UPPER_THRESHOLD = ByteSizeValue.ofGb(31).getBytes();
-    public static final long MINIMUM_AUTOMATIC_NODE_SIZE = ByteSizeValue.ofGb(1).getBytes();
+    public static final long MINIMUM_AUTOMATIC_NODE_SIZE = ByteSizeValue.ofMb(512).getBytes();
     private static final long OS_OVERHEAD = ByteSizeValue.ofMb(200).getBytes();
     // Memory size beyond which the JVM is given 10% of memory instead of 40%.
     // Must match the value used in MachineDependentHeap.MachineNodeRole.ML_ONLY.

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/autoscaling/MlMemoryAutoscalingDeciderTests.java

@@ -1217,8 +1217,8 @@ public void testScale_WithNoMlNodesButWaitingAnalytics() {
                 "requesting scale up as number of jobs in queues exceeded configured limit and there are no machine learning nodes"
             )
         );
-        assertThat(result.nodeSize(), equalTo(ByteSizeValue.ofGb(1)));
-        assertThat(result.tierSize(), equalTo(ByteSizeValue.ofGb(1)));
+        assertThat(result.nodeSize(), equalTo(ByteSizeValue.ofMb(714)));
+        assertThat(result.tierSize(), equalTo(ByteSizeValue.ofMb(714)));
     }
 
     private MlMemoryAutoscalingDecider buildDecider() {

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/utils/NativeMemoryCalculatorTests.java

@@ -108,15 +108,15 @@ public void testConsistencyInAutoCalculation() {
                     NativeMemoryCapacity nativeMemoryCapacity = new NativeMemoryCapacity(bytesForML, bytesForML, jvmSize);
 
                     MlMemoryAutoscalingCapacity capacity = nativeMemoryCapacity.autoscalingCapacity(30, true, Long.MAX_VALUE, 1).build();
-                    // We don't allow node sizes below 1GB, so we will always be at least that large
+                    // We don't allow node sizes below 0.5GB, so we will always be at least that large
                     // Also, allow 1 byte off for weird rounding issues
                     assertThat(
                         capacity.nodeSize().getBytes(),
-                        greaterThanOrEqualTo(Math.max(nodeSize, ByteSizeValue.ofGb(1).getBytes()) - 1L)
+                        greaterThanOrEqualTo(Math.max(nodeSize, ByteSizeValue.ofMb(512).getBytes()) - 1L)
                     );
                     assertThat(
                         capacity.tierSize().getBytes(),
-                        greaterThanOrEqualTo(Math.max(nodeSize, ByteSizeValue.ofGb(1).getBytes()) - 1L)
+                        greaterThanOrEqualTo(Math.max(nodeSize, ByteSizeValue.ofMb(512).getBytes()) - 1L)
                     );
                 }
             }