Reduce ML_ONLY heap size, so that direct memory is accounted for.

Author: jan-elastic

distribution/tools/server-cli/src/main/java/org/elasticsearch/server/cli/MachineDependentHeap.java

@@ -98,10 +98,17 @@ protected int getHeapSizeMb(Settings nodeSettings, MachineNodeRole role, long av
              * could result in ML processes crashing with OOM errors or repeated autoscaling up and down.
              */
             case ML_ONLY -> {
+                /*
+                 * An ML node used to have 40% of the total memory for the Java heap, and the remainder
+                 * for ML and overhead (the operating system). This did not account for the Java direct
+                 * memory, which equals half of the heap size (see JvmErgonomics).
+                 * Right now, a factor of 2/3 is applied to the heap size, leaving the ML memory formula
+                 * the same. That means it now also accounts for direct memory.
+                 */
                 if (availableMemory <= (GB * 16)) {
-                    yield mb((long) (availableMemory * .4), 4);
+                    yield mb((long) (availableMemory * .4 * 2/3), 4);
                 } else {
-                    yield mb((long) min((GB * 16) * .4 + (availableMemory - GB * 16) * .1, MAX_HEAP_SIZE), 4);
+                    yield mb((long) min(((GB * 16) * .4 + (availableMemory - GB * 16) * .1) * 2/3, MAX_HEAP_SIZE), 4);
                 }
             }
             /*