adjust global buffer defaults

Author: roberttoyonaga

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jfr/JfrOptionSet.java

@@ -75,12 +75,28 @@ public JfrOptionSet() {
     }
 
     public void validateAndAdjustMemoryOptions() {
+        maybeAdjustDefaults();
         ensureValidMinimumSizes();
         ensureValidMemoryRelations();
         adjustMemoryOptions();
         assert checkPostCondition();
     }
 
+    /**
+     * We expect that Native Image executables will require lesser defaults than JFR on the JVM. So
+     * to reduce JFR native memory consumption, we use lower default values. We should not adjust
+     * default values too early (at build time) because if any settings are user defined, we
+     * shouldn't change any values.
+     */
+    private void maybeAdjustDefaults() {
+        if (!globalBufferSize.isUserValue() && !globalBufferCount.isUserValue() && !memorySize.isUserValue()) {
+            globalBufferSize.setValue(globalBufferSize.defaultValue / 2);
+            globalBufferCount.setValue(globalBufferCount.defaultValue / 2);
+            // Update record of total size
+            memorySize.setValue(globalBufferSize.getValue() * globalBufferCount.getValue());
+        }
+    }
+
     private void ensureValidMinimumSizes() {
         if (memorySize.isUserValue() && memorySize.getValue() < MIN_MEMORY_SIZE) {
             throw new IllegalStateException("The value specified for the JFR option 'memorysize' is too low. Please use at least " + MIN_MEMORY_SIZE + " bytes.");