--- a/test/jdk/jdk/jfr/event/gc/collection/TestG1ParallelPhases.java	Tue Nov 12 16:13:16 2019 +0100
+++ b/test/jdk/jdk/jfr/event/gc/collection/TestG1ParallelPhases.java	Tue Nov 12 16:28:26 2019 +0100
@@ -176,6 +176,9 @@
     public static void provokeMixedGC(int g1HeapRegionSize) {
         final var arraySize = 20_000;
         var liveOldObjects = new ArrayList<byte[]>();
+
+        // Make sure the heap is in a known state.
+        getWhiteBox().fullGC();
         allocateOldObjects(liveOldObjects, g1HeapRegionSize, arraySize);
         waitTillCMCFinished(10);
         getWhiteBox().g1StartConcMarkCycle();