On my large TR 3970X, jtreg asks for MaxRAMFraction=0, which makes ZGC use 128M heap for some tests. These tests reliably fail with OOM in jdk-updates/jdk11u-dev:
 compiler/gcbarriers/UnsafeIntrinsicsTest.java
 compiler/loopopts/TestRangeCheckPredicatesControl.java

These tests are in tier1.

They are not failing in jdk/jdk, possibly due to JDK-8234338 improvements. Since these ZGC improvements are not easily backportable, I think setting the explicit heap size for these tests in 11u is the way to resolve tier1 failures:

diff -r ddbe957d9ff3 test/hotspot/jtreg/compiler/gcbarriers/UnsafeIntrinsicsTest.java
--- a/test/hotspot/jtreg/compiler/gcbarriers/UnsafeIntrinsicsTest.java Mon May 25 11:05:23 2020 +0200
+++ b/test/hotspot/jtreg/compiler/gcbarriers/UnsafeIntrinsicsTest.java Tue Aug 18 14:31:48 2020 +0200
@@ -25,11 +25,11 @@
  * @test
  * @bug 8059022
  * @modules java.base/jdk.internal.misc:+open
  * @summary Validate barriers after Unsafe getObject, CAS and swap (GetAndSet)
  * @requires vm.gc.Z & !vm.graal.enabled
- * @run main/othervm -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -XX:+UnlockDiagnosticVMOptions -XX:+ZUnmapBadViews -XX:ZCollectionInterval=1 -XX:-CreateCoredumpOnCrash -XX:CompileCommand=dontinline,*::mergeImpl* compiler.gcbarriers.UnsafeIntrinsicsTest
+ * @run main/othervm -Xmx256m -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -XX:+UnlockDiagnosticVMOptions -XX:+ZUnmapBadViews -XX:ZCollectionInterval=1 -XX:-CreateCoredumpOnCrash -XX:CompileCommand=dontinline,*::mergeImpl* compiler.gcbarriers.UnsafeIntrinsicsTest
  */
 
 package compiler.gcbarriers;
 
 import java.lang.reflect.Field;
diff -r ddbe957d9ff3 test/hotspot/jtreg/compiler/loopopts/TestRangeCheckPredicatesControl.java
--- a/test/hotspot/jtreg/compiler/loopopts/TestRangeCheckPredicatesControl.java Mon May 25 11:05:23 2020 +0200
+++ b/test/hotspot/jtreg/compiler/loopopts/TestRangeCheckPredicatesControl.java Tue Aug 18 14:31:48 2020 +0200
@@ -25,12 +25,12 @@
  * @test
  * @requires vm.gc.Z & !vm.graal.enabled
  * @bug 8237859
  * @summary A LoadP node has a wrong control input (too early) which results in an out-of-bounds read of an object array with ZGC.
  *
- * @run main/othervm -XX:+UnlockExperimentalVMOptions -XX:+UseZGC compiler.loopopts.TestRangeCheckPredicatesControl
- * @run main/othervm -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -XX:+UnlockDiagnosticVMOptions -XX:+StressGCM compiler.loopopts.TestRangeCheckPredicatesControl
+ * @run main/othervm -Xmx256m -XX:+UnlockExperimentalVMOptions -XX:+UseZGC compiler.loopopts.TestRangeCheckPredicatesControl
+ * @run main/othervm -Xmx256m -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -XX:+UnlockDiagnosticVMOptions -XX:+StressGCM compiler.loopopts.TestRangeCheckPredicatesControl
  */
 
 package compiler.loopopts;