8209920: runtime/logging/RedefineClasses.java fail with OOME with ZGC

[Per Liden]

Hi Leonid,

On 08/24/2018 10:44 PM, Leonid Mesnik wrote:
> Hi
> 
> Could you please review following fix. I think that fix could be 
> considered as trivial. So I would like to push it once I get Review to 
> avoid failures in tier3.
> bug: https://bugs.openjdk.java.net/browse/JDK-8209920
> diff:
> 
> diff -r 9d01ad46daef test/hotspot/jtreg/runtime/logging/RedefineClasses.java
> --- a/test/hotspot/jtreg/runtime/logging/RedefineClasses.java   Fri Aug 
> 24 15:49:21 2018 -0400
> +++ b/test/hotspot/jtreg/runtime/logging/RedefineClasses.java   Fri Aug 
> 24 13:23:17 2018 -0700
> @@ -26,6 +26,7 @@
>    * @bug 8197901 8209758
>    * @summary Redefine classes with enabling logging to verify 
> Klass:external_name() during GC.
>    * @comment This test is simplified version of 
> runtime/RedefineTests/RedefineRunningMethods.java.
> + * @requires vm.opt.final.ClassUnloading
>    * @library /test/lib
>    * @modules java.compiler
>    *          java.instrument
> 

I don't think this addresses the root cause. The test fails because 
we're running out of heap, not because we didn't unload classes. In 
other words, the test would still fail if you were running ZGC with 
class unloading enabled.

The problem is that the test is given 256m of heap, and the obj[] array 
keeps 20 * 10M alive. This will actually occupy 7*32M = 224M of medium 
ZPages on the heap, which leaves 32M for the rest. However, ZGC's heap 
reserve is 32M + 2M * nworkers, which means we're out of memory.

So, instead of adding the vm.opt.final.ClassUnloading attribute, I'd 
suggest we reduce the size of the obj[] array to 10 instead of 20. That 
would leave enough heap available for the test to pass.

diff --git a/test/hotspot/jtreg/runtime/logging/RedefineClasses.java 
b/test/hotspot/jtreg/runtime/logging/RedefineClasses.java
--- a/test/hotspot/jtreg/runtime/logging/RedefineClasses.java
+++ b/test/hotspot/jtreg/runtime/logging/RedefineClasses.java
@@ -42,7 +42,7 @@
  }

  public class RedefineClasses {
-    static Object[] obj = new Object[20];
+    static Object[] obj = new Object[10];
      public static String newB =
              "class RedefineClasses_B {" +
              "    public static void test() { " +
@@ -52,7 +52,7 @@
      public static void main(String[] args) throws Exception {
          RedefineClassHelper.redefineClass(RedefineClasses_B.class, newB);
          RedefineClasses_B.test();
-        for (int i = 0; i < 20 ; i++) {
+        for (int i = 0; i < obj.length; i++) {
              obj[i] = new byte[10 * 1024 * 1024];
              System.gc();
          }

On the other hand, it's not clear to me what the alloc+System.gc() 
for-loop is trying to accomplish and why it is needed. I can remove the 
whole for-loop and the test passes anyway, regardless of which GC i 
pick. Am I missing something?

cheers,
Per