Check JVM compressed oops option

even_better.md

try to stay below the threshold for zero-based compressed oops; the exact cutoff varies but 26 GB is safe on most systems, but can be as large as 30 GB on some systems. You can verify that you are under the limit by starting Elasticsearch with the JVM options -XX:+UnlockDiagnosticVMOptions -XX:+PrintCompressedOopsMode and looking for a line like the following

heap address: 0x000000011be00000, size: 27648 MB, zero based Compressed Oops

showing that zero-based compressed oops are enabled instead of

heap address: 0x0000000118400000, size: 28672 MB, Compressed Oops with base: 0x00000001183ff000

jvm_compressed_oops.sh

#!/bin/bash -ex

java -Xmx32766m -XX:+PrintFlagsFinal 2> /dev/null | grep UseCompressedOops

java -Xmx32767m -XX:+PrintFlagsFinal 2> /dev/null | grep UseCompressedOops

Memory.java

public class Memory {
 
     // Dummy Entity representing usual data objects
     private static class Entity {
         public String name;
         public String detail;
         public Double amount;
         public Integer age;
     }
 
     // Linked list offers table inserts and helps illustrating the issue by using multiple
     // references per entry
     public static java.util.LinkedList<Entity> entities = new java.util.LinkedList<>();
 
     // This threshold should be 2 times Xmn. It ensures the loop stops before a full GC happens.
     private static final int MB = 1024 * 1024;
     private static final int THRESHOLD = 100 * MB;
 
     public static void main(String... args) {
         System.out.println("Total Memory (in GB): " + Runtime.getRuntime().totalMemory() / 1024 / 1024 / 1024);
         System.out.println("Free Memory (in GB): " + Runtime.getRuntime().freeMemory() / 1024 / 1024 / 1024);
         System.out.println("Max Memory (in GB): " + Runtime.getRuntime().maxMemory() / 1024 / 1024 / 1024);
 
         while (true) {
             appendEntitiesToDataStructure();
             terminateBeforeFullGCorOOMEcanHappen();
         }
     }
 
     private static void appendEntitiesToDataStructure() {
         entities.add(new Entity());
     }
 
     private static void terminateBeforeFullGCorOOMEcanHappen() {
         if (Runtime.getRuntime().freeMemory() < THRESHOLD) {
             System.out.println("Elements created and added to LinkedList: " + entities.size());
             System.exit(0);
         }
     }
 
 }

run.sh

#!/bin/bash -ex

m=31 # change to 32 then compare the results

rm -rf *.class

javac Memory.java

java -Xms${m}g -Xmx${m}g -Xmn50m Memory