In certain JDK build configurations the binary files of the JDK can become quite large. One example is libjvm.so (the largest that I know of). If, for example the JDK is being built with --with-native-debug-symbols=internal in a fastdebug configuration, libjvm.so can become as large as 750MB on a x86_64 Linux system.

In order to generate a JEP 493 enabled runtime, the JimageDiffGenerator comes into play which compares all files originating from JMOD source to the optimized versions that have been processed by the jlink plug-in pipeline. Each and every file is being compared byte-by-byte and the current implementation is too memory hungry since it reads in both copies (of a file) into memory and then compares them using Arrays.equals().

This can cause jlink/runtimeImage test failures on some systems as witnessed below (and also in JDK-8344036). Note that for default JDK builds tests in runtimeImage folder generate a linkable runtime on-the-fly and then run the tests.

On linux-aarch64 platform: $ make test TEST="test/jdk/tools/jlink/runtimeImage"

Failing tests:
TEST: tools/jlink/runtimeImage/AddOptionsTest.java
TEST: tools/jlink/runtimeImage/BasicJlinkMissingJavaBase.java
TEST: tools/jlink/runtimeImage/BasicJlinkTest.java
TEST: tools/jlink/runtimeImage/CustomModuleJlinkTest.java
TEST: tools/jlink/runtimeImage/GenerateJLIClassesTest.java
TEST: tools/jlink/runtimeImage/KeepPackagedModulesFailTest.java
TEST: tools/jlink/runtimeImage/JavaSEReproducibleTest.java
TEST: tools/jlink/runtimeImage/ModifiedFilesExitTest.java
TEST: tools/jlink/runtimeImage/ModifiedFilesWarningTest.java
TEST: tools/jlink/runtimeImage/MultiHopTest.java
TEST: tools/jlink/runtimeImage/PackagedModulesVsRuntimeImageLinkTest.java
TEST: tools/jlink/runtimeImage/PatchedJDKModuleJlinkTest.java
TEST: tools/jlink/runtimeImage/SystemModulesTest.java
TEST: tools/jlink/runtimeImage/SystemModulesTest2.java

On linux-aarch64 platform: $ make test TEST="tools/jlink/runtimeImage/AddOptionsTest.java"

```
STDOUT:
STDERR:
jlink options: --output java-base-with-opts-jlink-tmp --add-modules jdk.jlink,java.base --generate-linkable-runtime --keep-packaged-modules=java-base-with-opts-jlink-tmp/jmods
Error: Java heap space
java.lang.OutOfMemoryError: Java heap space
        at java.base/java.io.InputStream.readNBytes(InputStream.java:447)
        at java.base/java.io.InputStream.readAllBytes(InputStream.java:348)
        at jdk.jlink/jdk.tools.jlink.plugin.ResourcePoolEntry.contentBytes(ResourcePoolEntry.java:127)
        at jdk.jlink/jdk.tools.jlink.internal.runtimelink.ResourcePoolReader.getResourceBytes(ResourcePoolReader.java:56)
        at jdk.jlink/jdk.tools.jlink.internal.runtimelink.JimageDiffGenerator.generateDiff(JimageDiffGenerator.java:91)
        at jdk.jlink/jdk.tools.jlink.internal.ImageFileCreator.addResourceDiffFiles(ImageFileCreator.java:333)
        at jdk.jlink/jdk.tools.jlink.internal.ImageFileCreator.generateJImage(ImageFileCreator.java:240)
        at jdk.jlink/jdk.tools.jlink.internal.ImageFileCreator.writeImage(ImageFileCreator.java:195)
        at jdk.jlink/jdk.tools.jlink.internal.ImageFileCreator.create(ImageFileCreator.java:126)
        at jdk.jlink/jdk.tools.jlink.internal.JlinkTask$ImageHelper.retrieve(JlinkTask.java:1015)
        at jdk.jlink/jdk.tools.jlink.internal.ImagePluginStack.operate(ImagePluginStack.java:194)
        at jdk.jlink/jdk.tools.jlink.internal.JlinkTask.createImage(JlinkTask.java:483)
        at jdk.jlink/jdk.tools.jlink.internal.JlinkTask.run(JlinkTask.java:293)
        at jdk.jlink/jdk.tools.jlink.internal.Main.run(Main.java:50)
        at jdk.jlink/jdk.tools.jlink.internal.Main$JlinkToolProvider.run(Main.java:67)
        at tests.JImageGenerator$JLinkTask.call(JImageGenerator.java:713)
        at AbstractLinkableRuntimeTest.createRuntimeLinkImage(AbstractLinkableRuntimeTest.java:274)
        at AbstractLinkableRuntimeTest.createJavaImageRuntimeLink(AbstractLinkableRuntimeTest.java:126)
        at AbstractLinkableRuntimeTest.createJavaImageRuntimeLink(AbstractLinkableRuntimeTest.java:121)
        at AddOptionsTest.runTest(AddOptionsTest.java:64)
        at AbstractLinkableRuntimeTest.run(AbstractLinkableRuntimeTest.java:62)
        at AddOptionsTest.main(AddOptionsTest.java:49)
        at java.base/java.lang.invoke.LambdaForm$DMH/0x000000f001002000.invokeStatic(LambdaForm$DMH)
        at java.base/java.lang.invoke.LambdaForm$MH/0x000000f001003400.invoke(LambdaForm$MH)
        at java.base/java.lang.invoke.Invokers$Holder.invokeExact_MT(Invokers$Holder)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invokeImpl(DirectMethodHandleAccessor.java:155)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
        at java.base/java.lang.reflect.Method.invoke(Method.java:565)
        at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
        at java.base/java.lang.Thread.runWith(Thread.java:1460)
        at java.base/java.lang.Thread.run(Thread.java:1447)

java.lang.AssertionError: Unexpected failure: 4
        at tests.Result.assertSuccess(Result.java:80)
        at AbstractLinkableRuntimeTest.createRuntimeLinkImage(AbstractLinkableRuntimeTest.java:274)
        at AbstractLinkableRuntimeTest.createJavaImageRuntimeLink(AbstractLinkableRuntimeTest.java:126)
        at AbstractLinkableRuntimeTest.createJavaImageRuntimeLink(AbstractLinkableRuntimeTest.java:121)
        at AddOptionsTest.runTest(AddOptionsTest.java:64)
        at AbstractLinkableRuntimeTest.run(AbstractLinkableRuntimeTest.java:62)
        at AddOptionsTest.main(AddOptionsTest.java:49)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
        at java.base/java.lang.reflect.Method.invoke(Method.java:565)
        at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
        at java.base/java.lang.Thread.run(Thread.java:1447)

JavaTest Message: Test threw exception: java.lang.AssertionError: Unexpected failure: 4
JavaTest Message: shutting down test

STATUS:Failed.`main' threw exception: java.lang.AssertionError: Unexpected failure: 4
```