On Thu, 22 Aug 2024 20:08:43 GMT, Roman Kennke <[email protected]> wrote:
>> This is the main body of the JEP 450: Compact Object Headers (Experimental).
>>
>> It is also a follow-up to #20640, which now also includes (and supersedes)
>> #20603 and #20605, plus the Tiny Class-Pointers parts that have been
>> previously missing.
>>
>> Main changes:
>> - Introduction of the (experimental) flag UseCompactObjectHeaders. All
>> changes in this PR are protected by this flag. The purpose of the flag is to
>> provide a fallback, in case that users unexpectedly observe problems with
>> the new implementation. The intention is that this flag will remain
>> experimental and opt-in for at least one release, then make it on-by-default
>> and diagnostic (?), and eventually deprecate and obsolete it. However, there
>> are a few unknowns in that plan, specifically, we may want to further
>> improve compact headers to 4 bytes, we are planning to enhance the Klass*
>> encoding to support virtually unlimited number of Klasses, at which point we
>> could also obsolete UseCompressedClassPointers.
>> - The compressed Klass* can now be stored in the mark-word of objects. In
>> order to be able to do this, we are add some changes to GC forwarding (see
>> below) to protect the relevant (upper 22) bits of the mark-word. Significant
>> parts of this PR deal with loading the compressed Klass* from the mark-word.
>> This PR also changes some code paths (mostly in GCs) to be more careful when
>> accessing Klass* (or mark-word or size) to be able to fetch it from the
>> forwardee in case the object is forwarded.
>> - Self-forwarding in GCs (which is used to deal with promotion failure) now
>> uses a bit to indicate 'self-forwarding'. This is needed to preserve the
>> crucial Klass* bits in the header. This also allows to get rid of
>> preserved-header machinery in SerialGC and G1 (Parallel GC abuses
>> preserved-marks to also find all other relevant oops).
>> - Full GC forwarding now uses an encoding similar to compressed-oops. We
>> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB,
>> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the
>> GC forwarding at all).
>> - Instances can now have their base-offset (the offset where the field
>> layouter starts to place fields) at offset 8 (instead of 12 or 16).
>> - Arrays will now store their length at offset 8.
>> - CDS can now write and read archives with the compressed header. However,
>> it is not possible to read an archive that has been written with an opposite
>> setting of UseCompactObjectHeaders. Some build machinery is added so that
>> _co...
>
> Roman Kennke has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Fix bit counts in GCForwarding
test/hotspot/jtreg/runtime/cds/appcds/TestZGCWithCDS.java line 59:
> 57: public static void main(String... args) throws Exception {
> 58: String zGenerational = args[0];
> 59: String compactHeaders = "-XX:" +
> (zGenerational.equals("-XX:+ZGenerational") ? "+" : "-") +
> "UseCompactObjectHeaders";
The test failing with
stdout: [[0.176s][info][cds] trying to map
/opt/mach5/mesos/work_dir/slaves/a20696e7-ae7d-4d37-8e9c-83f99ef002cb-S2261/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/f0801999-993f-4e08-b017-08b33a8ec44f/runs/34cc555e-ae8f-4a48-8175-e998194f204b/testoutput/test-support/jtreg_open_test_hotspot_jtreg_hotspot_cds_relocation/scratch/5/appcds-18h50m16s773.jsa
[0.176s][info][cds] Opened archive
/opt/mach5/mesos/work_dir/slaves/a20696e7-ae7d-4d37-8e9c-83f99ef002cb-S2261/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/f0801999-993f-4e08-b017-08b33a8ec44f/runs/34cc555e-ae8f-4a48-8175-e998194f204b/testoutput/test-support/jtreg_open_test_hotspot_jtreg_hotspot_cds_relocation/scratch/5/appcds-18h50m16s773.jsa.
[0.176s][info][cds] Archive was created with UseCompressedOops = 0,
UseCompressedClassPointers = 1
[0.176s][info][cds] The shared archive file's UseCompactObjectHeaders setting
(enabled) does not equal the current UseCompactObjectHeaders setting (disabled).
[0.176s][info][cds] Initialize static archive failed.
[0.176s][info][cds] Unable to map shared spaces
[0.176s][error][cds] An error has occurred while processing the shared archive
file.
[0.176s][error][cds] Unable to map shared spaces
Error occurred during initialization of VM
Unable to use shared archive.
];
stderr: []
exitValue = 1
java.lang.RuntimeException: 'Hello World' missing from stdout/stderr
at
jdk.test.lib.process.OutputAnalyzer.shouldContain(OutputAnalyzer.java:252)
at TestZGCWithCDS.main(TestZGCWithCDS.java:123)
at
java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:573)
at
com.sun.javatest.regtest.agent.MainActionHelper$AgentVMRunnable.run(MainActionHelper.java:333)
at java.base/java.lang.Thread.run(Thread.java:1575)
JavaTest Message: Test threw exception: java.lang.RuntimeException
JavaTest Message: shutting down test
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1729404477
