RFR (round 1), JDK-8214259: Implementation: JEP 189: Shenandoah: A Low-Pause Garbage Collector

Roman Kennke rkennke at redhat.com
Tue Nov 27 09:37:57 UTC 2018

Hi Vladimir,

> You need to check if shenandoahgc is disabled first - check
> DISABLED_JVM_FEATURES list (see code for jvmci).

Why? If Shenandoah is disabled, it will be on this list, and therefore
not be built (see JvmFeatures.gmk).

> Do you support 32-bit x86? You have OPENJDK_TARGET_CPU == xx86 check.
> Do you support all x86 OSs?

We can build with 32bit, but it will not run. It seems indeed curious to
enable it. It's probably only there to allow 32bit builds with
Shenandoah enabled, just to verify that we have all the relevant LP64
checks in code in place. I will check it with my collegues.

> Why you set VM CFLAGS when shenandoahgc is not enabled? It is in
> JvmFeatures.gmk.

This *disables* and excludes Shenandoah if not enabled.

+ifneq ($(call check-jvm-feature, shenandoahgc), true)
+  JVM_EXCLUDE_PATTERNS += gc/shenandoah

pretty much the same pattern as zgc and other features.

and then we add some flags when Shenandoah is enabled:


... which are required for building with Shenandoah enabled.

> I looked on C2 changes. It has INCLUDE_SHENANDOAHGC, checks and new gc
> specific nodes. This looks intrusive. I hope you clean it up.

There are a few places with INCLUDE_SHENANDOAHGC checks where it seemed
excessive to add a whole API just for a tiny, very GC specific check. We
still do have intrusive changes in loop*, which we are working on to
resolve. We declare+define all our GC specific nodes in
shenandoahBarrierSetC2.hpp and related Shenandoah-specific files. The
only things in shared code is the usual declarations in classes.hpp/cpp
and node.hpp to get is_ShenandoahBarrier() an similar queries. This
seems in-line with what other GCs do (look for e.g. LoadBarrier). Please
be specific which changes in opto you'd like to see resolved (and don't
look at loop* source files at this point ;-) ).

Thanks for reviewing!

> Thanks,
> Vladimir
> On 11/26/18 2:47 PM, Erik Joelsson wrote:
>> Build changes look ok to me.
>> /Erik
>> On 2018-11-26 13:39, Roman Kennke wrote:
>>> Hi,
>>> This is the first round of changes for including Shenandoah GC into
>>> mainline.
>>> I divided the review into parts that roughly correspond to the
>>> mailing lists
>>> that would normally review it, and I divided it into 'shared' code
>>> changes and
>>> 'shenandoah' code changes (actually, mostly additions). The intend is to
>>> eventually
>>> push them as single 'combined' changeset, once reviewed.
>>> JEP:
>>>    https://openjdk.java.net/jeps/189
>>> Bug entry:
>>>   https://bugs.openjdk.java.net/browse/JDK-8214259
>>> Webrevs:
>>>    http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/
>>> For those who want to see the full change, have a look at the
>>> shenandoah-complete
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shenandoah-complete/>
>>> directory,
>>> it contains the full combined webrev. Alternatively, there is the file
>>> shenandoah-master.patch
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shenandoah-master.patch>,
>>> which is what I intend to commit (and which should be equivalent to the
>>> 'shenandoah-complete' webrev).
>>> Sections to review (at this point) are the following:
>>>   *) shenandoah-gc
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shenandoah-gc/>
>>>      - Actual Shenandoah implementation, almost completely residing in
>>> gc/shenandoah
>>>   *) shared-gc
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shared-gc/>
>>>      - This is mostly boilerplate that is common to any GC
>>>      - referenceProcessor.cpp has a little change to make one assert not
>>> fail (next to CMS and G1)
>>>      - taskqueue.hpp has some small adjustments to enable subclassing
>>>   *) shared-serviceability
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shared-serviceability/>
>>>      - The usual code to support another GC
>>>   *) shared-runtime
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shared-runtime/>
>>>      - A number of friends declarations to allow Shenandoah iterators to
>>> hook up with,
>>>        e.g. ClassLoaderData, CodeCache, etc
>>>      - Warning and disabling JFR LeakProfiler
>>>      - fieldDescriptor.hpp added is_stable() accessor, for use in
>>> Shenandoah C2 optimizations
>>>      - Locks initialization in mutexLocker.cpp as usual
>>>      - VM operations defines for Shenandoah's VM ops
>>>      - globalDefinitions.hpp added UINT64_FORMAT_HEX_W for use in
>>> Shenandoah's logging
>>>      - The usual macros in macro.hpp
>>>   *) shared-build
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shared-build/>
>>>      - Add shenandoah feature, enabled by default, as agreed with
>>> Vladimir K. beforehand
>>>      - Some flags for shenandoah-enabled compilation to get
>>>        and SUPPORT_NOT_TO_SPACE_INVARIANT which is required for
>>> Shenandoah's barriers
>>>      - --param inline-unit-growth=1000 settings for 2 shenandoah source
>>> files, which is
>>>        useful to get the whole marking loop inlined (observed
>>> significant
>>> regression if we
>>>        don't)
>>>   *) shared-tests
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shared-tests/>
>>>      - Test infrastructure to support Shenandoah
>>>      - Shenandoah test groups
>>>      - Exclude Shenandoah in various tests that can be run with
>>> selected GC
>>>      - Enable/add configure for Shenandoah for tests that make sense to
>>> run with it
>>>   *) shenandoah-tests
>>> <http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/00/shenandoah-tests/>
>>>      - Shenandoah specific tests, most reside in gc/shenandoah
>>> subdirectory
>>>      - A couple of tests configurations have been added, e.g.
>>> TestGCBasherWithShenandoah.java
>>> I intentionally left out shared-compiler for now, because we have some
>>> work left to do
>>> there, but if you click around you'll find the patch anyway, in case you
>>> want to take
>>> a peek at it.
>>> We have regular builds on:
>>>    - {Linux} x {x86_64, x86_32, armhf, aarch64, ppc64el, s390x}
>>>    - {Windows} x {x86_64},
>>>    - {MacOS X} x {x86_64}
>>> This also routinely passes:
>>>    - the new Shenandoah tests
>>>    - jcstress with/without aggressive Shenandoah verification
>>>    - specjvm2008 with/without aggressive Shenandoah verification
>>> I'd like to thank my collegues at Red Hat: Christine Flood, she deserves
>>> the credit for being the original inventor of Shenandoah, Aleksey
>>> Shiplëv, Roland Westrelin & Zhengyu Gu for their countless
>>> contributions, everybody else in Red Hat's OpenJDK team for testing,
>>> advice and support, my collegues in Oracle's GC, runtime and compiler
>>> teams for tirelessly helping with and reviewing all the GC interface and
>>> related changes, and of course the many early adopters for reporting
>>> bugs and success stories and feature requests: we wouldn't be here
>>> without any of you!
>>> Best regards,
>>> Roman

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