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

Vladimir Kozlov vladimir.kozlov at oracle.com
Thu Nov 29 00:55:02 UTC 2018

About C2 changes.

.ad files. I am thinking may be we should put new code in separate <arch>_shenandoah.ad files and merge them only if 
Shenandoah is included in built [1]. I don't like #ifdefs - you still generate this mach nodes even without Shenandoah.

debug_final_field_at() and debug_stable_field_at() are not used - could be excluded from changes.

loopnode.cpp Sometimes you add #if INCLUDE_SHENANDOAHGC and sometimes don't. Be consistent. I don't know why you need 
#ifdef if such nodes should be generated without Shenandoah

compile.cpp - no need to include shenandoahBarrierSetC2.hpp

type.hpp - cast_to_nonconst() are not used.

phasetype.hpp, subnode.cpp - don't add files with only cleanup changes.

I wish loopnode.cpp and loopopts.cpp changes were more clean.

I like the idea where you expose only one ShenandoahBarrier node to C2 and hide all subnodes behind it. I really don't 
want to see a lot of Shenandoah*Node new classes in C2 shared code.


[1] http://hg.openjdk.java.net/jdk/jdk/file/157c1130b46e/make/hotspot/gensrc/GensrcAdlc.gmk#l122

On 11/28/18 2:29 PM, Roman Kennke wrote:
> Alright, we fixed:
> - The minor issues that Kim reported in shared-gc
> - A lot of fixes in shared-tests according to Leonid's review
> - Disabled SA heapdumping similar to ZGC as Per suggested
> Some notes:
> Leonid:  test/hotspot/jtreg/gc/TestFullGCCount.java was actually
> correct. The @requires there means to exclude runs with both CMS and
> ExplicitGCInvokesConcurrent at the same time, because that would be
> (expectedly) failing. It can run CMS, default GC and any other GC just
> fine. Adding the same clause for Shenandoah means the same, and filters
> the combination (+UseShenandoahGC)+(+ExplicitGCInvokesConcurrent). I
> made the condition a bit clearer by avoiding triple-negation.
> See:
> http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-November/008457.html
> Per: Disabling the SA part for heapdumping makes 2 tests fail:
> - test/hotspot/jtreg/serviceability/sa/ClhsdbJhisto.java
> - test/hotspot/jtreg/serviceability/sa/TestHeapDumpForLargeArray.java
> we filter them for Shenandoah now. I'm wondering: how do you get past
> those with ZGC?
> See:
> http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-November/008466.html
> (Note to Leonid and tests reviewers: I'll add those related filters in
> next round).
> Vladimir: Roland integrated a bunch of changes to make loop* code look
> better. I can tell that we're not done with C2 yet. Can you look over
> the code and see what is ok, and especially what is not ok, so that we
> can focus our efforts on the relevant parts?
> Updated set of webrevs:
> http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/01/
> Thanks,
> Roman
>> 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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