RFR: 8254598: StringDedupTable should use OopStorage [v2]

[Thomas Schatzl]

On Fri, 7 May 2021 08:28:43 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Please review this change to the String Deduplication facility.  It is being
>> changed to use OopStorage to hold weak references to relevant objects,
>> rather than bespoke data structures requiring dedicated processing phases by
>> supporting GCs.
>> 
>> (The Shenandoah update was contributed by Zhengyu Gu.)
>> 
>> This change significantly simplifies the interface between a given GC and
>> the String Deduplication facility, which should make it much easier for
>> other GCs to opt in.  However, this change does not alter the set of GCs
>> providing support; currently only G1 and Shenandoah support string
>> deduplication.  Adding support by other GCs will be in followup RFEs.
>> 
>> Reviewing via the diffs might not be all that useful for some parts, as
>> several files have been essentially completely replaced, and a number of
>> files have been added or eliminated.  The full webrev might be a better
>> place to look.
>> 
>> The major changes are in gc/shared/stringdedup/* and in the supporting
>> collectors, but there are also some smaller changes in other places, most
>> notably classfile/{stringTable,javaClasses}.
>> 
>> This change is additionally labeled for review by core-libs, although there
>> are no source changes there.  This change injects a byte field of bits into
>> java.lang.String, using one of the previously unused padding bytes in that
>> class.  (There were two unused bytes, now only one.)
>> 
>> Testing:
>> mach5 tier1-2 with and without -XX:+UseStringDeduplication
>> 
>> Locally (linux-x64) ran all of the existing tests that use string
>> deduplication with both G1 and Shenandoah.  Note that
>> TestStringDeduplicationInterned.java is disabled for shenandoah, as it
>> currently fails, for reasons I haven't figured out but suspect are test
>> related.
>> 
>> - gc/stringdedup/   -- these used to be in gc/g1
>> - runtime/cds/SharedStringsDedup.java
>> - runtime/cds/appcds/cacheObject/DifferentHeapSizes.java
>> - runtime/cds/appcds/sharedStrings/*
>> 
>> shenandoah-only:
>> - gc/shenandoah/jvmti/TestHeapDump.java
>> - gc/shenandoah/TestStringDedup.java
>> - gc/shenandoah/TestStringDedupStress.java
>> 
>> Performance tested baseline, baseline + stringdedup, new with stringdedup,
>> finding no significant differences.
>
> Kim Barrett has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - more comment improvements
>  - improve naming and comments around injected String flags
>  - fix some typos in comments

Lgtm.

src/hotspot/share/gc/shared/stringdedup/stringDedup.hpp line 82:

> 80: // Doing so would counteract C2 optimizations on string literals.  But an
> 81: // interned string might later become a deduplication candidate through the
> 82: // normal GC discovery mechanism.  To prevent such modificatoins, the

s/modificatoins/modifications

-------------

Marked as reviewed by tschatzl (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3662