On Fri, 23 Apr 2021 19:48:47 GMT, Kim Barrett <kbarr...@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.
I looked at the runtime code, which looks fine. I didn't read the GC code.
src/hotspot/share/classfile/javaClasses.inline.hpp line 77:
> 75:
> 76: uint8_t* java_lang_String::flags_addr(oop java_string) {
> 77: assert(_initialized, "Mut be initialized");
typo: must
src/hotspot/share/classfile/stringTable.cpp line 784:
> 782: SharedStringIterator iter(f);
> 783: _shared_table.iterate(&iter);
> 784: }
So with this change (somewhere below) do you no longer deduplicate strings from
the shared string table? ie
// The CDS archive does not include the string deduplication table. Only the
string
// table is saved in the archive. The shared strings from CDS archive need to be
// added to the string deduplication table before deduplication occurs. That is
// done in the beginning of the StringDedupThread (see
StringDedupThread::do_deduplication()).
void StringDedupThread::deduplicate_shared_strings(StringDedupStat* stat) {
StringDedupSharedClosure sharedStringDedup(stat);
StringTable::shared_oops_do(&sharedStringDedup);
}
Asking @iklam to have a look then.
src/hotspot/share/runtime/globals.hpp line 1994:
> 1992:
> \
> 1993: product(uint64_t, StringDeduplicationHashSeed, 0, DIAGNOSTIC,
> \
> 1994: "Seed for the table hashing function; 0 requests computed
> seed") \
Should these two really be develop() options?
src/hotspot/share/runtime/mutexLocker.cpp line 239:
> 237: def(StringDedupQueue_lock , PaddedMonitor, leaf, true,
> _safepoint_check_never);
> 238: def(StringDedupTable_lock , PaddedMutex , leaf + 1, true,
> _safepoint_check_never);
> 239: }
Why weren't these duplicate definitions? This is disturbing.
-------------
Marked as reviewed by coleenp (Reviewer).
PR: https://git.openjdk.java.net/jdk/pull/3662
