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List:       openjdk-serviceability-dev
Subject:    Re: RFR: 8305566: ZGC: gc/stringdedup/TestStringDeduplicationFullGC.java#Z failed with SIGSEGV in
From:       Thomas Schatzl <tschatzl () openjdk ! org>
Date:       2023-04-26 14:57:54
Message-ID: jgdht8TLtGXkVcj_SU_twnigJjRNs2uMbziXLN_QWR0=.a0289fd9-bbe8-4682-8818-eb424431cb6c () github ! com
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On Mon, 24 Apr 2023 09:25:01 GMT, Kim Barrett <kbarrett@openjdk.org> wrote:

> > Please review this change to the string deduplication thread to make it a kind
> > of JavaThread rather than a ConcurrentGCThread.  There are several pieces to
> > this change:
> > 
> > (1) New class StringDedupThread (derived from JavaThread), separate from
> > StringDedup::Processor (which is now just a CHeapObj instead of deriving from
> > ConcurrentGCThread).  The thread no longer needs to or supports being stopped,
> > like other similar threads.  It also needs to be started later, once Java
> > threads are supported.  Also don't need an explicit visitor, since it will be
> > in the normal Java threads list.  This separation made the changeover a little
> > cleaner to develop, and made the servicability support a little cleaner too.
> > 
> > (2) The Processor now uses the ThreadBlockInVM idiom to be safepoint polite,
> > instead of using the SuspendibleThreadSet facility.
> > 
> > (3) Because we're using ThreadBlockInVM, which has a different usage style
> > from STS, the tracking of time spent by the processor blocked for safepoints
> > doesn't really work.  It's not very important anyway, since normal thread
> > descheduling can also affect the normal processing times being gathered and
> > reported.  So we just drop the so-called "blocked" time and associated
> > infrastructure, simplifying Stat tracking a bit.  Also renamed the
> > "concurrent" stat to be "active", since it's all in a JavaThread now.
> > 
> > (4) To avoid #include problems, moved the definition of
> > JavaThread::is_active_Java_thread from the .hpp file to the .inline.hpp file,
> > where one of the functions it calls also is defined.
> > 
> > (5) Added servicability support for the new thread.
> > 
> > Testing:
> > mach5 tier1-3 with -XX:+UseStringDeduplication.
> > The test runtime/cds/DeterministicDump.java fails intermittently with that
> > option, which is not surprising - see JDK-8306712.
> > 
> > I was never able to reproduce the failure; it's likely quite timing sensitive.
> > The fix of changing the type is based on StefanK's comment that ZResurrection
> > doesn't expect a non-Java thread to perform load-barriers.
> 
> Kim Barrett has updated the pull request incrementally with one additional commit \
> since the last revision: 
> fix include order

Please improve the bug name as suggested by @shipilev before pushing.

Looks good otherwise.

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

Marked as reviewed by tschatzl (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13607#pullrequestreview-1402152540


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