[PATCH] Exploit Empty Regions in Young Gen to Enhance PS Full GC Performance
Stefan Johansson
stefan.johansson at oracle.com
Thu Oct 24 12:16:03 UTC 2019
Hi Haoyu,
On 2019-10-23 17:15, Haoyu Li wrote:
> Hi Stefan,
>
> Thanks for your constructive feedback. I've addressed all the issues you
> mentioned, and the updated patch is attached in this email.
Nice, I will look at the patch next week, but I'll shortly answer your
questions right away.
>
> During refining the patch, I have a couple of questions:
> 1) Now the MoveAndUpdateClosure and ShadowClosure assume the destination
> address is the very beginning of a region, instead of an arbitrary
> address like what it used to be. However, there is an unused function
> named PSParallelCompact::move_and_update() uses the MoveAndUpdateClosure
> to process a region from its middle, which conflicts with the
> assumption. I notice that you removed this function in your patch, and
> so did I in the updated patch. Does it matter?
Yes, I found this function during my code review and it should be
removed, but I think that should be handled as a separate issue. We can
do this removal before this patch goes in.
> 2) Using the same do_addr() in MoveAndUpdateClosure and ShadowClosure is
> doable, but it does not reuse all the code neatly. Because storing the
> address of the shadow region in _destination requires extra virtual
> functions to handle allocating blocks in the start_array and setting
> addresses of deferred objects. In particular, allocate_blocks() and
> set_deferred_object_for() in both closures are added. Is it worth
> avoiding to use _offset to calculate the shadow_destination?
Ok, sounds like it might be better to have specific do_addr() functions
then. I'll think some more around this when reviewing the new patch in
depth.
>
> If there are any problems with this patch, please contact me anytime.
> I'm more than happy to keep improving the code. Thanks again for reviewing.
>
Sound good, thanks,
Stefan
> Best,
> Haoyu Li
>
>
> Stefan Johansson <stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>> 于2019年10月22日周二 下午9:42写道：
>
> Hi Haoyu,
>
> I've reviewed the patch now and have some comments and questions.
>
> To simplify the review and have a common base to look at I've created a
> webrev at:
> http://cr.openjdk.java.net/~sjohanss/8220465/00/
>
> One general note first, most of the new code uses four space
> indentation, in hotspot the standard is two spaces, please change this.
> Below are some file by file comments.
>
> src/hotspot/share/gc/parallel/psCompactionManager.cpp
> ---
> 53 GrowableArray<size_t >* ParCompactionManager::_free_shadow = new
> (ResourceObj::C_HEAP, mtInternal) GrowableArray<size_t >(10, true);
> 54 Monitor* ParCompactionManager::_monitor = NULL;
>
> Set _free_shadow to NULL here like the other statics and then create
> the
> GrowableArray in initialize(). I also think _shadow_region_array or
> something like that would be a better name and the monitor should also
> be named something that signals that it is used for this array.
> ---
> 70 if (_monitor == NULL) {
> 71 _monitor = new Monitor(Mutex::barrier, "CompactionManager
> monitor",
> 72 Mutex::_allow_vm_block_flag,
> Monitor::_safepoint_check_never);
> 73 }
>
> Instead of doing the monitor creation here having to check for NULL, do
> it in initialize() below together with the array creation.
> ---
>
> src/hotspot/share/gc/parallel/psParallelCompact.cpp
> ---
> 2974 if (cur->push()) {
>
> Correct me if I'm wrong, if this call to push() returns true it means
> that nobody else has "stolen" it (used a shadow region to prepare it)
> and we mark it as pushed. But when pushed in this code path this is the
> end state for this RegionData? If this is the case I think it would be
> easier to understand the code if we added another function and state
> for
> when we "steal" it. Haven't thought very much about the names but I
> think you understand what I want to achieve:
> Normal path:
> UNUSED -> push() -> NORMAL
> Steal path:
> UNUSED -> steal() -> STOLEN -> fill() -> FILLED -> copy() -> SHADOW
>
> We could then also assert in set_completed() that the state is either
> NORMAL or SHADOW (or if they have a shared end state DONE). As I said
> the names can be improved (both for the states and the functions) but I
> think we should have names and not just numbers.
> ---
>
> 3060 template <class T>
> 3061 void PSParallelCompact::fill_region(ParCompactionManager* cm,
> size_t region_idx, size_t shadow, size_t offset)
>
> As I told you this was a big improvement from the first patch, but I
> think there is room for even more improvements around the way we
> pass in
> ignored parameters to MoveAndUpdateClosure. Explaining my idea in text
> is harder than code, so I created a patch, what do you think about this?
> http://cr.openjdk.java.net/~sjohanss/8220465/00-alt/
>
> This alternative is based on 00 and does not take my other comments
> into
> consideration. So it might have to be altered a bit if you address some
> of my other comments/questions.
> ---
>
> 3196 void PSParallelCompact::copy_back(HeapWord *region_addr, HeapWord
> *shadow_addr) {
>
> I think the paramenter should change place, so that it corresponds with
> the copy below.
> ---
>
> 3200 bool PSParallelCompact::steal_shadow_region(ParCompactionManager*
> cm, size_t ®ion_idx) {
> 3201 size_t& record = cm->shadow_record();
>
> Did you consider to just let shadow_record() be a simple getter instead
> of getting a reference and then have a next_shadow_record() which
> advances it by active_workers?
> ---
>
> 3236 void PSParallelCompact::initialize_steal_record(uint which) {
>
> I'm having a hard time understanding the details here, or I get that
> all
> threads should have a separate shadow record, but I'm not sure why
> it is
> not enough to just do:
> size_t record = _summary_data.addr_to_region_idx(
> _space_info[old_space_id].dense_prefix());
> cm->set_shadow_record(record + which);
>
> As you can see I'm also suggesting adding a setter for shadow_record.
> ---
>
> 3434 ParMarkBitMapClosure::IterationStatus
> 3435 ShadowClosure::do_addr(HeapWord* addr, size_t words) {
> 3436 HeapWord* shadow_destination = destination() + _offset;
>
> Using an offset instead of a given address feels a bit backwards, did
> you consider letting the closure keep and update a _shadow_destination
> instead? Or would it even be possible to just set destination to be the
> shadow region address? In that case it should be possible to just use
> the do_addr and other functions from the MoveAndUpdateClosure.
>
> I see from looking at this particular function that there is one assert
> that would have to change:
> 3408
> assert(PSParallelCompact::summary_data().calc_new_pointer(source(),
> compaction_manager()) ==
> 3409 destination(), "wrong destination");
>
> This should be easily fixed by adding a virtual function
> check_destination, that has a special implementation for the
> ShadowClosure.
> ---
>
> src/hotspot/share/gc/parallel/psParallelCompact.hpp
> ---
> 333 // Preempt the region to avoid double processes
> 334 inline bool push();
> 335 // Mark the region as filled and ready to be copied back
> 336 inline bool fill();
> 337 // Preempt the region to copy the shadow region content back
> 338 inline bool copy();
>
> As mentioned, I think there might be better names for those functions
> and the comments. Maybe adding a prefix would make the code more self
> explaining. try_push(), mark_filled(), try_copy() and the new
> try_steal().
> ---
>
> Thanks again for providing this patch, I look forward to see an updated
> version.
>
> Cheers,
> Stefan
>
>
> On 2019-10-14 15:00, Stefan Johansson wrote:
> > Thanks for the quick update Haoyu,
> >
> > This is a great improvement and I will try to find time to look
> into the
> > patch in more detail the coming weeks.
> >
> > Thanks,
> > Stefan
> >
> > On 2019-10-11 14:49, Haoyu Li wrote:
> >> Hi Stefan,
> >>
> >> Thanks for your suggestion! It is very redundant that
> >> PSParallelCompact::fill_shadow_region() copies most code from
> >> PSParallelCompact::fill_region(), and therefore I've refactored
> these
> >> two functions to share code as many as possible. And the
> attachment is
> >> the updated patch.
> >>
> >> Specifically, the closure, which moves objects, in
> >> PSParallelCompact::fill_region() is now declared as a template of
> >> either MoveAndUpdateClosure or ShadowClosure. So by controlling the
> >> type of closure when invoking the function, we can decide whether to
> >> fill a normal region or a shadow one. Thus, almost all code in
> >> PSParallelCompact::fill_region() can be reused.
> >>
> >> Besides, a virtual function named complete_region() is added in both
> >> closures to do some work after the filling, such setting states and
> >> copying the shadow region back.
> >>
> >> Thanks again for reviewing the patch, looking forward to your
> insights
> >> and suggestions!
> >>
> >> Best Regards,
> >> Haoyu Li
> >>
> >> 2019-10-10 21:50 GMT+08:00, Stefan Johansson
> >> <stefan.johansson at oracle.com <mailto:stefan.johansson at oracle.com>>:
> >>> Thanks for the clarification =)
> >>>
> >>> Moving on to the next part, the code in the patch. So this
> won't be a
> >>> full review of the patch but just an initial comment that I
> would like
> >>> to be addressed first.
> >>>
> >>> The new function PSParallelCompact::fill_shadow_region() is
> more or less
> >>> a copy of PSParallelCompact::fill_region() and I understand
> that from a
> >>> proof of concept point of view it was the easy (and right) way
> to do it.
> >>> I would prefer if the code could be refactored so that
> fill_region() and
> >>> fill_shadow_region() share more code. There might be reasons
> that I've
> >>> missed, that prevents it, but we should at least explore how
> much code
> >>> can be shared.
> >>>
> >>> Thanks,
> >>> Stefan
> >>>
> >>> On 2019-10-10 15:10, Haoyu Li wrote:
> >>>> Hi Stefan,
> >>>>
> >>>> Thanks for your quick response! As to your concern about the
> OCA, I am
> >>>> the sole author of the patch. And it is the case as what the
> agreement
> >>>> states.
> >>>> Best Regrads,
> >>>> Haoyu Li,
> >>>>
> >>>>
> >>>> Stefan Johansson <stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>
> >>>> <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>>> 于2019年10月10日周四 下午8:37
> >>>> 写道：
> >>>>
> >>>> Hi,
> >>>>
> >>>> On 2019-10-10 13:06, Haoyu Li wrote:
> >>>> > Hi Stefan,
> >>>> >
> >>>> > Thanks for your testing! One possible reason for the
> >>>> regressions
> >>>> in
> >>>> > simple tests is that the region dependencies maybe not
> heavy
> >>>> enough.
> >>>> > Because the locality of shadow regions is lower than
> that of
> >>>> heap
> >>>> > regions, writing to shadow regions will be slower than to
> >>>> normal
> >>>> > regions, and this is a part of the reason why I reuse
> shadow
> >>>> regions.
> >>>> > Therefore, if only a few shadow regions are created
> and not
> >>>> reused, the
> >>>> > overhead may not be amortized.
> >>>>
> >>>> I guess it is something like this. I thought that for
> "easy" heaps
> >>>> the
> >>>> shadow regions won't be used at all, and should therefor not
> >>>> really
> >>>> cost
> >>>> anything.
> >>>>
> >>>> >
> >>>> > As to the OCA, it is the case that I'm the only person
> >>>> signing the
> >>>> > agreement. Please let me know if you have any further
> >>>> questions.
> >>>> Thanks
> >>>> > again!
> >>>>
> >>>> Ok, so you are the sole author of the patch. The important
> >>>> part, as
> >>>> the
> >>>> agreement states, is:
> >>>> "no other person or entity, including my employer, has or
> will
> >>>> have
> >>>> rights with respect my contributions"
> >>>>
> >>>> Is that the case?
> >>>>
> >>>> Thanks,
> >>>> Stefan
> >>>>
> >>>> >
> >>>> > Best Regrads,
> >>>> > Haoyu Li
> >>>> >
> >>>> > Stefan Johansson <stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>
> >>>> <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>>
> >>>> > <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>
> >>>> <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>>>> 于2019年10月8日周二 下午
> >>>> 6:49
> >>>> 写道：
> >>>> >
> >>>> > Hi Haoyu,
> >>>> >
> >>>> > I've done some more testing and I haven't seen any
> issues
> >>>> with the
> >>>> > patch
> >>>> > so far and the performance looks promising in most
> >>>> cases. For
> >>>> simple
> >>>> > tests I've seen some regressions, but I'm not
> really sure
> >>>> why. Will do
> >>>> > some more digging.
> >>>> >
> >>>> > To move forward with this the first thing we need
> to do is
> >>>> making sure
> >>>> > that you being covered by the Oracle Contributor
> >>>> Agreement is
> >>>> enough.
> >>>> > From what we can see it is only you as an
> individual that
> >>>> has signed
> >>>> > the OCA and in that case it is important that this
> >>>> statement
> >>>> from the
> >>>> > OCA is fulfilled: "no other person or entity,
> including my
> >>>> employer,
> >>>> > has
> >>>> > or will have rights with respect my contributions"
> >>>> >
> >>>> > Is this the case for this contribution or should
> we have
> >>>> the
> >>>> university
> >>>> > sign the OCA as well? For more information
> regarding the
> >>>> OCA
> >>>> please
> >>>> > refer to:
> >>>> > https://www.oracle.com/technetwork/oca-faq-405384.pdf
> >>>> >
> >>>> > Thanks,
> >>>> > Stefan
> >>>> >
> >>>> > On 2019-09-16 16:02, Haoyu Li wrote:
> >>>> > > FYI, the evaluation results on OpenJDK 14 are
> plotted in
> >>>> the
> >>>> > attachment.
> >>>> > > I compute the full GC throughput by dividing
> the heap
> >>>> size
> >>>> before
> >>>> > full
> >>>> > > GC by the GC pause time, and the results are
> arithmetic
> >>>> mean
> >>>> > values of
> >>>> > > ten runs after a warm-up run. The evaluation is
> >>>> conducted on
> >>>> a
> >>>> > machine
> >>>> > > with dual Intel ®XeonTM E5-2618L v3 CPUs (2
> sockets, 16
> >>>> physical
> >>>> > cores
> >>>> > > with SMT enabled) and 64G DRAM.
> >>>> > >
> >>>> > > Best Regrads,
> >>>> > > Haoyu Li,
> >>>> > > Institute of Parallel and Distributed
> Systems(IPADS),
> >>>> > > School of Software,
> >>>> > > Shanghai Jiao Tong University
> >>>> > >
> >>>> > >
> >>>> > > Stefan Johansson <stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>
> >>>> <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>>
> >>>> > <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>
> >>>> <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>>>
> >>>> > > <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>
> >>>> <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>>
> >>>> > <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>
> >>>> <mailto:stefan.johansson at oracle.com
> <mailto:stefan.johansson at oracle.com>>>>> 于2019年9月12日周四 上
> >>>> 午5:34
> >>>> > 写道：
> >>>> > >
> >>>> > > Hi Haoyu,
> >>>> > >
> >>>> > > I recently came across your patch and I would
> >>>> like to
> >>>> pick up on
> >>>> > > some of the things Kim mentioned in his
> mails. I
> >>>> especially want
> >>>> > > evaluate and investigate if this is a
> technique
> >>>> we can
> >>>> use to
> >>>> > > improve the other GCs as well. To start
> that work I
> >>>> want to
> >>>> > take the
> >>>> > > patch for a spin in our internal performance
> >>>> testing.
> >>>> The patch
> >>>> > > doesn’t apply clean to the latest JDK
> repository, so
> >>>> if you could
> >>>> > > provide an updated patch that would be very
> helpful.
> >>>> > >
> >>>> > > It would also be great if you could share
> some more
> >>>> information
> >>>> > > around the results presented in the paper. For
> >>>> example,
> >>>> it
> >>>> > would be
> >>>> > > good to get the full command lines for the
> different
> >>>> > benchmarks so
> >>>> > > we can run them locally and reproduce the
> >>>> results you’ve seen.
> >>>> > >
> >>>> > > Thanks,
> >>>> > > Stefan
> >>>> > >
> >>>> > >> 12 mars 2019 kl. 03:21 skrev Haoyu Li
> >>>> <leihouyju at gmail.com <mailto:leihouyju at gmail.com>
> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>>
> >>>> > <mailto:leihouyju at gmail.com
> <mailto:leihouyju at gmail.com> <mailto:leihouyju at gmail.com
> <mailto:leihouyju at gmail.com>>>
> >>>> > >> <mailto:leihouyju at gmail.com
> <mailto:leihouyju at gmail.com>
> >>>> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>>
> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>
> >>>> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>>>>>:
> >>>> > >>
> >>>> > >> Hi Kim,
> >>>> > >>
> >>>> > >> Thanks for reviewing and testing the
> patch. If
> >>>> there
> >>>> are any
> >>>> > >> failures or performance degradation
> relevant to the
> >>>> work, please
> >>>> > >> let me know and I'll be very happy to keep
> >>>> improving
> >>>> it.
> >>>> > Also, any
> >>>> > >> suggestions about code improvements are well
> >>>> appreciated.
> >>>> > >>
> >>>> > >> I'm not quite sure if both G1 and Shenandoah
> >>>> have the
> >>>> similar
> >>>> > >> region dependency issue, since I haven't
> studied
> >>>> their
> >>>> GC
> >>>> > >> behaviors before. If they have, I'm also
> willing to
> >>>> propose
> >>>> > a more
> >>>> > >> general optimization.
> >>>> > >>
> >>>> > >> As to the memory overhead, I believe it
> will be low
> >>>> because this
> >>>> > >> patch exploits empty regions in the young
> space
> >>>> rather than
> >>>> > >> off-heap memory to allocate shadow
> regions, and
> >>>> also
> >>>> reuses the
> >>>> > >> /_source_region/ field of each /RegionData
> /to
> >>>> record
> >>>> the
> >>>> > >> correspongding shadow region index. We only
> >>>> introduce
> >>>> a new
> >>>> > >> integer filed /_shadow /in the RegionData
> class to
> >>>> indicate the
> >>>> > >> status of a region, a global /GrowableArray
> >>>> _free_shadow/ to
> >>>> > store
> >>>> > >> the indices of shadow regions, and a global
> >>>> /Monitor/ to protect
> >>>> > >> the array. These information might help if
> the
> >>>> memory
> >>>> overhead
> >>>> > >> need to be evaluated.
> >>>> > >>
> >>>> > >> Looking forward to your insight.
> >>>> > >>
> >>>> > >> Best Regrads,
> >>>> > >> Haoyu Li,
> >>>> > >> Institute of Parallel and Distributed
> >>>> Systems(IPADS),
> >>>> > >> School of Software,
> >>>> > >> Shanghai Jiao Tong University
> >>>> > >>
> >>>> > >>
> >>>> > >> Kim Barrett <kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>>
> >>>> > <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com <mailto:kim.barrett at oracle.com>>>
> >>>> > >> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>>
> >>>> > <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>>>>> 于2019年3月12日周二 上午6:11
> >>>> 写道：
> >>>> > >>
> >>>> > >> > On Mar 11, 2019, at 1:45 AM, Kim Barrett
> >>>> > >> <kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>>>
> >>>> > <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>
> >>>> <mailto:kim.barrett at oracle.com
> <mailto:kim.barrett at oracle.com>>>>> wrote:
> >>>> > >> >
> >>>> > >> >> On Jan 24, 2019, at 3:58 AM, Haoyu Li
> >>>> > <leihouyju at gmail.com <mailto:leihouyju at gmail.com>
> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>>
> >>>> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>
> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>>>
> >>>> > >> <mailto:leihouyju at gmail.com
> <mailto:leihouyju at gmail.com>
> >>>> <mailto:leihouyju at gmail.com <mailto:leihouyju at gmail.com>>
> >>>> > <mailto:leihouyju at gmail.com
> <mailto:leihouyju at gmail.com> <mailto:leihouyju at gmail.com
> <mailto:leihouyju at gmail.com>>>>>
> >>>> wrote:
> >>>> > >> >>
> >>>> > >> >> Hi Kim,
> >>>> > >> >>
> >>>> > >> >> I have ported my patch to OpenJDK 13
> >>>> according
> >>>> to your
> >>>> > >> instructions in your last mail, and the
> >>>> patch is
> >>>> attached in
> >>>> > >> this mail. The patch does not change
> much since
> >>>> PSGC is
> >>>> > indeed
> >>>> > >> pretty stable.
> >>>> > >> >>
> >>>> > >> >> Also, I evaluate the correctness and
> >>>> performance of
> >>>> > PS full
> >>>> > >> GC with benchmarks from DaCapo,
> SPECjvm2008,
> >>>> and
> >>>> JOlden
> >>>> > �� suits
> >>>> > >> on a machine with dual Intel Xeon
> E5-2618L v3
> >>>> CPUs(16
> >>>> > physical
> >>>> > >> cores), 64G DRAM and linux kernel
> 4.17. The
> >>>> evaluation
> >>>> > result,
> >>>> > >> indicating 1.9X GC throughput
> improvement on
> >>>> average, is
> >>>> > >> attached, too.
> >>>> > >> >>
> >>>> > >> >> However, I have no idea how to
> further test
> >>>> this
> >>>> > patch for
> >>>> > >> both correctness and performance. Can
> I please
> >>>> get any
> >>>> > >> guidance from you or some sponsor?
> >>>> > >> >
> >>>> > >> > Sorry I missed that you had sent an
> updated
> >>>> version of the
> >>>> > >> patch.
> >>>> > >> >
> >>>> > >> > I’ve run the full regression suite
> across
> >>>> Oracle-supported
> >>>> > >> platforms. There are some
> >>>> > >> > failures, but there are almost
> always some
> >>>> failures in the
> >>>> > >> later tiers right now. I’ll start
> >>>> > >> > looking at them tomorrow to figure out
> >>>> whether
> >>>> any of them
> >>>> > >> are relevant.
> >>>> > >> >
> >>>> > >> > I’m also planning to run some of our
> >>>> performance
> >>>> > benchmarks.
> >>>> > >> >
> >>>> > >> > I’ve lightly skimmed the proposed
> changes.
> >>>> There might be
> >>>> > >> some code improvements
> >>>> > >> > to be made.
> >>>> > >> >
> >>>> > >> > I’m also wondering if this technique
> >>>> applies to
> >>>> other
> >>>> > >> collectors. It seems like both G1 and
> >>>> > >> > Shenandoah full gc’s might have similar
> >>>> issues? If so, a
> >>>> > >> solution that is ParallelGC-specific
> >>>> > >> > is less interesting than one that
> has broader
> >>>> > >> applicability. Though maybe this
> optimization
> >>>> > >> > is less important for G1 and
> Shenandoah,
> >>>> since
> >>>> they
> >>>> > actively
> >>>> > >> try to avoid full gc’s.
> >>>> > >> >
> >>>> > >> > I’m also not clear on how much
> additional
> >>>> memory might be
> >>>> > >> temporarily allocated by this
> >>>> > >> > mechanism.
> >>>> > >>
> >>>> > >> I’ve created a CR for this:
> >>>> > >> https://bugs.openjdk.java.net/browse/JDK-8220465
> >>>> > >>
> >>>> > >
> >>>> >
> >>>>
> >>>
> >>
> >>
>
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