RFR (S): 8201326: Renaming ThreadLocalAllocationBuffer end to fast_path_end
Vladimir Kozlov
vladimir.kozlov at oracle.com
Tue Apr 17 23:12:00 UTC 2018
Hi JC,
Yes, from the beginning such changes should be discussed on common hotspot-dev list since all
Hotspot's parts are affected.
Graal specific question could be send to hotspot-compiler-dev list with [Graal] in subject.
I looked on JEP's changes http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.02/ to understand how
it works.
Few questions about proposed JEP changes so I can understand it.
You introducing new TLAB end for sampling and adjust it so that allocations in JITed code and
Interpreter it will trigger slow path allocation where you do sampling. Right?
Do all changes to _end, _actual_end and other TLAB fields happen during slow allocation?
I am concern about concurrent accesses to these fields from other threads if you have them (I don't
see).
Renaming. I am fine with renaming if it helps to understand code better. I agree with proposed
changes to fields name:
_current_end
_allocation_end
But, as Dean, I would suggest to keep names of access functions. This way we can say that allocation
code in Interpreter and JITed code stay the same.
You may need only new method to access _allocation_end in places which look for real end of TLAB.
Regards,
Vladimir
On 4/16/18 4:42 PM, JC Beyler wrote:
> Hi Dean,
>
> I think perhaps this is also an attempt to figure out the naming of all
> this because naming (or renaming like here) is often the start of big
> conversations :).
>
> Originally, in the JEP-331 work, I had left the _end field as is and, by
> doing so, this whole renaming webrev goes away. However, if we do that,
> then the TLAB code contains:
>
> _end: the fast path end, can be the allocation end or the to-be-sampled end
> _allocation_end: the actual allocation end of the current TLAB
> hard_end: _allocation_end + reserved space
>
> With an early iteration of a webrev for JEP-331, a conversation occurred
> about whether or not that was clear or not and it was determined that this
> renaming was more clear:
>
> _current_end: the fast path end
> _allocation_end: the actual allocation end of the current TLAB
> reserved_end: _allocation_end + reserved space
>
> Because I'm trying to reduce the footprint of files changed, I pulled out
> the renaming changes into this webrev. While talking about it with the GC
> team, the renaming suggested became:
>
> _fast_path_end: the fast path end
> _allocation_end: the actual allocation end of the current TLAB
> hard_end: _allocation_end + reserved space
>
> Now, to be honest, any renaming or no renaming is fine by me. I like not
> renaming the fields to not change the code of every backend and Graal; I
> also like the fast_path_end rename due to it making the backend code easy
> to read as mentioned in the GC mailing lis thread.
>
> So there are two questions really:
> - Should we rename the _end field and thus provoke the changes in this
> webrev?
> - If we do want to change the field, should/could it go in an initial
> webrev or should it go in the JEP-331 webrev whenever/ifever it goes in?
> And if we do wait, could we at least converge on a renaming now so that
> this does not become a point of contention for that webrev's review?
>
> If I read your answer correctly:
> - You are saying that we should keep the _end field altogether; or at
> least only rename the field not the methods to access it, thus reducing the
> change scope.
> - You are also saying to wait for the JEP-331 webrev's final iteration
> and integrate it in one step
>
> Have I understood your answer correctly?
>
> Again, I am fine with renaming to whatever or not renaming at all. I just
> am trying to get forward progress here :)
>
> Thanks for your help!
> Jc
>
> On Mon, Apr 16, 2018 at 3:29 PM <dean.long at oracle.com> wrote:
>
>> Hi JC. Others might disagree, but I would vote "no" on doing this
>> renaming now, before the JEP, and even in the context of the JEP, I
>> don't think renaming the field requires renaming all the uses. The
>> compiler code is only interested in the fast path, so it's implicitly
>> understood. Also, if there is a fast_path_end(), then isn't it logical
>> to have fast_path_start() paired with it? ("start" is already
>> overloaded, but nobody is suggesting adding
>> allocation_start()/current_start()/hard_start() are they?). My opinion
>> is that it's fine the way it is.
>>
>> dl
>>
>> On 4/16/18 1:43 PM, JC Beyler wrote:
>>> Hi all,
>>>
>>> I've left the mail thread from the hotspot-gc-dev below for tracking
>>> purposes but will start a new request here.
>>>
>>> - I'm trying to push a renaming of a TLAB field to make my work for
>> JEP-331
>>> <http://openjdk.java.net/jeps/331> easier
>>> - There is an understanding that if JEP-331 does not make it, this
>> might
>>> be useless and I'll revert if that is what the community wants; however
>> the
>>> new name seems better anyway so perhaps not?
>>>
>>> - The webrev renames a field used across the various back-ends and Graal
>>> - The webrev is here:
>>> http://cr.openjdk.java.net/~jcbeyler/8201326/webrev.04/
>>>
>>> Could I please get some feedback on this?
>>>
>>> Thanks all for your help,
>>> Jc
>>>
>>>
>>>
>>> On Fri, Apr 13, 2018 at 2:37 AM Stefan Johansson <
>>> stefan.johansson at oracle.com> wrote:
>>>
>>>> Hi JC,
>>>>
>>>> I don't have a name, but I've looked at bit more at the failures and I
>>>> think they are unrelated and one of the local compiler engineers agree.
>>>>
>>>> I also ran some local testing and was not able to get any error with you
>>>> latest changes, but verified that it doens't work without the graal
>>>> parts. So they seem good. It might still be good to switch this over to
>>>> the general hotspot-dev list to let someone with Graal knowledge to look
>>>> at the change and make sure everything is correct.
>>>>
>>>> Thanks,
>>>> Stefan
>>>>
>>>>
>>>> On 2018-04-12 17:26, JC Beyler wrote:
>>>>> Hi Stefan,
>>>>>
>>>>> Thanks for testing :). I've renamed the bug title in the JBS and will
>>>>> emit a new webrev shortly. Do you have the name of a compiler engineer
>>>>> in mind or should I perhaps now move this conversation to the general
>>>>> hotspot-dev mailing list and ask there?
>>>>>
>>>>> The alternative is to add the compiler-mailing list to this email
>> thread
>>>>> and ask there before sending to the general list. What do you think is
>>>> best?
>>>>> Thanks for all your help,
>>>>> Jc
>>>>>
>>>>> On Thu, Apr 12, 2018 at 2:09 AM Stefan Johansson
>>>>> <stefan.johansson at oracle.com <mailto:stefan.johansson at oracle.com>>
>>>> wrote:
>>>>>
>>>>>
>>>>> On 2018-04-11 17:48, JC Beyler wrote:
>>>>> > Hi Stefan,
>>>>> >
>>>>> > Sorry about that, I must have missed adding the files or
>>>>> something. Here
>>>>> > is a webrev that added the changes for the SA file:
>>>>> > http://cr.openjdk.java.net/~jcbeyler/8201326/webrev.03/
>>>>> >
>>>>> No problem, this looks good. I kicked of a run in our test system
>> to
>>>>> get
>>>>> some more coverage and have tried to include some Graal testing.
>> I'll
>>>>> let you know the results once they are done.
>>>>>
>>>>> Please also update the bug title both in JBS and the changeset.
>>>>>
>>>>> Cheers,
>>>>> Stefan
>>>>>
>>>>> > I changed the method name, which propagated a change to:
>>>>> >
>>>>>
>>>>
>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/ObjectHeap.java
>>>>> >
>>>>> > I tried testing your test file. It exists in my branch (if the
>>>>> same) under:
>>>>> > hotspot/jtreg/serviceability/sa/ClhsdbJhisto.java
>>>>> >
>>>>> > and interestingly (which generally means I did something
>> wrong),
>>>> it
>>>>> > passed before/after the change so I could not verify that this
>> is
>>>>> a test
>>>>> > showing that all is well in the world on my side. Any ideas of
>>>>> what I
>>>>> > did wrong?
>>>>> >
>>>>> > I did again test it for hotspot/jtreg/compiler/aot/ and
>>>>> > hotspot/jtreg/serviceability/jvmti and it passes those.
>>>>> >
>>>>> > Thanks for all your help,
>>>>> > Jc
>>>>> >
>>>>> >
>>>>> >
>>>>> > On Wed, Apr 11, 2018 at 7:44 AM 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>>> wrote:
>>>>> >
>>>>> > Hi JC,
>>>>> >
>>>>> > On 2018-04-11 00:56, JC Beyler wrote:
>>>>> > > Small update:
>>>>> > >
>>>>> > > Here is the fixed webrev for the '{' that were out of
>>>>> alignment.
>>>>> > This
>>>>> > > passed release build JTREG
>>>>> for hotspot/jtreg/compiler/jvmti (just
>>>>> > to run
>>>>> > > something as a smoke screen)
>>>>> and hotspot/jtreg/compiler/aot/ (to
>>>>> > test
>>>>> > > Graal).
>>>>> > > http://cr.openjdk.java.net/~jcbeyler/8201326/webrev.02/
>>>>> > >
>>>>> > I think this looks better, I agree that leaving _end is
>>>>> tempting to
>>>>> > avoid a lot of change, but I think this will be better in
>> the
>>>>> long run.
>>>>> >
>>>>> > I still miss the changes to make the SA work. To see a
>>>>> problem you
>>>>> > can run:
>>>>> > make CONF=fast run-test
>>>>> >
>> TEST=open/test/hotspot/jtreg/serviceability/ClhsdbJhisto.java
>>>>> >
>>>>> > Cheers,
>>>>> > Stefan
>>>>> >
>>>>> > > Let me know what you think,
>>>>> > > Jc
>>>>> > >
>>>>> > > On Tue, Apr 10, 2018 at 3:21 PM JC Beyler
>>>>> <jcbeyler at google.com <mailto:jcbeyler at google.com>
>>>>> > <mailto:jcbeyler at google.com <mailto:jcbeyler at google.com>>
>>>>> > > <mailto:jcbeyler at google.com <mailto:jcbeyler at google.com
>>>
>>>>> <mailto:jcbeyler at google.com <mailto:jcbeyler at google.com>>>>
>> wrote:
>>>>> > >
>>>>> > > Hi Karen and Stefan,
>>>>> > >
>>>>> > > @Stefan: Naming is hard :)
>>>>> > > @Karen: thanks for the Graal comment, I fixed it in
>>>>> the new
>>>>> > webrev,
>>>>> > > let me know what you think :)
>>>>> > >
>>>>> > > I think the naming convention suggested in this
>> webrev
>>>>> came from
>>>>> > > conversations in for JEP-331 and I am actually
>>>> relatively
>>>>> > > indifferent to the final result (as long as we have
>>>>> some form of
>>>>> > > forward progress :)). To be honest, I'd also be
>> happy
>>>>> to just
>>>>> > leave
>>>>> > > _end as is for all architectures and Graal and have
>> a
>>>> new
>>>>> > > _allocation_end. However, during initial reviews of
>>>>> JEP-331
>>>>> > it was
>>>>> > > deemed complicated to understand:
>>>>> > >
>>>>> > > _end -> the _end or sampling end
>>>>> > > _allocation_end -> end pointer for the last possible
>>>>> allocation
>>>>> > > hard_end -> allocation end + reserved space
>>>>> > >
>>>>> > > That is how this naming came up and why hard_end
>> went
>>>> to
>>>>> > "reserved_end".
>>>>> > >
>>>>> > > I'm really indifferent, so I offer as a perusal:
>>>>> > > http://cr.openjdk.java.net/~jcbeyler/8201326/webrev.01/
>>>>> > >
>>>>> > > I noticed a few problems of alignement of '{' so
>> I'll
>>>>> go fix
>>>>> > that.
>>>>> > > Basically this webrev does the following:
>>>>> > >
>>>>> > > - Uses fast_path_end instead of end
>>>>> > > - Reverts hard_end back to where it was
>>>>> > > - Adds the changes to Graal; there is a bunch of
>>>>> changes in Graal
>>>>> > > because Graal still contains a bit of code doing
>>>>> fasttlabrefills.
>>>>> > >
>>>>> > > Let me know what you think!
>>>>> > >
>>>>> > > Thanks,
>>>>> > > Jc
>>>>> > >
>>>>> > >
>>>>> > > On Tue, Apr 10, 2018 at 6:56 AM Karen Kinnear
>>>>> > > <karen.kinnear at oracle.com
>>>>> <mailto:karen.kinnear at oracle.com> <mailto:
>> karen.kinnear at oracle.com
>>>>> <mailto:karen.kinnear at oracle.com>>
>>>>> > <mailto:karen.kinnear at oracle.com
>>>>> <mailto:karen.kinnear at oracle.com> <mailto:
>> karen.kinnear at oracle.com
>>>>> <mailto:karen.kinnear at oracle.com>>>>
>>>>> > wrote:
>>>>> > >
>>>>> > > Hi JC,
>>>>> > >
>>>>> > > A comment about Graal. The impact on Graal for
>> this
>>>>> > particular
>>>>> > > change would be to break it - so you’ll need
>>>>> > > to complete the Graal changes for this renaming.
>>>>> That isn’t
>>>>> > > optional or something that could be a
>> follow-on. It
>>>>> > > is not ok to break a feature, even an
>> experimental
>>>>> one.
>>>>> > We will
>>>>> > > discuss in the other thread potential phasing of
>>>>> adding
>>>>> > sampling.
>>>>> > >
>>>>> > > I did not do a thorough search -you can do that
>> -
>>>>> I did find
>>>>> > > src/jdk.internal.vm.compiler/share/classes/
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > public final int threadTlabOffset =
>>>>> > > getFieldOffset("Thread::_tlab", Integer.class,
>>>>> > > "ThreadLocalAllocBuffer");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>>> threadLocalAllocBufferStartOffset =
>>>>> > > getFieldOffset("ThreadLocalAllocBuffer::_start",
>>>>> > Integer.class,
>>>>> > > "HeapWord*");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>> threadLocalAllocBufferEndOffset =
>>>>> > > getFieldOffset("ThreadLocalAllocBuffer::_end",
>>>>> Integer.class,
>>>>> > > "HeapWord*");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>> threadLocalAllocBufferTopOffset =
>>>>> > > getFieldOffset("ThreadLocalAllocBuffer::_top",
>>>>> Integer.class,
>>>>> > > "HeapWord*");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>>> threadLocalAllocBufferPfTopOffset =
>>>>> > >
>> getFieldOffset("ThreadLocalAllocBuffer::_pf_top",
>>>>> > Integer.class,
>>>>> > > "HeapWord*");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>>> > threadLocalAllocBufferSlowAllocationsOffset
>>>>> > > =
>>>>> getFieldOffset("ThreadLocalAllocBuffer::_slow_allocations",
>>>>> > > Integer.class, "unsigned");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>>> > threadLocalAllocBufferFastRefillWasteOffset
>>>>> > > =
>>>>> >
>> getFieldOffset("ThreadLocalAllocBuffer::_fast_refill_waste",
>>>>> > > Integer.class, "unsigned");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>>> > threadLocalAllocBufferNumberOfRefillsOffset
>>>>> > > =
>>>>> >
>> getFieldOffset("ThreadLocalAllocBuffer::_number_of_refills",
>>>>> > > Integer.class, "unsigned");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>>> > > threadLocalAllocBufferRefillWasteLimitOffset =
>>>>> > >
>>>>> getFieldOffset("ThreadLocalAllocBuffer::_refill_waste_limit",
>>>>> > > Integer.class, "size_t");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > private final int
>>>>> > threadLocalAllocBufferDesiredSizeOffset =
>>>>> > >
>>>>> getFieldOffset("ThreadLocalAllocBuffer::_desired_size",
>>>>> > > Integer.class, "size_t");
>>>>> > >
>>>>> >
>>>>>
>>>>
>> org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/GraalHotSpotVMConfig.java:
>>>>> > > public final int tlabAlignmentReserve =
>>>>> > >
>>>>> >
>>>>>
>>>>
>> getFieldValue("CompilerToVM::Data::ThreadLocalAllocBuffer_alignment_reserve",
>>>>> > > Integer.class, "size_t”);
>>>>> > >
>>>>> > > hope this helps,
>>>>> > > Karen
>>>>> > >
>>>>> > >> On Apr 10, 2018, at 7:04 AM, 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>>>> wrote:
>>>>> > >>
>>>>> > >> Hi JC,
>>>>> > >>
>>>>> > >> I realize that the names have been discussed
>>>>> before but I'm
>>>>> > >> leaning towards suggesting something new. We
>>>>> discussed this
>>>>> > >> briefly here in the office and others might
>> have
>>>>> different
>>>>> > >> opinions. One point that came up is that if we
>> do
>>>>> this
>>>>> > change
>>>>> > >> and change all usages of
>>>>> JavaThread::tlab_end_offset() it
>>>>> > >> would be good to make sure the new name is
>> good.
>>>>> To us
>>>>> > >> _current_end is not very descriptive, but
>> naming
>>>>> is hard and
>>>>> > >> the best we could come up with is
>> _fast_path_end
>>>>> which would
>>>>> > >> give the code:
>>>>> > >> cmpptr(end, Address(thread,
>>>>> > >> JavaThread::tlab_fast_path_end_offset()));
>>>>> > >> jcc(Assembler::above, slow_case);
>>>>> > >>
>>>>> > >> I think this reads pretty good and is fairly
>>>>> clear. If we do
>>>>> > >> this rename I think you can re-use _end in
>> JEP-331
>>>>> > instead of
>>>>> > >> calling it _allocation_end. But that is a later
>>>>> review :)
>>>>> > >>
>>>>> > >> Also, is there a good reason for renaming
>>>>> hard_end() to
>>>>> > >> reserved_end()?
>>>>> > >>
>>>>> > >> One additional thing, you need to update the SA
>>>>> to reflect
>>>>> > >> this change. I think the only place you need to
>>>>> fix is in:
>>>>> > >>
>>>>> >
>>>>>
>>>>
>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/runtime/ThreadLocalAllocBuffer.java
>>>>> > >>
>>>>> > >> Thanks,
>>>>> > >> Stefan
>>>>> > >>
>>>>> > >> On 2018-04-09 19:24, JC Beyler wrote:
>>>>> > >>> Hi all,
>>>>> > >>> Small pre-amble to this request:
>>>>> > >>> In my work to try to get a heap sampler in
>>>>> OpenJDK (via JEP
>>>>> > >>> 331
>>>>> > <https://bugs.openjdk.java.net/browse/JDK-8171119>), I'm
>>>>> > >>> trying to reduce the footprint of my change so
>>>>> that the
>>>>> > >>> integration can be easier. I was told that
>>>>> generally a JEP
>>>>> > >>> webrev should be feature complete and go in
>>>> at-once.
>>>>> > However,
>>>>> > >>> with the change touching quite a bit of
>> various
>>>> code
>>>>> > pieces,
>>>>> > >>> I was trying to figure out what could be
>>>>> separated as not
>>>>> > >>> "part of the feature".
>>>>> > >>> I asked around and said that perhaps a
>> solution
>>>>> would be to
>>>>> > >>> cut up the renaming of TLAB's end field that I
>>>>> do in that
>>>>> > >>> webrev. Because I'm renaming a field in TLAB
>>>> used by
>>>>> > various
>>>>> > >>> backends for that work, I have to update every
>>>>> architecture
>>>>> > >>> dependent code to reflect it.
>>>>> > >>> I entirely understand that perhaps this is not
>>>>> in the
>>>>> > habits
>>>>> > >>> and very potentially might not be the way
>> things
>>>> are
>>>>> > >>> generally done. If so, I apologize and let me
>>>>> know if you
>>>>> > >>> would not want this to go in separately :)
>>>>> > >>> Final note: there is still a chance JEP-331
>> does
>>>>> not go in.
>>>>> > >>> If it does not, we can leave the new name in
>>>>> place or I'll
>>>>> > >>> happily revert it. I can even create an issue
>> to
>>>>> track this
>>>>> > >>> if that makes it easier for all.
>>>>> > >>> End of the pre-amble.
>>>>> > >>> The 33-line change webrev in question is here:
>>>>> > >>>
>> http://cr.openjdk.java.net/~jcbeyler/8201326/webrev.00/
>>>>> > >>> I fixed all the architectures and JVMCI and
>> ran
>>>>> a few
>>>>> > sanity
>>>>> > >>> tests to ensure I had not missed anything.
>>>>> > >>> Thanks for your help and I hope this is not
>> too
>>>> much
>>>>> > trouble,
>>>>> > >>> Jc
>>>>> > >>> Ps: there is a graal change that needs to
>> happen
>>>>> but I was
>>>>> > >>> not sure who/where
>> <https://teams.googleplex.com/u/where>
>>>> <https://teams.googleplex.com/u/where>
>>>>> <https://teams.googleplex.com/u/where>
>>>>> > <https://teams.googleplex.com/u/where>
>>>>> > <https://teams.googleplex.com/u/where> to
>>>>> > >>> ask about it. I was told it could happen in a
>>>>> separate
>>>>> > >>> webrev. Can anyone point me to the right
>>>> direction?
>>>>> > Should it
>>>>> > >>> just be hotspot-compiler-dev?
>>>>> > >
>>>>> >
>>>>>
>>
>>
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