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List: openjdk-serviceability-dev
Subject: Re: JDK-8171119: Low-Overhead Heap Profiling
From: JC Beyler <jcbeyler () google ! com>
Date: 2018-02-15 6:34:48
Message-ID: CAF9BGBzUuT3qVZubqyQHB53Q2Abgvy_fNYHB7G4wVc0u2Gn1LQ () mail ! gmail ! com
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Hi Erik,
I inlined my answers, which the last one seems to answer Robbin's concerns
about the same thing (adding things to Thread).
On Wed, Feb 14, 2018 at 2:51 AM, Erik Österlund <erik.osterlund@oracle.com>
wrote:
> Hi JC,
>
> Comments are inlined below.
>
>
> On 2018-02-13 06:18, JC Beyler wrote:
>
> Hi Erik,
>
> Thanks for your answers, I've now inlined my own answers/comments.
>
> I've done a new webrev here:
> http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.08/
>
> The incremental is here:
> http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.07_08/
>
> Note to all:
> - I've been integrating changes from Erin/Serguei/David comments so this
> webrev incremental is a bit an answer to all comments in one. I apologize
> for that :)
>
>
> On Mon, Feb 12, 2018 at 6:05 AM, Erik Österlund <erik.osterlund@oracle.com
> > wrote:
>
> > Hi JC,
> >
> > Sorry for the delayed reply.
> >
> > Inlined answers:
> >
> >
> > On 2018-02-06 00:04, JC Beyler wrote:
> >
> > > Hi Erik,
> > >
> > > (Renaming this to be folded into the newly renamed thread :))
> > >
> > > First off, thanks a lot for reviewing the webrev! I appreciate it!
> > >
> > > I updated the webrev to:
> > > http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.05a/
> > >
> > > And the incremental one is here:
> > > http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.04_05a/
> > >
> > > It contains:
> > > - The change for since from 9 to 11 for the jvmti.xml
> > > - The use of the OrderAccess for initialized
> > > - Clearing the oop
> > >
> > > I also have inlined my answers to your comments. The biggest question
> > > will come from the multiple *_end variables. A bit of the logic there
> > > is due to handling the slow path refill vs fast path refill and
> > > checking that the rug was not pulled underneath the slowpath. I
> > > believe that a previous comment was that TlabFastRefill was going to
> > > be deprecated.
> > >
> > > If this is true, we could revert this code a bit and just do a : if
> > > TlabFastRefill is enabled, disable this. And then deprecate that when
> > > TlabFastRefill is deprecated.
> > >
> > > This might simplify this webrev and I can work on a follow-up that
> > > either: removes TlabFastRefill if Robbin does not have the time to do
> > > it or add the support to the assembly side to handle this correctly.
> > > What do you think?
> > >
> >
> > I support removing TlabFastRefill, but I think it is good to not depend
> > on that happening first.
> >
> >
>
> I'm slowly pushing on the FastTLABRefill (
> <https://bugs.openjdk.java.net/browse/JDK-8194084>
> https://bugs.openjdk.java.net/browse/JDK-8194084), I agree on keeping
> both separate for now though so that we can think of both differently
>
>
>
> > Now, below, inlined are my answers:
> > >
> > > On Fri, Feb 2, 2018 at 8:44 AM, Erik Österlund
> > > <erik.osterlund@oracle.com> wrote:
> > >
> > > > Hi JC,
> > > >
> > > > Hope I am reviewing the right version of your work. Here goes...
> > > >
> > > > src/hotspot/share/gc/shared/collectedHeap.inline.hpp:
> > > >
> > > > 159 AllocTracer::send_allocation_outside_tlab(klass, result,
> > > > size *
> > > > HeapWordSize, THREAD);
> > > > 160
> > > > 161 THREAD->tlab().handle_sample(THREAD, result, size);
> > > > 162 return result;
> > > > 163 }
> > > >
> > > > Should not call tlab()->X without checking if (UseTLAB) IMO.
> > > >
> > > > Done!
> > >
> >
> > More about this later.
> >
> >
> >
> > > src/hotspot/share/gc/shared/threadLocalAllocBuffer.cpp:
> > > >
> > > > So first of all, there seems to quite a few ends. There is an "end", a
> > > > "hard
> > > > end", a "slow path end", and an "actual end". Moreover, it seems like
> > > > the
> > > > "hard end" is actually further away than the "actual end". So the "hard
> > > > end"
> > > > seems like more of a "really definitely actual end" or something. I
> > > > don't
> > > > know about you, but I think it looks kind of messy. In particular, I
> > > > don't
> > > > feel like the name "actual end" reflects what it represents, especially
> > > > when
> > > > there is another end that is behind the "actual end".
> > > >
> > > > 413 HeapWord* ThreadLocalAllocBuffer::hard_end() {
> > > > 414 // Did a fast TLAB refill occur?
> > > > 415 if (_slow_path_end != _end) {
> > > > 416 // Fix up the actual end to be now the end of this TLAB.
> > > > 417 _slow_path_end = _end;
> > > > 418 _actual_end = _end;
> > > > 419 }
> > > > 420
> > > > 421 return _actual_end + alignment_reserve();
> > > > 422 }
> > > >
> > > > I really do not like making getters unexpectedly have these kind of side
> > > > effects. It is not expected that when you ask for the "hard end", you
> > > > implicitly update the "slow path end" and "actual end" to new values.
> > > >
> > > > As I said, a lot of this is due to the FastTlabRefill. If I make this
> > > not supporting FastTlabRefill, this goes away. The reason the system
> > > needs to update itself at the get is that you only know at that get if
> > > things have shifted underneath the tlab slow path. I am not sure of
> > > really better names (naming is hard!), perhaps we could do these
> > > names:
> > >
> > > - current_tlab_end // Either the allocated tlab end or a sampling
> > > point
> > > - last_allocation_address // The end of the tlab allocation
> > > - last_slowpath_allocated_end // In case a fast refill occurred the
> > > end might have changed, this is to remember slow vs fast past refills
> > >
> > > the hard_end method can be renamed to something like:
> > > tlab_end_pointer() // The end of the lab including a bit of
> > > alignment reserved bytes
> > >
> >
> > Those names sound better to me. Could you please provide a mapping from
> > the old names to the new names so I understand which one is which please?
> >
> > This is my current guess of what you are proposing:
> >
> > end -> current_tlab_end
> > actual_end -> last_allocation_address
> > slow_path_end -> last_slowpath_allocated_end
> > hard_end -> tlab_end_pointer
> >
> >
> Yes that is correct, that was what I was proposing.
>
>
> > I would prefer this naming:
> >
> > end -> slow_path_end // the end for taking a slow path; either due to
> > sampling or refilling
> > actual_end -> allocation_end // the end for allocations
> > slow_path_end -> last_slow_path_end // last address for slow_path_end (as
> > opposed to allocation_end)
> > hard_end -> reserved_end // the end of the reserved space of the TLAB
> >
> > About setting things in the getter... that still seems like a very
> > unpleasant thing to me. It would be better to inspect the call hierarchy
> > and explicitly update the ends where they need updating, and assert in the
> > getter that they are in sync, rather than implicitly setting various ends
> > as a surprising side effect in a getter. It looks like the call hierarchy
> > is very small. With my new naming convention, reserved_end() would
> > presumably return _allocation_end + alignment_reserve(), and have an assert
> > checking that _allocation_end == _last_slow_path_allocation_end,
> > complaining that this invariant must hold, and that a caller to this
> > function, such as make_parsable(), must first explicitly synchronize the
> > ends as required, to honor that invariant.
> >
> >
>
> I've renamed the variables to how you preferred it except for the _end
> one. I did:
> current_end
> last_allocation_address
> tlab_end_ptr
>
> The reason is that the architecture dependent code use the thread.hpp API
> and it already has tlab included into the name so it becomes
> tlab_current_end (which is better that tlab_current_tlab_end in my opinion).
>
> I also moved the update into a separate method with a TODO that says to
> remove it when FastTLABRefill is deprecated
>
>
> This looks a lot better now. Thanks.
>
> Note that the following comment now needs updating accordingly in
> threadLocalAllocBuffer.hpp:
>
> 41 // Heap sampling is performed via the end/actual_end fields. 42 // \
> actual_end contains the real end of the tlab allocation, 43 // whereas \
> end can be set to an arbitrary spot in the tlab to 44 // trip the \
> return and sample the allocation. 45 // slow_path_end is used to track \
> if a fast tlab refill occured 46 // between slowpath calls.
> There might be other comments too, I have not looked in detail.
>
This was the only spot that still had an actual_end, I fixed it now. I'll
do a sweep to double check other comments.
>
>
>
>
>
> >
> > Not sure it's better but before updating the webrev, I wanted to try
> > > to get input/consensus :)
> > >
> > > (Note hard_end was always further off than end).
> > >
> > > src/hotspot/share/prims/jvmti.xml:
> > > >
> > > > 10357 <capabilityfield id="can_sample_heap" since="9">
> > > > 10358 <description>
> > > > 10359 Can sample the heap.
> > > > 10360 If this capability is enabled then the heap sampling
> > > > methods
> > > > can be called.
> > > > 10361 </description>
> > > > 10362 </capabilityfield>
> > > >
> > > > Looks like this capability should not be "since 9" if it gets integrated
> > > > now.
> > > >
> > > Updated now to 11, crossing my fingers :)
> > >
> > >
> > > src/hotspot/share/runtime/heapMonitoring.cpp:
> > > >
> > > > 448 if (is_alive->do_object_b(value)) {
> > > > 449 // Update the oop to point to the new object if it is
> > > > still
> > > > alive.
> > > > 450 f->do_oop(&(trace.obj));
> > > > 451
> > > > 452 // Copy the old trace, if it is still live.
> > > > 453 _allocated_traces->at_put(curr_pos++, trace);
> > > > 454
> > > > 455 // Store the live trace in a cache, to be served up on
> > > > /heapz.
> > > > 456 _traces_on_last_full_gc->append(trace);
> > > > 457
> > > > 458 count++;
> > > > 459 } else {
> > > > 460 // If the old trace is no longer live, add it to the list
> > > > of
> > > > 461 // recently collected garbage.
> > > > 462 store_garbage_trace(trace);
> > > > 463 }
> > > >
> > > > In the case where the oop was not live, I would like it to be explicitly
> > > > cleared.
> > > >
> > > Done I think how you wanted it. Let me know because I'm not familiar
> > > with the RootAccess API. I'm unclear if I'm doing this right or not so
> > > reviews of these parts are highly appreciated. Robbin had talked of
> > > perhaps later pushing this all into a OopStorage, should I do this now
> > > do you think? Or can that wait a second webrev later down the road?
> > >
> >
> > I think using handles can and should be done later. You can use the
> > Access API now.
> > I noticed that you are missing an #include "oops/access.inline.hpp" in
> > your heapMonitoring.cpp file.
> >
> >
> The missing header is there for me so I don't know, I made sure it is
> present in the latest webrev. Sorry about that.
>
>
>
> > + Did I clear it the way you wanted me to or were you thinking of
> > > something else?
> > >
> >
> > That is precisely how I wanted it to be cleared. Thanks.
> >
> > + Final question here, seems like if I were to want to not do the
> > > f->do_oop directly on the trace.obj, I'd need to do something like:
> > >
> > > f->do_oop(&value);
> > > ...
> > > trace->store_oop(value);
> > >
> > > to update the oop internally. Is that right/is that one of the
> > > advantages of going to the Oopstorage sooner than later?
> > >
> >
> > I think you really want to do the do_oop on the root directly. Is there a
> > particular reason why you would not want to do that?
> > Otherwise, yes - the benefit with using the handle approach is that you
> > do not need to call do_oop explicitly in your code.
> >
> >
> There is no reason except that now we have a load_oop and a get_oop_addr,
> I was not sure what you would think of that.
>
>
> That's fine.
>
>
>
> >
> > > Also I see a lot of concurrent-looking use of the following field:
> > > > 267 volatile bool _initialized;
> > > >
> > > > Please note that the "volatile" qualifier does not help with reordering
> > > > here. Reordering between volatile and non-volatile fields is completely
> > > > free
> > > > for both compiler and hardware, except for windows with MSVC, where
> > > > volatile
> > > > semantics is defined to use acquire/release semantics, and the hardware
> > > > is
> > > > TSO. But for the general case, I would expect this field to be stored
> > > > with
> > > > OrderAccess::release_store and loaded with OrderAccess::load_acquire.
> > > > Otherwise it is not thread safe.
> > > >
> > > Because everything is behind a mutex, I wasn't really worried about
> > > this. I have a test that has multiple threads trying to hit this
> > > corner case and it passes.
> > >
> > > However, to be paranoid, I updated it to using the OrderAccess API
> > > now, thanks! Let me know what you think there too!
> > >
> >
> > If it is indeed always supposed to be read and written under a mutex,
> > then I would strongly prefer to have it accessed as a normal non-volatile
> > member, and have an assertion that given lock is held or we are in a
> > safepoint, as we do in many other places. Something like this:
> >
> > assert(HeapMonitorStorage_lock->owned_by_self() ||
> > (SafepointSynchronize::is_at_safepoint() &&
> > Thread::current()->is_VM_thread()), "this should not be accessed
> > concurrently");
> >
> > It would be confusing to people reading the code if there are uses of
> > OrderAccess that are actually always protected under a mutex.
> >
> >
> Thank you for the exact example to be put in the code! I put it around
> each access/assignment of the _initialized method and found one case where
> yes you can touch it and not have the lock. It actually is "ok" because you
> don't act on the storage until later and only when you really want to
> modify the storage (see the object_alloc_do_sample method which calls the
> add_trace method).
>
> But, because of this, I'm going to put the OrderAccess here, I'll do some
> performance numbers later and if there are issues, I might add a "unsafe"
> read and a "safe" one to make it explicit to the reader. But I don't think
> it will come to that.
>
>
> Okay. This double return in heapMonitoring.cpp looks wrong:
>
> 283 bool initialized() {
> 284 return OrderAccess::load_acquire(&_initialized) != 0;
> 285 return _initialized;
> 286 }
>
> Since you said object_alloc_do_sample() is the only place where you do not
> hold the mutex while reading initialized(), I had a closer look at that. It
> looks like in its current shape, the lack of a mutex may lead to a memory
> leak. In particular, it first checks if (initialized()). Let's assume this
> is now true. It then allocates a bunch of stuff, and checks if the number
> of frames were over 0. If they were, it calls \
> StackTraceStorage::storage()->add_trace() seemingly hoping that after grabbing the \
> lock in there, initialized() will still return true. But it could now return false \
> and skip doing anything, in which case the allocated stuff will never be freed.
>
I fixed this now by making add_trace return a boolean and checking for
that. It will be in the next webrev. Thanks, the truth is that in our
implementation the system is always on or off, so this never really occurs
> ). In this version though, that is not true and it's important to handle
so thanks again!
>
> So the analysis seems to be that _initialized is only used outside of the
> mutex in once instance, where it is used to perform double-checked locking,
> that actually causes a memory leak.
>
> I am not proposing how to fix that, just raising the issue. If you still
> want to perform this double-checked locking somehow, then the use of
> acquire/release still seems odd. Because the memory ordering restrictions
> of it never comes into play in this particular case. If it ever did, then
> the use of destroy_stuff(); release_store(_initialized, 0) would be broken
> anyway as that would imply that whatever concurrent reader there ever was
> would after reading _initialized with load_acquire() could *never* read the
> data that is concurrently destroyed anyway. I would be biased to think that
> RawAccess<MO_RELAXED>::load/store looks like a more appropriate solution,
> given that the memory leak issue is resolved. I do not know how painful it
> would be to not perform this double-checked locking.
>
So I agree with this entirely. I looked also a bit more and the difference
and code really stems from our internal version. In this version however,
there are actually a lot of things going on that I did not go entirely
through in my head but this comment made me ponder a bit more on it.
Since every object_alloc_do_sample is protected by a check to
HeapMonitoring::enabled(), there is only a small chance that the call is
happening when things have been disabled. So there is no real need to do a
first check on the initialized, it is a rare occurence that a call happens
to object_alloc_do_sample and the initialized of the storage returns false.
(By the way, even if you did call object_alloc_do_sample without looking at
HeapMonitoring::enabled(), that would be ok too. You would gather the
stacktrace and get nowhere at the add_trace call, which would return false;
so though not optimal performance wise, nothing would break).
Furthermore, the add_trace is really the moment of no return and we have
the mutex lock and then the initialized check. So, in the end, I did two
things: I removed that first check and then I removed the OrderAccess for
the storage initialized. I think now I have a better grasp and
understanding why it was done in our code and why it is not needed here.
Thanks for pointing it out :). This now still passes my JTREG tests,
especially the threaded one.
>
>
>
>
>
> > As a kind of meta comment, I wonder if it would make sense to add sampling
> > > > for non-TLAB allocations. Seems like if someone is rapidly allocating a
> > > > whole bunch of 1 MB objects that never fit in a TLAB, I might still be
> > > > interested in seeing that in my traces, and not get surprised that the
> > > > allocation rate is very high yet not showing up in any profiles.
> > > >
> > > > That is handled by the handle_sample where you wanted me to put a
> > > UseTlab because you hit that case if the allocation is too big.
> > >
> >
> > I see. It was not obvious to me that non-TLAB sampling is done in the
> > TLAB class. That seems like an abstraction crime.
> > What I wanted in my previous comment was that we do not call into the
> > TLAB when we are not using TLABs. If there is sampling logic in the TLAB
> > that is used for something else than TLABs, then it seems like that logic
> > simply does not belong inside of the TLAB. It should be moved out of the
> > TLAB, and instead have the TLAB call this common abstraction that makes
> > sense.
> >
> >
> So in the incremental version:
> http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.07_08/, this is still
> a "crime". The reason is that the system has to have the bytes_until_sample
> on a per-thread level and it made "sense" to have it with the TLAB
> implementation. Also, I was not sure how people felt about adding something
> to the thread instance instead.
>
> Do you think it fits better at the Thread level? I can see how difficult
> it is to make it happen there and add some logic there. Let me know what
> you think.
>
>
> We have an unfortunate situation where everyone that has some fields that
> are thread local tend to dump them right into Thread, making the size and
> complexity of Thread grow as it becomes tightly coupled with various
> unrelated subsystems. It would be desirable to have a separate class for
> this instead that encapsulates the sampling logic. That class could
> possibly reside in Thread though as a value object of Thread.
>
I imagined that would be the case but was not sure. I will look at the
example that Robbin is talking about (ThreadSMR) and will see how to
refactor my code to use that.
Thanks again for your help,
Jc
>
>
>
>
>
> > Hope I have answered your questions and that my feedback makes sense to
> > you.
> >
> >
> You have and thank you for them, I think we are getting to a cleaner
> implementation and things are getting better and more readable :)
>
>
> Yes it is getting better.
>
> Thanks,
> /Erik
>
>
> Thanks for your help!
> Jc
>
>
>
> > Thanks,
> > /Erik
> >
> >
> > I double checked by changing the test
> > > http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.05a/raw_
> > > files/new/test/hotspot/jtreg/serviceability/jvmti/HeapMonito
> > > r/MyPackage/HeapMonitorStatObjectCorrectnessTest.java
> > >
> > > to use a smaller Tlab (2048) and made the object bigger and it goes
> > > through that and passes.
> > >
> > > Thanks again for your review and I look forward to your pointers for
> > > the questions I now have raised!
> > > Jc
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > Thanks,
> > > > /Erik
> > > >
> > > >
> > > > On 2018-01-26 06:45, JC Beyler wrote:
> > > >
> > > > > Thanks Robbin for the reviews :)
> > > > >
> > > > > The new full webrev is here:
> > > > > http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.03/
> > > > > The incremental webrev is here:
> > > > > http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.02_03/
> > > > >
> > > > > I inlined my answers:
> > > > >
> > > > > On Thu, Jan 25, 2018 at 1:15 AM, Robbin Ehn < <robbin.ehn@oracle.com>
> > > > > robbin.ehn@oracle.com> wrote:
> > > > >
> > > > > > Hi JC, great to see another revision!
> > > > > >
> > > > > > ####
> > > > > > heapMonitoring.cpp
> > > > > >
> > > > > > StackTraceData should not contain the oop for 'safety' reasons.
> > > > > > When StackTraceData is moved from _allocated_traces:
> > > > > > L452 store_garbage_trace(trace);
> > > > > > it contains a dead oop.
> > > > > > _allocated_traces could instead be a tupel of oop and StackTraceData
> > > > > > thus
> > > > > > dead oops are not kept.
> > > > > >
> > > > > Done I used inheritance to make the copier work regardless but the
> > > > > idea is the same.
> > > > >
> > > > > You should use the new Access API for loading the oop, something like
> > > > > > this:
> > > > > > RootAccess<ON_PHANTOM_OOP_REF | AS_NO_KEEPALIVE>::load(...)
> > > > > > I don't think you need to use Access API for clearing the oop, but it
> > > > > > would
> > > > > > look nicer. And you shouldn't probably be using:
> > > > > > Universe::heap()->is_in_reserved(value)
> > > > > >
> > > > > I am unfamiliar with this but I think I did do it like you wanted me
> > > > > to (all tests pass so that's a start). I'm not sure how to clear the
> > > > > oop exactly, is there somewhere that does that, which I can use to do
> > > > > the same?
> > > > >
> > > > > I removed the is_in_reserved, this came from our internal version, I
> > > > > don't know why it was there but my tests work without so I removed it
> > > > > > )
> > > > >
> > > > >
> > > > > The lock:
> > > > > > L424 MutexLocker mu(HeapMonitorStorage_lock);
> > > > > > Is not needed as far as I can see.
> > > > > > weak_oops_do is called in a safepoint, no TLAB allocation can happen
> > > > > > and
> > > > > > JVMTI thread can't access these data-structures. Is there something
> > > > > > more
> > > > > > to
> > > > > > this lock that I'm missing?
> > > > > >
> > > > > Since a thread can call the JVMTI getLiveTraces (or any of the other
> > > > > ones), it can get to the point of trying to copying the
> > > > > _allocated_traces. I imagine it is possible that this is happening
> > > > > during a GC or that it can be started and a GC happens afterwards.
> > > > > Therefore, it seems to me that you want this protected, no?
> > > > >
> > > > >
> > > > > ####
> > > > > > You have 6 files without any changes in them (any more):
> > > > > > g1CollectedHeap.cpp
> > > > > > psMarkSweep.cpp
> > > > > > psParallelCompact.cpp
> > > > > > genCollectedHeap.cpp
> > > > > > referenceProcessor.cpp
> > > > > > thread.hpp
> > > > > >
> > > > > > Done.
> > > > >
> > > > > ####
> > > > > > I have not looked closely, but is it possible to hide heap sampling in
> > > > > > AllocTracer ? (with some minor changes to the AllocTracer API)
> > > > > >
> > > > > > I am imagining that you are saying to move the code that does the
> > > > > sampling code (change the tlab end, do the call to HeapMonitoring,
> > > > > etc.) into the AllocTracer code itself? I think that is right and I'll
> > > > > look if that is possible and prepare a webrev to show what would be
> > > > > needed to make that happen.
> > > > >
> > > > > ####
> > > > > > Minor nit, when declaring pointer there is a little mix of having the
> > > > > > pointer adjacent by type name and data name. (Most hotspot code is by
> > > > > > type
> > > > > > name)
> > > > > > E.g.
> > > > > > heapMonitoring.cpp:711 jvmtiStackTrace *trace = ....
> > > > > > heapMonitoring.cpp:733 Method* m = vfst.method();
> > > > > > (not just this file)
> > > > > >
> > > > > > Done!
> > > > >
> > > > > ####
> > > > > > HeapMonitorThreadOnOffTest.java:77
> > > > > > I would make g_tmp volatile, otherwise the assignment in loop may
> > > > > > theoretical be skipped.
> > > > > >
> > > > > > Also done!
> > > > >
> > > > > Thanks again!
> > > > > Jc
> > > > >
> > > >
> > > >
> >
>
>
[Attachment #3 (text/html)]
<div dir="ltr">Hi Erik,<div><br></div><div>I inlined my answers, which the last one \
seems to answer Robbin's concerns about the same thing (adding things to \
Thread).</div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Feb 14, \
2018 at 2:51 AM, Erik Österlund <span dir="ltr"><<a \
href="mailto:erik.osterlund@oracle.com" \
target="_blank">erik.osterlund@oracle.com</a>></span> wrote:<br><blockquote \
class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
Hi JC,<br>
<br>
Comments are inlined below.<div><div class="h5"><br>
<br>
<div class="m_-513143501369679108moz-cite-prefix">On 2018-02-13 06:18, JC Beyler \
wrote:<br> </div>
<blockquote type="cite">
<div dir="ltr">Hi Erik,
<div><br>
</div>
<div>Thanks for your answers, I've now inlined my own
answers/comments.</div>
<div><br>
</div>
<div>I've done a new webrev here:</div>
<div><a href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.08/" \
target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/webrev.08/</a><br> \
</div> <div><br>
</div>
<div>The incremental is here:</div>
<div><a href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.07_08/" \
target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/webrev.07_08/</a><br>
</div>
<div><br>
</div>
<div>Note to all:</div>
<div> - I've been integrating changes from Erin/Serguei/David
comments so this webrev incremental is a bit an answer to all
comments in one. I apologize for that :)</div>
<div><br>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Feb 12, 2018 at 6:05 AM, Erik
Österlund <span dir="ltr"><<a href="mailto:erik.osterlund@oracle.com" \
target="_blank">erik.osterlund@oracle.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi JC,<br> <br>
Sorry for the delayed reply.<br>
<br>
Inlined answers:
<div>
<div \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379h5"><br> \
<br> On 2018-02-06 00:04, JC Beyler wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Hi Erik,<br>
<br>
(Renaming this to be folded into the newly renamed
thread :))<br>
<br>
First off, thanks a lot for reviewing the webrev! I
appreciate it!<br>
<br>
I updated the webrev to:<br>
<a \
href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.05a/" rel="noreferrer" \
target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/webrev.05a/</a><br> \
<br> And the incremental one is here:<br>
<a \
href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.04_05a/" rel="noreferrer" \
target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/webrev.04_05a/</a><br>
<br>
It contains:<br>
- The change for since from 9 to 11 for the
jvmti.xml<br>
- The use of the OrderAccess for initialized<br>
- Clearing the oop<br>
<br>
I also have inlined my answers to your comments. The
biggest question<br>
will come from the multiple *_end variables. A bit
of the logic there<br>
is due to handling the slow path refill vs fast path
refill and<br>
checking that the rug was not pulled underneath the
slowpath. I<br>
believe that a previous comment was that
TlabFastRefill was going to<br>
be deprecated.<br>
<br>
If this is true, we could revert this code a bit and
just do a : if<br>
TlabFastRefill is enabled, disable this. And then
deprecate that when<br>
TlabFastRefill is deprecated.<br>
<br>
This might simplify this webrev and I can work on a
follow-up that<br>
either: removes TlabFastRefill if Robbin does not
have the time to do<br>
it or add the support to the assembly side to handle
this correctly.<br>
What do you think?<br>
</blockquote>
<br>
</div>
</div>
I support removing TlabFastRefill, but I think it is good
to not depend on that happening first.<span><br>
<br>
</span></blockquote>
<div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379gmail-Apple-interchange-newline">
I'm slowly pushing on the FastTLABRefill (<a \
href="https://bugs.openjdk.java.net/browse/JDK-8194084" target="_blank"></a><a \
class="m_-513143501369679108moz-txt-link-freetext" \
href="https://bugs.openjdk.java.net" \
target="_blank">https://bugs.openjdk.java.net</a><wbr>/browse/JDK-8194084), I agree \
on keeping both separate for now though so that we can think of both \
differently</span><br> </div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Now, below, inlined \
are my answers:<br> <br>
On Fri, Feb 2, 2018 at 8:44 AM, Erik Österlund<br>
<<a href="mailto:erik.osterlund@oracle.com" \
target="_blank">erik.osterlund@oracle.com</a>> wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Hi JC,<br>
<br>
Hope I am reviewing the right version of your work.
Here goes...<br>
<br>
src/hotspot/share/gc/shared/co<wbr>llectedHeap.inline.hpp:<br>
<br>
159 \
AllocTracer::send_allocation_<wbr>outside_tlab(klass, result, size *<br>
HeapWordSize, THREAD);<br>
160<br>
161 THREAD->tlab().handle_sample(<wbr>THREAD,
result, size);<br>
162 return result;<br>
163 }<br>
<br>
Should not call tlab()->X without checking if
(UseTLAB) IMO.<br>
<br>
</blockquote>
Done!<br>
</blockquote>
<br>
</span>
More about this later.
<div>
<div \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379h5"><br> \
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> <br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
\
src/hotspot/share/gc/shared/th<wbr>readLocalAllocBuffer.cpp:<br> <br>
So first of all, there seems to quite a few ends.
There is an "end", a "hard<br>
end", a "slow path end", and an "actual \
end". Moreover, it seems like the<br>
"hard end" is actually further away than the
"actual end". So the "hard end"<br>
seems like more of a "really definitely actual
end" or something. I don't<br>
know about you, but I think it looks kind of
messy. In particular, I don't<br>
feel like the name "actual end" reflects what it
represents, especially when<br>
there is another end that is behind the "actual
end".<br>
<br>
413 HeapWord* ThreadLocalAllocBuffer::hard_e<wbr>nd()
{<br>
414 // Did a fast TLAB refill occur?<br>
415 if (_slow_path_end != _end) {<br>
416 // Fix up the actual end to be now the
end of this TLAB.<br>
417 _slow_path_end = _end;<br>
418 _actual_end = _end;<br>
419 }<br>
420<br>
421 return _actual_end + alignment_reserve();<br>
422 }<br>
<br>
I really do not like making getters unexpectedly
have these kind of side<br>
effects. It is not expected that when you ask for
the "hard end", you<br>
implicitly update the "slow path end" and \
"actual end" to new values.<br>
<br>
</blockquote>
As I said, a lot of this is due to the
FastTlabRefill. If I make this<br>
not supporting FastTlabRefill, this goes away. The
reason the system<br>
needs to update itself at the get is that you only
know at that get if<br>
things have shifted underneath the tlab slow path. I
am not sure of<br>
really better names (naming is hard!), perhaps we
could do these<br>
names:<br>
<br>
- current_tlab_end // Either the allocated
tlab end or a sampling point<br>
- last_allocation_address // The end of the tlab
allocation<br>
- last_slowpath_allocated_end // In case a fast
refill occurred the<br>
end might have changed, this is to remember slow vs
fast past refills<br>
<br>
the hard_end method can be renamed to something
like:<br>
tlab_end_pointer() // The end of the lab
including a bit of<br>
alignment reserved bytes<br>
</blockquote>
<br>
</div>
</div>
Those names sound better to me. Could you please provide a
mapping from the old names to the new names so I
understand which one is which please?<br>
<br>
This is my current guess of what you are proposing:<br>
<br>
end -> current_tlab_end<br>
actual_end -> last_allocation_address<br>
slow_path_end -> last_slowpath_allocated_end<br>
hard_end -> tlab_end_pointer<br>
<br>
</blockquote>
<div><br>
</div>
<div>Yes that is correct, that was what I was proposing.</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> I would prefer this \
naming:<br> <br>
end -> slow_path_end // the end for taking a slow path;
either due to sampling or refilling<br>
actual_end -> allocation_end // the end for allocations<br>
slow_path_end -> last_slow_path_end // last address for
slow_path_end (as opposed to allocation_end)<br>
hard_end -> reserved_end // the end of the reserved
space of the TLAB<br>
<br>
About setting things in the getter... that still seems
like a very unpleasant thing to me. It would be better to
inspect the call hierarchy and explicitly update the ends
where they need updating, and assert in the getter that
they are in sync, rather than implicitly setting various
ends as a surprising side effect in a getter. It looks
like the call hierarchy is very small. With my new naming
convention, reserved_end() would presumably return
_allocation_end + alignment_reserve(), and have an assert
checking that _allocation_end ==
_last_slow_path_allocation_end<wbr>, complaining that this
invariant must hold, and that a caller to this function,
such as make_parsable(), must first explicitly synchronize
the ends as required, to honor that invariant.
<div>
<div \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379h5"><br> \
</div> </div>
</blockquote>
<div><br>
</div>
<div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><br \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379gmail-Apple-interchange-newline">
I've renamed the variables to how you preferred it
except for the _end one. I did:</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">current_end</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-st \
yle:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;lett \
er-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:nor \
mal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-dec \
oration-color:initial;float:none;display:inline">last_allocation_address</span><br> \
</div> <div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;f \
ont-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:40 \
0;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-spa \
ce:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">tlab_end_ptr</span><br>
</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br>
</span></div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The
reason is that the architecture dependent code use the
thread.hpp API and it already has tlab included into
the name so it becomes tlab_current_end (which is
better that tlab_current_tlab_end in my opinion).</span></div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">I
also moved the update into a separate method with a TODO
that says to remove it when FastTLABRefill is deprecated<br>
</div>
</div>
</div>
</div>
</div>
</blockquote>
<br></div></div>
This looks a lot better now. Thanks.<br>
<br>
Note that the following comment now needs updating accordingly in
threadLocalAllocBuffer.hpp:<br>
<br>
<pre><span class="m_-513143501369679108new"> 41 // Heap sampling is \
performed via the end/actual_end fields.</span> <span \
class="m_-513143501369679108new"> 42 // actual_end contains the real end \
of the tlab allocation,</span> <span class="m_-513143501369679108new"> 43 // \
whereas end can be set to an arbitrary spot in the tlab to</span> <span \
class="m_-513143501369679108new"> 44 // trip the return and sample the \
allocation.</span> <span class="m_-513143501369679108new"> 45 // \
slow_path_end is used to track if a fast tlab refill occured</span> <span \
class="m_-513143501369679108new"> 46 // between slowpath \
calls.</span></pre> There might be other comments too, I have not looked in \
detail.</div></blockquote><div><br></div><div>This was the only spot that still had \
an actual_end, I fixed it now. I'll do a sweep to double check other \
comments.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 \
.8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" \
text="#000000"><div><div class="h5"><br> <br>
<blockquote type="cite">
<div dir="ltr">
<div class="gmail_extra">
<div class="gmail_quote">
<div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> <div>
<div \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379h5"> \
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
Not sure it's better but before updating the webrev,
I wanted to try<br>
to get input/consensus :)<br>
<br>
(Note hard_end was always further off than end).<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> \
src/hotspot/share/prims/jvmti.<wbr>xml:<br> <br>
10357 <capabilityfield
id="can_sample_heap" since="9"><br>
10358 <description><br>
10359 Can sample the heap.<br>
10360 If this capability is enabled then
the heap sampling methods<br>
can be called.<br>
10361 </description><br>
10362 </capabilityfield><br>
<br>
Looks like this capability should not be "since 9"
if it gets integrated<br>
now.<br>
</blockquote>
Updated now to 11, crossing my fingers :)<br>
<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
src/hotspot/share/runtime/heap<wbr>Monitoring.cpp:<br>
<br>
448 if (is_alive->do_object_b(value)) {<br>
449 // Update the oop to point to the
new object if it is still<br>
alive.<br>
450 f->do_oop(&(trace.obj));<br>
451<br>
452 // Copy the old trace, if it is
still live.<br>
453 \
_allocated_traces->at_put(cur<wbr>r_pos++, trace);<br>
454<br>
455 // Store the live trace in a cache,
to be served up on /heapz.<br>
456 \
_traces_on_last_full_gc->appe<wbr>nd(trace);<br> 457<br>
458 count++;<br>
459 } else {<br>
460 // If the old trace is no longer
live, add it to the list of<br>
461 // recently collected garbage.<br>
462 store_garbage_trace(trace);<br>
463 }<br>
<br>
In the case where the oop was not live, I would
like it to be explicitly<br>
cleared.<br>
</blockquote>
Done I think how you wanted it. Let me know because
I'm not familiar<br>
with the RootAccess API. I'm unclear if I'm doing
this right or not so<br>
reviews of these parts are highly appreciated.
Robbin had talked of<br>
perhaps later pushing this all into a OopStorage,
should I do this now<br>
do you think? Or can that wait a second webrev later
down the road?<br>
</blockquote>
<br>
</div>
</div>
I think using handles can and should be done later. You
can use the Access API now.<br>
I noticed that you are missing an #include
"oops/access.inline.hpp" in your heapMonitoring.cpp \
file.<span><br> <br>
</span></blockquote>
<div><br>
</div>
<div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial;background-color:rgb(255,255,255)"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The
missing header is there for me so I don't know, I made
sure it is present in the latest webrev. Sorry about
that.</span><br \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379gmail-Apple-interchange-newline">
<br>
</div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial;background-color:rgb(255,255,255)"> \
<br> </div>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> + Did I clear it the \
way you wanted me to or were you thinking of<br>
something else?<br>
</blockquote>
<br>
</span>
That is precisely how I wanted it to be cleared. Thanks.<span><br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> + Final question \
here, seems like if I were to want to not do the<br>
f->do_oop directly on the trace.obj, I'd need to do
something like:<br>
<br>
f->do_oop(&value);<br>
...<br>
trace->store_oop(value);<br>
<br>
to update the oop internally. Is that right/is that
one of the<br>
advantages of going to the Oopstorage sooner than
later?<br>
</blockquote>
<br>
</span>
I think you really want to do the do_oop on the root
directly. Is there a particular reason why you would not
want to do that?<br>
Otherwise, yes - the benefit with using the handle
approach is that you do not need to call do_oop explicitly
in your code.<span><br>
<br>
</span></blockquote>
<div><br>
</div>
<div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial;background-color:rgb(255,255,255)">There
is no reason except that now we have a load_oop and a
get_oop_addr, I was not sure what you would think of
that.</div>
<br>
</div>
</div>
</div>
</div>
</blockquote>
<br></div></div>
That's fine.<span class=""><br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div class="gmail_extra">
<div class="gmail_quote">
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> <br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Also I see a lot of \
concurrent-looking use of the following field:<br>
267 volatile bool _initialized;<br>
<br>
Please note that the "volatile" qualifier does not
help with reordering<br>
here. Reordering between volatile and non-volatile
fields is completely free<br>
for both compiler and hardware, except for windows
with MSVC, where volatile<br>
semantics is defined to use acquire/release
semantics, and the hardware is<br>
TSO. But for the general case, I would expect this
field to be stored with<br>
OrderAccess::release_store and loaded with
OrderAccess::load_acquire.<br>
Otherwise it is not thread safe.<br>
</blockquote>
Because everything is behind a mutex, I wasn't really
worried about<br>
this. I have a test that has multiple threads trying
to hit this<br>
corner case and it passes.<br>
<br>
However, to be paranoid, I updated it to using the
OrderAccess API<br>
now, thanks! Let me know what you think there too!<br>
</blockquote>
<br>
</span>
If it is indeed always supposed to be read and written
under a mutex, then I would strongly prefer to have it
accessed as a normal non-volatile member, and have an
assertion that given lock is held or we are in a
safepoint, as we do in many other places. Something like
this:<br>
<br>
assert(HeapMonitorStorage_lock<wbr>->owned_by_self() ||
(SafepointSynchronize::is_at_s<wbr>afepoint() &&
Thread::current()->is_VM_threa<wbr>d()), "this should
not be accessed concurrently");<br>
<br>
It would be confusing to people reading the code if there
are uses of OrderAccess that are actually always protected
under a mutex.<span><br>
<br>
</span></blockquote>
<div><br>
</div>
<div>Thank you for the exact example to be put in the code!
I put it around each access/assignment of the _initialized
method and found one case where yes you can touch it and
not have the lock. It actually is "ok" because you don't
act on the storage until later and only when you really
want to modify the storage (see the object_alloc_do_sample
method which calls the add_trace method).</div>
<div><br>
</div>
<div>But, because of this, I'm going to put the OrderAccess
here, I'll do some performance numbers later and if there
are issues, I might add a "unsafe" read and a \
"safe" one to make it explicit to the reader. But I don't think it
will come to that.</div>
</div>
</div>
</div>
</blockquote>
<br></span>
Okay. This double return in heapMonitoring.cpp looks wrong:<br>
<br>
283 bool initialized() {<br>
284 return OrderAccess::load_acquire(&_<wbr>initialized) != 0;<br>
285 return _initialized;<br>
286 }<br>
<br>
Since you said object_alloc_do_sample() is the only place where you
do not hold the mutex while reading initialized(), I had a closer
look at that. It looks like in its current shape, the lack of a
mutex may lead to a memory leak. In particular, it first checks if
(initialized()). Let's assume this is now true. It then allocates a
bunch of stuff, and checks if the number of frames were over 0. If
they were, it calls StackTraceStorage::storage()-><wbr>add_trace()
seemingly hoping that after grabbing the lock in there,
initialized() will still return true. But it could now return false
and skip doing anything, in which case the allocated stuff will
never be freed.<br></div></blockquote><div><br></div><div>I fixed this now by \
making add_trace return a boolean and checking for that. It will be in the next \
webrev. Thanks, the truth is that in our implementation the system is always on or \
off, so this never really occurs :). In this version though, that is not true and \
it's important to handle so thanks again!</div><div><br></div><div> \
</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"> <br>
So the analysis seems to be that _initialized is only used outside
of the mutex in once instance, where it is used to perform
double-checked locking, that actually causes a memory leak.<br>
<br>
I am not proposing how to fix that, just raising the issue. If you
still want to perform this double-checked locking somehow, then the
use of acquire/release still seems odd. Because the memory ordering
restrictions of it never comes into play in this particular case. If
it ever did, then the use of destroy_stuff();
release_store(_initialized, 0) would be broken anyway as that would
imply that whatever concurrent reader there ever was would after
reading _initialized with load_acquire() could *never* read the data
that is concurrently destroyed anyway. I would be biased to think
that RawAccess<MO_RELAXED>::load/<wbr>store looks like a more
appropriate solution, given that the memory leak issue is resolved.
I do not know how painful it would be to not perform this
double-checked locking.</div></blockquote><div><br></div><div>So I agree with \
this entirely. I looked also a bit more and the difference and code really stems from \
our internal version. In this version however, there are actually a lot of things \
going on that I did not go entirely through in my head but this comment made me \
ponder a bit more on it.</div><div><br></div><div>Since every object_alloc_do_sample \
is protected by a check to HeapMonitoring::enabled(), there is only a small chance \
that the call is happening when things have been disabled. So there is no real need \
to do a first check on the initialized, it is a rare occurence that a call happens to \
object_alloc_do_sample and the initialized of the storage returns \
false.</div><div><br></div><div>(By the way, even if you did call \
object_alloc_do_sample without looking at HeapMonitoring::enabled(), that would be ok \
too. You would gather the stacktrace and get nowhere at the add_trace call, which \
would return false; so though not optimal performance wise, nothing would \
break).</div><div><br></div><div>Furthermore, the add_trace is really the moment of \
no return and we have the mutex lock and then the initialized check. So, in the end, \
I did two things: I removed that first check and then I removed the OrderAccess for \
the storage initialized. I think now I have a better grasp and understanding why it \
was done in our code and why it is not needed here. Thanks for pointing it out :). \
This now still passes my JTREG tests, especially the threaded one. \
</div><div><br></div><div><br></div><div><br></div><div> </div><blockquote \
class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><span class=""><br> \
<br> <blockquote type="cite">
<div dir="ltr">
<div class="gmail_extra">
<div class="gmail_quote">
<div>
<div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:s \
mall;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-wei \
ght:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;whi \
te-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial;background-color:rgb(255,255,255)"><br>
</div>
</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> As a kind of meta \
comment, I wonder if it would make sense to add sampling<br>
for non-TLAB allocations. Seems like if someone is
rapidly allocating a<br>
whole bunch of 1 MB objects that never fit in a
TLAB, I might still be<br>
interested in seeing that in my traces, and not get
surprised that the<br>
allocation rate is very high yet not showing up in
any profiles.<br>
<br>
</blockquote>
That is handled by the handle_sample where you wanted
me to put a<br>
UseTlab because you hit that case if the allocation is
too big.<br>
</blockquote>
<br>
</span>
I see. It was not obvious to me that non-TLAB sampling is
done in the TLAB class. That seems like an abstraction
crime.<br>
What I wanted in my previous comment was that we do not
call into the TLAB when we are not using TLABs. If there
is sampling logic in the TLAB that is used for something
else than TLABs, then it seems like that logic simply does
not belong inside of the TLAB. It should be moved out of
the TLAB, and instead have the TLAB call this common
abstraction that makes sense.<br>
<br>
</blockquote>
<div><br>
</div>
<div>So in the incremental version:</div>
<div><a href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.07_08/" \
target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/webrev.07_08/</a>,
this is still a "crime". The reason is that the system has
to have the bytes_until_sample on a per-thread level and
it made "sense" to have it with the TLAB implementation.
Also, I was not sure how people felt about adding
something to the thread instance instead.<br>
</div>
<div><br>
</div>
<div>Do you think it fits better at the Thread level? I can
see how difficult it is to make it happen there and add
some logic there. Let me know what you think.</div>
</div>
</div>
</div>
</blockquote>
<br></span>
We have an unfortunate situation where everyone that has some fields
that are thread local tend to dump them right into Thread, making
the size and complexity of Thread grow as it becomes tightly coupled
with various unrelated subsystems. It would be desirable to have a
separate class for this instead that encapsulates the sampling
logic. That class could possibly reside in Thread though as a value
object of Thread.</div></blockquote><div><br></div><div>I imagined that would be \
the case but was not sure. I will look at the example that Robbin is talking about \
(ThreadSMR) and will see how to refactor my code to use \
that.</div><div><br></div><div>Thanks again for your help,</div><div>Jc</div><div> \
</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><span class=""><br> \
<br> <blockquote type="cite">
<div dir="ltr">
<div class="gmail_extra">
<div class="gmail_quote">
<div><br>
</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Hope I have answered \
your questions and that my feedback makes sense to you.<br>
<br>
</blockquote>
<div><br>
</div>
<div>You have and thank you for them, I think we are getting
to a cleaner implementation and things are getting better
and more readable :)</div>
</div>
</div>
</div>
</blockquote>
<br></span>
Yes it is getting better.<br>
<br>
Thanks,<br>
/Erik<div><div class="h5"><br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div class="gmail_extra">
<div class="gmail_quote">
<div>Thanks for your help!</div>
<div>Jc</div>
<div><br>
</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Thanks,<br>
/Erik
<div class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379HOEnZb">
<div \
class="m_-513143501369679108m_-3432361388973563659gmail-m_2261932697128216379h5"><br> \
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> I double checked by \
changing the test<br> <a \
href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.05a/raw_files/new/test/hot \
spot/jtreg/serviceability/jvmti/HeapMonitor/MyPackage/HeapMonitorStatObjectCorrectnessTest.java" \
rel="noreferrer" target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/web \
rev.05a/raw_<wbr>files/new/test/hotspot/jtreg/s<wbr>erviceability/jvmti/HeapMonito<wbr>r/MyPackage/HeapMonitorStatObj<wbr>ectCorrectnessTest.java</a><br>
<br>
to use a smaller Tlab (2048) and made the object
bigger and it goes<br>
through that and passes.<br>
<br>
Thanks again for your review and I look forward to
your pointers for<br>
the questions I now have raised!<br>
Jc<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Thanks,<br>
/Erik<br>
<br>
<br>
On 2018-01-26 06:45, JC Beyler wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Thanks Robbin for \
the reviews :)<br> <br>
The new full webrev is here:<br>
<a \
href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.03/" rel="noreferrer" \
target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/webrev.03/</a><br> \
The incremental webrev is here:<br> <a \
href="http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.02_03/" rel="noreferrer" \
target="_blank">http://cr.openjdk.java.net/~jc<wbr>beyler/8171119/webrev.02_03/</a><br>
<br>
I inlined my answers:<br>
<br>
On Thu, Jan 25, 2018 at 1:15 AM, Robbin Ehn <<a \
href="mailto:robbin.ehn@oracle.com" target="_blank"></a><a \
class="m_-513143501369679108moz-txt-link-abbreviated" \
href="mailto:robbin.ehn@oracle.com" target="_blank">robbin.ehn@oracle.com</a>> \
wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Hi JC, great to see \
another revision!<br> <br>
####<br>
heapMonitoring.cpp<br>
<br>
StackTraceData should not contain the oop for
'safety' reasons.<br>
When StackTraceData is moved from
_allocated_traces:<br>
L452 store_garbage_trace(trace);<br>
it contains a dead oop.<br>
_allocated_traces could instead be a tupel of
oop and StackTraceData thus<br>
dead oops are not kept.<br>
</blockquote>
Done I used inheritance to make the copier work
regardless but the<br>
idea is the same.<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> You should use the \
new Access API for loading the oop, something like<br>
this:<br>
RootAccess<ON_PHANTOM_OOP_REF |
AS_NO_KEEPALIVE>::load(...)<br>
I don't think you need to use Access API for
clearing the oop, but it<br>
would<br>
look nicer. And you shouldn't probably be
using:<br>
Universe::heap()->is_in_reserv<wbr>ed(value)<br>
</blockquote>
I am unfamiliar with this but I think I did do
it like you wanted me<br>
to (all tests pass so that's a start). I'm not
sure how to clear the<br>
oop exactly, is there somewhere that does that,
which I can use to do<br>
the same?<br>
<br>
I removed the is_in_reserved, this came from our
internal version, I<br>
don't know why it was there but my tests work
without so I removed it<br>
:)<br>
<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> The lock:<br>
L424 MutexLocker
mu(HeapMonitorStorage_lock);<br>
Is not needed as far as I can see.<br>
weak_oops_do is called in a safepoint, no TLAB
allocation can happen and<br>
JVMTI thread can't access these
data-structures. Is there something more<br>
to<br>
this lock that I'm missing?<br>
</blockquote>
Since a thread can call the JVMTI getLiveTraces
(or any of the other<br>
ones), it can get to the point of trying to
copying the<br>
_allocated_traces. I imagine it is possible that
this is happening<br>
during a GC or that it can be started and a GC
happens afterwards.<br>
Therefore, it seems to me that you want this
protected, no?<br>
<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> ####<br>
You have 6 files without any changes in them
(any more):<br>
g1CollectedHeap.cpp<br>
psMarkSweep.cpp<br>
psParallelCompact.cpp<br>
genCollectedHeap.cpp<br>
referenceProcessor.cpp<br>
thread.hpp<br>
<br>
</blockquote>
Done.<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> ####<br>
I have not looked closely, but is it possible
to hide heap sampling in<br>
AllocTracer ? (with some minor changes to the
AllocTracer API)<br>
<br>
</blockquote>
I am imagining that you are saying to move the
code that does the<br>
sampling code (change the tlab end, do the call
to HeapMonitoring,<br>
etc.) into the AllocTracer code itself? I think
that is right and I'll<br>
look if that is possible and prepare a webrev to
show what would be<br>
needed to make that happen.<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> ####<br>
Minor nit, when declaring pointer there is a
little mix of having the<br>
pointer adjacent by type name and data name.
(Most hotspot code is by<br>
type<br>
name)<br>
E.g.<br>
heapMonitoring.cpp:711 jvmtiStackTrace
*trace = ....<br>
heapMonitoring.cpp:733 Method* m =
vfst.method();<br>
(not just this file)<br>
<br>
</blockquote>
Done!<br>
<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> ####<br>
HeapMonitorThreadOnOffTest.jav<wbr>a:77<br>
I would make g_tmp volatile, otherwise the
assignment in loop may<br>
theoretical be skipped.<br>
<br>
</blockquote>
Also done!<br>
<br>
Thanks again!<br>
Jc<br>
</blockquote>
<br>
</blockquote>
</blockquote>
<br>
</div>
</div>
</blockquote>
</div>
<br>
</div>
</div>
</blockquote>
<br>
</div></div></div>
</blockquote></div><br></div></div>
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