JDK-8171119: Low-Overhead Heap Profiling

Erik Österlund erik.osterlund at oracle.com
Wed Feb 14 10:51:24 UTC 2018

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/ 
> <http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.08/>
> The incremental is here:
> http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.07_08/ 
> <http://cr.openjdk.java.net/%7Ejcbeyler/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 at oracle.com <mailto:erik.osterlund at 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/
>         <http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.05a/>
>         And the incremental one is here:
>         http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.04_05a/
>         <http://cr.openjdk.java.net/%7Ejcbeyler/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 at oracle.com <mailto:erik.osterlund at 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 

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.

>         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.

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.

>             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/ 
> <http://cr.openjdk.java.net/%7Ejcbeyler/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.

>     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 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/HeapMonitor/MyPackage/HeapMonitorStatObjectCorrectnessTest.java
>         <http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.05a/raw_files/new/test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/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/ <http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.03/>
>                 The incremental webrev is here:
>                 http://cr.openjdk.java.net/~jcbeyler/8171119/webrev.02_03/
>                 <http://cr.openjdk.java.net/%7Ejcbeyler/8171119/webrev.02_03/>
>                 I inlined my answers:
>                 On Thu, Jan 25, 2018 at 1:15 AM, Robbin Ehn
>                 <robbin.ehn at oracle.com <mailto:robbin.ehn at 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

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