(original) (raw)

Hi all,

I apologize for the delay but I wanted to add an event system and that took a bit longer than expected and I also reworked the code to take into account the deprecation of FastTLABRefill.

This update has four parts:

A) I moved the implementation from Thread to ThreadHeapSampler inside of Thread. Would you prefer it as a pointer inside of Thread or like this works for you? Second question would be would you rather have an association outside of Thread altogether that tries to remember when threads are live and then we would have something like:

ThreadHeapSampler::get\_sampling\_size(this\_thread);

I worry about the overhead of this but perhaps it is not too too bad?

B) I also have been working on the Allocation event system that sends out a notification at each sampled event. This will be practical when wanting to do something at the allocation point. I'm also looking at if the whole heapMonitoring code could not reside in the agent code and not in the JDK. I'm not convinced but I'm talking to Serguei about it to see/assess :)

- Also added two tests for the new event subsystem

C) Removed the slow\_path fields inside the TLAB code since now FastTLABRefill is deprecated

D) Updated the JVMTI documentation and specification for the methods.

So the incremental webrev is here:

http://cr.openjdk.java.net/\~jcbeyler/8171119/webrev.09\_10/

and the full webrev is here:

http://cr.openjdk.java.net/\~jcbeyler/8171119/webrev.10

I believe I have updated the various JIRA issues that track this :)

Thanks for your input,

Jc

On Wed, Feb 14, 2018 at 10:34 PM, JC Beyler <jcbeyler@google.com> wrote:

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), 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
10358
10359 Can sample the heap.
10360 If this capability is enabled then the heap sampling methods
can be called.
10361
10362

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::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/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/
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> 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::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