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.osterl...@oracle.com <mailto:erik.osterl...@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.osterl...@oracle.com <mailto:erik.osterl...@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.
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,
/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
<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....@oracle.com <mailto:robbin....@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
