Hi JC, (dropping compiler on mail thread)
On 01/26/2018 06:45 AM, 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/
Thanks!
I got this compile issue:
/home/rehn/source/jdk/small/open/src/hotspot/share/gc/shared/threadLocalAllocBuffer.hpp:
In static member function 'static void
G1ResManTLABCache::put(ThreadLocalAllocBuffer&, AllocationContext_t)':
/home/rehn/source/jdk/small/open/src/hotspot/share/gc/shared/threadLocalAllocBuffer.hpp:97:13:
error: 'HeapWord* ThreadLocalAllocBuffer::hard_end()' is private
HeapWord* hard_end();
^
/home/rehn/source/jdk/small/closed/src/hotspot/share/gc/g1/g1ResManTLABCache.cpp:52:50:
error: within this context
size_t remaining = pointer_delta(tlab.hard_end(), tlab.top());
I inlined my answers:
On Thu, Jan 25, 2018 at 1:15 AM, Robbin Ehn <[email protected]> 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.
Looks good.
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
:)
I talked a bit with GC folks about this today.
So actually the oop should be in the oopStorage and your should have a
weakhandle to that oop (at least in the near future).
But this should work for now...
In the future when we have the oop in oppStorage you will not get called, so you
will not know when the oops are dead, either GC must trigger a concurrent vm
operation (e.g. _no_ safepoint operation) so we can clear dead oops or do
periodic scanning.
It would be good with some input here from Thomas that knows what you are doing
and knows GC.
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?
A thread calling getLiveTraces will be stopped in the HeapThreadTransition.
A safepoint don't allow any threads to be in_vm or to be in_java during
safepoint. Only threads in native can execute, but they will be stopped on any
transition. If a thread is in_vm the safepoint waits to a transition (locking a
mutex is also transition to blocked). So if weak_oops is called you have an
guarantee that no threads are executing inside the VM or executing Java code.
(not counting GC threads of course)
This also means that the lock can never be contented when weak_oops is called,
so it's not harmful.
####
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.
Sounds good.
####
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!
Looks good, thanks!
/Robbin
Thanks again!
Jc
