RFR(M/L): 6484982: G1: process references during evacuation pauses

John Cuthbertson john.cuthbertson at oracle.com
Wed Sep 14 22:24:12 UTC 2011

Hi Everyone,

A new webrev that incorporates (most of) the suggestions made by 
everyone can be found at: 

  * Added explanation about the _is_alive_non_header value being optional.
  * removed the G1_DEBUG code
  * Clarified the comment in ReferenceProcessor::weak_oops_do etc to say 
that the code in the comment is an alternative implmentation of the loop.
  * Moved the assert that _discovering_refs is false and that there are 
no refs discovered into ReferenceProcessor::enable_discovery. 
Bengt/Stefan  were correct in that I needed to flags to enable_discovery 
which had a knock on effect.
  * Added a helper routine to set the discovered field for use in 
ReferenceProcessor::balance_queues(). Note that in the other calls to 
set_discovered we either really need the barrier or we don't care except 
when enqueuing on to a discovered list or balancing the discovered 
lists, where we explicitly do not want a barrier.

I'll be submitting RFEs for the other valid suggestions that were made.

Sanity Testing: The entire GC test suite with G1 with a low marking 
threshold; for the other collectors (serial, parallel, par old, CMS w/ 
ParNew, CMS w/o ParNew) I ran the GC test suite version of specjvm98.



On 09/09/11 14:19, John Cuthbertson wrote:
> Hi Everyone,
> I have sync'ed these changes up to and including the changeset 
> 2660:3bddbf0f57d6 which was for "7087717: G1: make the 
> G1PrintRegionLivenessInfo parameter diagnostic" including merging with 
> Stefan's and Ramki's reference processor changes. As part of merging I 
> changed to code that preserves objects that referenced from the 
> discovered lists of the concurrent mark's reference processor to use 
> the DiscoveredListIterator class. This necessitated moving 
> DiscoveredListIterator and some of its methods referenceProcessor.hpp.
> The new webrev can be found at: 
> http://cr.openjdk.java.net/~johnc/6484982/webrev.3/
> Testing: the GC test suite with a marking threshold of 20% (with heap 
> verification enabled) and KitchenSink.
> JohnC
> On 08/17/11 11:15, John Cuthbertson wrote:
>> Hi Everyone,
>> A new webrev for these changes can be found at: 
>> http://cr.openjdk.java.net/~johnc/6484982/webrev.2/
>> The changes in this webrev reverse the order of preserving objects 
>> referenced from the concurrent mark ref processor's discovered lists 
>> and processing references discovered by the STW ref processor. 
>> Preserving the objects referenced from the concurrent mark ref 
>> processor's discovered lists comes first so that any object that 
>> needs to be copied is done so before reference processing.
>> JohnC
>> On 08/03/11 10:44, John Cuthbertson wrote:
>>> Hi Everyone,
>>> A new webrev incorporating some feedback from Ramki can be found at: 
>>> http://cr.openjdk.java.net/~johnc/6484982/webrev.1/
>>> Thanks,
>>> JohnC
>>> On 06/23/11 14:35, John Cuthbertson wrote:
>>>> Hi Everyone,
>>>> I would like to get a couple of volunteers to review the code 
>>>> changes for this CR - the webrev can be found at 
>>>> http://cr.openjdk.java.net/~johnc/6484982/webrev.0/
>>>> Summary:
>>>> G1 now contains 2 instances of the reference processor class - one 
>>>> for concurrent marking and the other for STW GCs (both full and 
>>>> incremental evacuation pauses). For evacuation pauses, during 
>>>> object scanning and RSet scanning I embed the STW reference 
>>>> processor into the OopClosures used to scan objects. This causes 
>>>> reference objects to be 'discovered' by the reference processor. 
>>>> Towards the end of the evacuation pause (just prior to retiring the 
>>>> the GC alloc regions) I have added the code to process these 
>>>> discovered reference objects, preserving (and copying) referent 
>>>> objects (and their reachable graphs) as appropriate. The code that 
>>>> does this makes extensive use of the existing copying oop closures 
>>>> and the G1ParScanThreadState structure (to handle to-space 
>>>> allocation).
>>>> The code changes also include a couple of fixes that were exposed 
>>>> by the reference processing:
>>>>  * In satbQueue.cpp, the routine 
>>>> SATBMarkQueueSet::par_iterate_closure_all_threads() was claiming 
>>>> all JavaThreads (giving them one parity value) but skipping the 
>>>> VMThread. In a subsequent call to 
>>>> Thread::possibly_parallel_oops_do, the Java threads were 
>>>> successfully claimed but the VMThread was not. This could cause the 
>>>> VMThread's handle area to be skipped during the root scanning.
>>>>  * There were a couple of assignments to the discovered field of 
>>>> Reference objects that were not guarded by _discovery_needs_barrier 
>>>> resulting in the G1 C++ write-barrier to dirty the card spanning 
>>>> the Reference object's discovered field. This was causing the card 
>>>> table verification (during card table clearing) to fail.
>>>>  * There were also a couple of assignments of NULL to the next 
>>>> field of Reference objects causing the same symptom.
>>>> Testing: The GC test suite (32/64 bit) (+UseG1GC, +UseG1GC 
>>>> +ExplicitGCInvokesConcurrent, +UseG1GC 
>>>> InitiatingHeapOccupancyPercent=5, +UseG1GC 
>>>> +ParallelRefProcEnabled), KitchenSink (48 hour runs with +UseG1GC, 
>>>> +UseG1GC +ExplicitGCInvokesConcurrent), OpenDS (+UseG1GC, +UseG1GC 
>>>> +ParallelRefProcEnabled), nsk GC and compiler tests, and jprt. 
>>>> Testing was conducted with the _is_alive_non_header field in the 
>>>> STW ref procssor both cleared and set (when cleared, more reference 
>>>> objects are 'discovered').
>>>> Thanks,
>>>> JohnC

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