CRR (S/M): 7092309: G1: introduce old region set

Tony Printezis tony.printezis at
Fri Oct 7 12:59:07 UTC 2011


Np. Your suggestion definitely made the conditions in the guarantees 
simpler to read and understand. They had got a bit out of hand...


On 10/7/2011 8:55 AM, Bengt Rutisson wrote:
> Looks good. Thanks for fixing the other check_mt_safety()  methods too.
> Bengt
> On 2011-10-06 18:42, Tony Printezis wrote:
>> Latest webrev based on Bengt's suggestions (I'll need to test the 
>> changes to make sure I didn't mess up the checks, but I'll do that in 
>> a few days after I've finished testing another changeset):
>> The only differences wrt the first webrev are the changes in the 
>> check_mt_safety() methods in heapRegionSets.cpp.
>> Tony
>> Tony Printezis wrote:
>>> Bengt,
>>> Thanks for looking at this! See inline.
>>> On 10/6/2011 7:04 AM, Bengt Rutisson wrote:
>>>> Hi Tony,
>>>> This looks good. It will be great to have the old region set for 
>>>> the CR regarding humungous allocation and concurrent mark start 
>>>> that I am working on.
>>> Yes, this would be a nice benefit of this change and make the cycle 
>>> initiation code a bit more straightforward.
>>>> Some comments regarding MasterOldRegionSet::check_mt_safety():
>>>> The code would be easier (at least for me) to read like this:
>>>>   if (SafepointSynchronize::is_at_safepoint()) {
>>>>     guarantee(Thread::current()->is_VM_thread() ||
>>>>              (_phase == HRSPhaseEvacuation && 
>>>> FreeList_lock->owned_by_self()) ||
>>>>              (_phase == HRSPhaseCleanup && 
>>>> OldSets_lock->owned_by_self()), "Master old set MT safety protocol: 
>>>> safepoint invariants violated");
>>>>   } else {
>>>>     guarantee(Heap_lock->owned_by_self(), "Master old set MT safety 
>>>> protocol: Heap lock must be held if we are not at a safepoint");
>>>>   }
>>>> It would also align nicely with how you structured the comment just 
>>>> above the guarantee.
>>> This is a good suggestion. I'll make the change and also update the 
>>> other check_mt_safety() methods in that file this way too.
>>>> Also, the only call to check_mt_safety() that I could find is in 
>>>> hrs_assert_mt_safety_ok() and there we call it from an assert. If 
>>>> that is correct, why are then the check_mt_safety() implementations 
>>>> using guarantees and not asserts?
>>> The idiom:
>>> assert(check_something(), "...");
>>> with check_something() doing various checks (guarantees, explicitly 
>>> tests, etc.) and returns true is common in HotSpot as a way to do 
>>> those checks only in a non-product build (instead of creating a 
>>> non-product method which is a nop in the product). Having them as 
>>> guarantees also allows us to turn the assert in 
>>> hrs_assert_mt_safety_ok() into a guarantee and enable those checks 
>>> in a product build, if we're looking for a race that it's only 
>>> reproduced with product builds.
>>> Tony
>>>> Thanks,
>>>> Bengt
>>>> On 2011-10-04 21:54, Tony Printezis wrote:
>>>>> Hi all,
>>>>> This change adds to G1 the facility to keep track of the allocated 
>>>>> old regions with the HeapRegionSet abstraction and moves us one 
>>>>> step closer to having all regions being tracked by the same 
>>>>> abstraction (after this, only the survivor / eden regions remain). 
>>>>> I decided to work on the old region set now as it was a relatively 
>>>>> easy change to implement and will make some follow-on CRs easier 
>>>>> to implement. The webrev is here:
>>>>> Most of the changes were straightforward (I largely followed how 
>>>>> we maintain the humongous region set). The only part which I had 
>>>>> to slightly revamp was the code that tears down / rebuilds the 
>>>>> region sets. This small refactoring, though, was worthwhile as it 
>>>>> makes that code easier to extend in the future (when we introduce 
>>>>> the eden / survivor lists).
>>>>> Tony

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