RFR: 8079275: Remove CollectedHeap::use_parallel_gc_threads

Stefan Johansson stefan.johansson at oracle.com
Tue May 5 08:12:55 UTC 2015


Hi Stefan,

On 2015-05-04 17:52, Stefan Karlsson wrote:
> Hi,
>
> On 2015-05-04 17:28, Stefan Johansson wrote:
>> Hi Stefan,
>>
>> On 2015-05-04 15:34, Stefan Karlsson wrote:
>>> Hi,
>>>
>>> Please review this patch to remove 
>>> CollectedHeap::use_parallel_gc_threads.
>>>
>>> http://cr.openjdk.java.net/~stefank/8079275/webrev.01/
>> Change looks good, nice cleanup.
>>
>> If I'm not missing anything you could remove the typedef in 
>> concurrentMarkSweepGeneration.cpp:
>>      typedef CMSParGCThreadState* CMSParGCThreadStatePtr;
>> -    _par_gc_thread_states =
>> -      NEW_C_HEAP_ARRAY(CMSParGCThreadStatePtr, ParallelGCThreads, 
>> mtGC);
>> +  _par_gc_thread_states = NEW_C_HEAP_ARRAY(CMSParGCThreadStatePtr, 
>> ParallelGCThreads, mtGC);
>>
>> The typedef is only used for in the NEW_C_HEAP_ARRAY-call and I can't 
>> see any good reason for keeping it.
>
> OK. I removed it. I also removed the unused _modUnionClosure.
>
> http://cr.openjdk.java.net/~stefank/8079275/webrev.02.delta/
> http://cr.openjdk.java.net/~stefank/8079275/webrev.02/
>
Looks good,

StefanJ
> Thanks,
> StefanK
>
>>
>> Thanks,
>> Stefan
>>
>>> https://bugs.openjdk.java.net/browse/JDK-8079275
>>>
>>> This function is used from:
>>>
>>> 1) CMS - CMS can't be started with -XX:ParallelGCThreads=0 anymore, 
>>> so all checks against use_parallel_gc_threads will always be true.
>>>
>>> 2) GenCollectedHeap::process_roots - This usage could be replaced 
>>> with n_par_threads(), the same check the other parallel task 
>>> (Threads::possibly_parallel_oops_do) uses.
>>>
>>> This will change the executed code for CMS but not Serial.
>>>
>>> Serial will always return false for 
>>> CollectedHeap::use_parallel_gc_threads and 
>>> GenCollectedHeap:n_par_threads(). See:
>>> void GenCollectedHeap::set_par_threads(uint t) {
>>>   assert(t == 0 || !UseSerialGC, "Cannot have parallel threads");
>>>   CollectedHeap::set_par_threads(t);
>>>
>>> CMS will always return true for 
>>> CollectedHeap::use_parallel_gc_threads, but can now return either 
>>> true or false for n_par_threads() depending on how n_par_threads was 
>>> setup for the currently executing task. This means that some CMS 
>>> paths will now call StringTable::oops_do instead of 
>>> StringTable::possibly_parallel_oops_do, when running single threaded.
>>>
>>> Thanks,
>>> StefanK
>>
>



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