RFR: 8079275: Remove CollectedHeap::use_parallel_gc_threads

Stefan Karlsson stefan.karlsson at oracle.com
Mon May 4 15:52:05 UTC 2015


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/

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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