RFR: 8138920: Refactor the sampling thread from ConcurrentG1RefineThread

Bengt Rutisson bengt.rutisson at oracle.com
Mon Oct 19 19:17:54 UTC 2015



Hi Derek,

On 2015-10-17 03:32, Derek White wrote:
> 3rd Webrev for review please:
>
> This version does away with the common abstract base case 
> ConcurrentServiceThread, and pushes the code down to the concrete 
> classes. This may get cleaned up in a later fix to ConcurrentGCThread.
>
>
> RFE: JDK-8138920 <https://bugs.openjdk.java.net/browse/JDK-8138920> 
> Refactor the sampling thread from ConcurrentG1RefineThread
>
> Webrev: http://cr.openjdk.java.net/~drwhite/8138920/webrev.03/

I think this version is much good.

I talked a bit to Per about the latest webrev and he pointed out that we 
don't really need the run_service()/stop_service() abstraction now that 
the threads implement all this themselves. The change would be smaller 
if we left that change out.

I'll leave it up to you if you want to keep it in there or not. Do you 
plan on following this work up by starting to work on JDK-8138920 pretty 
soon? In that case I think the run_service()/stop_service() abstraction 
will have to be handled there anyway which makes it a smaller issue now 
in my opinion.

Thanks,
Bengt

>
> Passed jprt (finally!).
>
> Thanks for looking!
>
>  - Derek
>
> On 10/9/15 12:01 PM, Bengt Rutisson wrote:
>>
>> Hi Derek,
>>
>> Comments inlined.
>>
>> On 2015-10-09 00:29, Derek White wrote:
>>> Another call for review:
>>>
>>> 2nd webrev:
>>> http://cr.openjdk.java.net/~drwhite/8138920/webrev.02/
>>>
>>> See changes and comments below:
>>>
>>> On 10/8/15 2:47 AM, Bengt Rutisson wrote:
>>>>
>>>> Hi Derek,
>>>>
>>>> On 2015-10-07 17:19, Derek White wrote:
>>>>> Hi Bengt,
>>>>>
>>>>> On 10/7/15 4:02 AM, Bengt Rutisson wrote:
>>>>>> Hi Derek,
>>>>>>
>>>>>> Thanks for fixing this!
>>>>>>
>>>>>> On 2015-10-06 19:51, Derek White wrote:
>>>>>>> Refactor and cleanup the G1 concurrent thread classes:
>>>>>>>  - Pull out a sampling thread class (now 
>>>>>>> ConcurrentG1SampleThread) from ConcurrentG1RefineThread.
>>>>>>>  - Factor out an abstract base class ConcurrentG1ServiceThread 
>>>>>>> that is used by:
>>>>>>>     - ConcurrentG1RefineThread
>>>>>>>     - ConcurrentG1SampleThread
>>>>>>>     - ConcurrentMarkThread
>>>>>>>  - Made the handling of the "primary" refinement thread more 
>>>>>>> explicit.
>>>>>>>  - Updated obsolete and confusing comments
>>>>>>>
>>>>>>> This is tech debt that also will allow disabling concurrent 
>>>>>>> refinement (if desired) and also fixes a P4 bug.
>>>>>>> Patch started by Thomas and improved and/or mangled myself.
>>>>>>>
>>>>>>> RFE: JDK-8138920 
>>>>>>> <https://bugs.openjdk.java.net/browse/JDK-8138920> Refactor the 
>>>>>>> sampling thread from ConcurrentG1RefineThread
>>>>>>>
>>>>>>> Webrev: http://cr.openjdk.java.net/~drwhite/8138920/webrev.01/
>>>>>>
>>>>>> Overall this looks really good to me.
>>>>>>
>>>>>> Some comments:
>>>>>>
>>>>>> No one seems to depend on 
>>>>>> ConcurrentG1ServiceThread::vtime_accum(). All uses have the 
>>>>>> specific subclass available. So, I don't think the pure virtual 
>>>>>> declaration in ConcurrentG1ServiceThread is needed. I'd just 
>>>>>> remove that and also make the corresponding methods in the 
>>>>>> subclasses non-virtual.
>>>>>
>>>>> OK. At some point we need to systematic rewrite of timing, but 
>>>>> that can wait.
>>>>
>>>> Quite agree.
>>>>
>>>>>> That more or less only leaves the run() and stop() methods in 
>>>>>> ConcurrentG1ServiceThread. It is kind of nice for the subclasses 
>>>>>> to get help with this, but I wonder if it is not possible to 
>>>>>> improve the ConcurrentGCThread class to help with this instead. 
>>>>>> Basically I guess I am a little unsure if the extra class 
>>>>>> ConcurrentG1ServiceThread is really needed.
>>>>>>
>>>>> I'll look at ConcurrentGCThread to see how well it could cover 
>>>>> these cases.
>>>>
>>>> Thanks. I think it is worth a try. If it doesn't turn out well we 
>>>> can keep the intermediate class, but I think it is worth exploring.
>>>
>>> I looked at pushing ConcurrentServiceThread up into 
>>> ConcurrentGCThread, but things got a little complicated. 
>>> ConcurrentG1RefineThread, ConcurrentMarkThread, and 
>>> ConcurrentSampleThread have a very "regularized" implementation of 
>>> the "termination protocol". G1StringDedupThread is slightly off from 
>>> this, and ConcurrentMarkSweepThread more so.  Pushing the shared 
>>> code up into ConcurrentGCThread but not using it in 
>>> G1StringDedupThread and ConcurrentMarkSweepThread seems confusing.
>>>
>>> There's a tension between providing a framework that handles all 
>>> shared factorizable code, but can become a straight jacket for 
>>> future code, and everyone doing everything separately and 
>>> differently. This webrev is somewhere in the middle. Some of the 
>>> changes between webrev.01 and .02 are to make the duplicated code 
>>> more similar, even though it's not shared.
>>
>> I see. Thanks for investigating it!
>>
>> I think I agree with Per, though. The value of 
>> ConcurrentServiceThread in its current form is IMHO not really worth 
>> the extra complexity of having it. I would prefer to just duplicate 
>> the code in ConcurrentG1RefineThread, ConcurrentMarkThread, and 
>> ConcurrentSampleThread for now and then have a separate change to try 
>> to clean this part up.
>>
>>>
>>>>>> Naming. The naming in G1 is a bit inconsistent. Some files and 
>>>>>> classes are prefixed with G1 and some are not. But if they are 
>>>>>> called something with G1 it is normally a prefix. So, I would 
>>>>>> prefer the new classes to be called G1Concurrent* instead of 
>>>>>> ConcurrentG1*.
>>>>>
>>>>> So we have:
>>>>>     - ConcurrentG1RefineThread
>>>>>     - ConcurrentMarkThread
>>>>>
>>>>> And I added:
>>>>>     - ConcurrentG1SampleThread
>>>>>     - ConcurrentG1ServiceThread
>>>>>
>>>>> And maybe I'm removing ConcurrentG1ServiceThread. In that case I'd 
>>>>> be inclined to rename:
>>>>>      ConcurrentG1SampleThread => ConcurrentSampleThread 
>>>>
>>>> Sounds good. The ConcurrentG1Refine* classes are really the only 
>>>> oddly named G1 classes, so I think it is better not to let that 
>>>> naming spread.
>>> This version includes the class renaming. 
>>
>> Thanks for fixing this.
>>
>> If we do decide to keep ConcurrentServiceThread around I think it 
>> would be better to move ConcurrentSampleThread into its own file. It 
>> is a separate enough entity to warrant its own file I think.
>>
>>>>
>>>>>> You write "and also fixes a P4 bug". Which bug is that?
>>>>> JDK-8136856 <https://bugs.openjdk.java.net/browse/JDK-8136856> G1 
>>>>> makes two concurrent refinement threads with 
>>>>> -XX:G1ConcRefinementThreads=1
>>>>> (because the sampling thread "is-a" concurrent refinement thread.
>>>>
>>>> Ah. I see. Makes sense. Thanks.
>>>>
>>>> But it is still not possible to turn refinement off by setting 
>>>> -XX:G1ConcRefinementThreads=0 since that is considered the default, 
>>>> right?
>>>
>>> I'm not sure about this. If I recall correctly, Thomas implied that 
>>> it was hard to disable concurrent refinement without disabling the 
>>> sampling thread too.
>>
>> I'm thinking about this code in Arguments::set_g1_gc_flags():
>>
>>   if (G1ConcRefinementThreads == 0) {
>>     FLAG_SET_DEFAULT(G1ConcRefinementThreads, ParallelGCThreads);
>>   }
>>
>> Could we change that, so that you could turn off refinement by 
>> setting -XX:G1ConcRefinementThreads=0 ? It should probably be handled 
>> as a separate fix though.
>>
>> Thanks,
>> Bengt
>>
>>>
>>>>> I cant' recall how to mark a bug as blocking another bug.
>>>>
>>>> You add a link (More->Link) to the other bug and choose "block" or 
>>>> "is blocked by".
>>>>
>>>> Thanks,
>>>> Bengt
>>> Thanks for the tip!
>>>
>>> - Derek
>>>
>>
>

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