CRR (M): 7097002: G1: remove a lot of unused / redundant code from the G1CollectorPolicy class

Tue Nov 1 13:28:45 UTC 2011

Tony,

On 2011-10-31 18:04, Tony Printezis wrote:
> Bengt,
>
> On 10/31/2011 9:12 AM, Bengt Rutisson wrote:
>>
>> Hi Tony,
>>
>> I think this looks really good. Thanks for cleaning it up!
>
> Np and thanks for looking at it.
>
>> Some minor suggestions:
>>
>> The method G1CollectorPolicy::add_to_collection_set() is only used 
>> for adding old regions to the collection set. How about renaming it 
>> to something like add_old_region_to_collection_set()?
>
> Good point and will do. Quick "history": we used to add all regions to 
> the CSet with that method. But, since John worked on the incremental 
> CSet building, it's not needed any more for eden / survivors and it's 
> currently only used for old regions. But, you're right, we should 
> rename it.

Good. And thanks for the background info.

>
>> I would like to consider "cset" a prefix for the methods that return 
>> information about the collection set. That would mean that methods like
>>
>> young_cset_region_num()
>> old_cset_region_num()
>> eden_cset_region_num()
>> survivor_cset_region_num()
>> init_cset_region_nums()
>>
>> could be called something like:
>>
>> cset_young_region_num()
>> cset_old_region_num()
>> cset_eden_region_num()
>> cset_survivor_region_num()
>> cset_init_region_nums()
>
> Well, it depends on how you see it. If you consider "cset_region" as a 
> "region in the collection set", then young_cset_region is "a young 
> region in the collection set" which is what I went for. I also like 
> having the young_, old_, etc. as the first word in the name as (at 
> least IMHO) it makes a bit clearer which regions we are referring to. 
> Compare:
>
> foo = cset_young_region_num() + cset_old_region_num()
>
> to:
>
> foo = young_cset_region_num() + old_cset_region_num()
>
> I think the "young" and "old" in the names pop out a bit more in the 
> latter version. I'm inclined to leave these names as they are.

Ok. That's fine.

>> If you do this change there are also some local variables that are 
>> used to call init_cset_region_nums() that would benefit from the same 
>> kind of renaming.
>>
>> Finally, to me the *_num() name could indicate something else than 
>> the count(), size() or lenght(). How about renaming them *_num() 
>> mehods (and corresponding variables) to *_length() ? I am more used 
>> to seeing the "length" name when we use lists of regions in the G1 code.
>
> (half -seriously) a lot of thought went into that name. :-) :-) :-) 
> Let's see:
>
> - count : This would be OK, but I don't think we use "count" in this 
> context much in HotSpot.
> - size : This is used instead of length / number in HotSpot and, IMHO, 
> is misleading. A count of regions is not a size (the size would be how 
> much space they take up).
> - length : It implies a list to me and, yes, we do use lists to keep 
> track of the CSet regions but maybe we'll do that in a different way 
> in the future. (Note that the policy class should not really know how 
> the regions are maintained; it should only care about how many of them 
> there are.)
>
> ...which is why I used "num". Having said that, the HeapRegionSetBase 
> class uses length for the number of regions so I'll replace num with 
> length to be consistent with that. Sounds good?

Sounds good. :-)

Bengt

>> Anyway, these were all tiny nit picks. Overall it looks good. Ship it!
>
> Thanks!
>
> Tony
>
>> Bengt
>>
>>
>>
>>
>> On 2011-10-14 12:58, Tony Printezis wrote:
>>> Hi again,
>>>
>>> While working on the prediction code I found another 200 lines that 
>>> can be removed (I think this was part of the original prediction 
>>> code which has been superseded by the current version). I'd like to 
>>> piggy-back this removal on this CR so that I don't open a new one. 
>>> It also fits within the "cleanup" scope of this CR anyway. Here's 
>>> the latest overall webrev:
>>>
>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.2/webrev.all/
>>>
>>> The first two parts are exactly what I had below:
>>>
>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.2/webrev.0.G1PredRefactorVerboseRemoval/ 
>>>
>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.2/webrev.1.G1PredRefactorOtherRemoval/ 
>>>
>>>
>>> The new part is this:
>>>
>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.2/webrev.2.G1PredRefactorExtra/ 
>>>
>>>
>>> Thanks!
>>>
>>> Tony
>>>
>>> On 10/12/2011 02:22 PM, Tony Printezis wrote:
>>>> Hi all,
>>>>
>>>> Thanks to John Cuthbertson for looking at this. Here are the 
>>>> updated webrevs based on his comments:
>>>>
>>>> overall:
>>>>
>>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.1/webrev.all/
>>>>
>>>> part 1:
>>>>
>>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.1/webrev.0.G1PredRefactorVerboseRemoval/ 
>>>>
>>>>
>>>> part 2:
>>>>
>>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.1/webrev.1.G1PredRefactorOtherRemoval/ 
>>>>
>>>>
>>>> Tony
>>>>
>>>> On 10/06/2011 04:03 PM, Tony Printezis wrote:
>>>>> Hi all,
>>>>>
>>>>> This is G1CollectorPolicy cleanup #2, now I got the first one 
>>>>> (7088680) code reviewed (thanks to Ramki and Bengt). BTW, the 
>>>>> webrevs below were done on top of the changes for 7088680. So if 
>>>>> you want to apply them to your workspace you have to apply the 
>>>>> patch for 7088680 first (at least until it's pushed to the GC 
>>>>> repo). And the webrevs do not include the changes for 7088680 
>>>>> despite what the top-level webrev pages say (I checked).
>>>>>
>>>>> The motivation for these changes was that there's a lot of unused 
>>>>> and redundant code in the G1CollectorPolicy class that's been 
>>>>> complicating some other changes I've been working on (simplify the 
>>>>> pause prediction model, extend the ergo decision logging, revamp 
>>>>> the eden / survivor lists, etc.). So, I thought I'd do this 
>>>>> cleanup first so that we can get it tested separately (as it's not 
>>>>> trivial, at least wrt its size) and to also keep the code reviews 
>>>>> somewhat smaller and simpler.
>>>>>
>>>>> The overall webrev is this one:
>>>>>
>>>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.all/
>>>>>
>>>>> Here it is in two parts in case it makes the code reviews easier. 
>>>>> BTW, I could be persuaded to push each part as a separate 
>>>>> changeset if you think they are kind of unrelated. That'd be easy 
>>>>> to do (I have them as two separate MQ patches anyway).
>>>>>
>>>>> 1) Remove methods and fields only used by the PREDICTIONS_VERBOSE 
>>>>> switch and G1PolicyVerbose flag, both of which are also removed.
>>>>>
>>>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.0.G1PredRefactorVerboseRemoval/ 
>>>>>
>>>>>
>>>>> We'll replace the functionality of PREDICTIONS_VERBOSE and 
>>>>> G1PolicyVerbose with extra logging using G1's ergo decision 
>>>>> logging framework. So, it's good to get rid of all the code that 
>>>>> is otherwise not used. This is a pure code removal change, I don't 
>>>>> believe I added or modified anything else.
>>>>>
>>>>> 2) Avoid redundant / unnecessary region counting.
>>>>>
>>>>> http://cr.openjdk.java.net/~tonyp/7097002/webrev.1.G1PredRefactorOtherRemoval/ 
>>>>>
>>>>>
>>>>> It turns out that we have no less than six (!!!) ways of counting 
>>>>> regions in the CSet:
>>>>>
>>>>> a) YoungList : maintains the count of survivor and eden regions 
>>>>> that are added to it
>>>>> b) _inc_cset_size : the number of young regions added to the 
>>>>> incremental CSet, which is the same as the total count the 
>>>>> YoungList maintains
>>>>> c) _collection_set_size : the number of regions added to the CSet, 
>>>>> which is the same as _inc_cset_size plus any old regions we add to 
>>>>> the CSet
>>>>> d) _inc_cset_young_index : used to set the young CSet index of the 
>>>>> young regions (0 for the oldest region, 1 for the second oldest 
>>>>> region, etc.) which is again essentially the same as _inc_cset_size
>>>>> e) _young_cset_length : the number of young regions in the CSet, 
>>>>> which is already counted by the YoungList
>>>>> f) _recorded_young_regions / _recorded_non_young_regions : the 
>>>>> number of young / old regions in the CSet, again the former is 
>>>>> counted by the YoungList and the latter is included in 
>>>>> _collection_set_size
>>>>>
>>>>> I replaced all this by three fields that reflect the number of 
>>>>> eden, survivor, and old regions in the CSet and a few accessor 
>>>>> methods. The first two are set using the information the YoungList 
>>>>> maintains, the third one is incremented every time we add an old 
>>>>> region to the CSet.
>>>>>
>>>>> The changes are mostly straightforward: remove the unnecessary 
>>>>> fields and methods and replace them with the new accessor methods. 
>>>>> The only slightly tricky part was to make sure the young CSet 
>>>>> indexes were set correctly now that we don't maintain the 
>>>>> _inc_cset_young_index (this just needed a small refactoring).
>>>>>
>>>>> In addition, we maintain counts for the number of mutator 
>>>>> allocation regions that we allocate. Given that we used to allow 
>>>>> such allocations to be satisfied out of the old generation, we 
>>>>> maintain two counts: one for young allocations and one for tenured 
>>>>> allocations. It was helpful to know what percentage of allocations 
>>>>> were satisfied out of the old gen. Given that all mutator 
>>>>> allocation regions are now allocated out of the young gen, this 
>>>>> information is irrelevant. So, I also removed those fields and 
>>>>> associated code.
>>>>>
>>>>> Tony
>>>>>
>>>>>
>>