RFR(S): 7119908: G1: Cache CSet start region for each worker for subsequent reuse

John Cuthbertson john.cuthbertson at oracle.com
Wed Dec 14 22:23:09 UTC 2011

Hi Everyone,

Based upon some further comments from Tony, I have another new webrev 
for this change. It can be found at: 

Changes in this version include:

* some minor name changes
* the code that calculates the starting region was using total_workers() 
when it should have been using active_workers()
* minor tweaks to the start and end index values to get a slightly 
better distribution.

GC test suite



On 12/13/11 10:39, John Cuthbertson wrote:
> Hi Everyone,
> I have a new webrev based upon comments from Tony, which can be found 
> at: http://cr.openjdk.java.net/~johnc/7119908/webrev.1/
> Changes in this version include:
> * Using the value of the existing G1CollectedHeap::_gc_time_stamp 
> field, rather than the value of _total_collections, as the TS values.
> * A small optimization that may reduce the number of iterations while 
> walking the collection set to calculate the starting region for a 
> worker. We check the entry for the previous worker and if that is 
> valid then we starting iterating over the collection set from that region.
> Testing:
> GC test suite; jprt.
> Thanks,
> JohnC
> On 12/09/11 11:18, John Cuthbertson wrote:
>> Hi Everyone,
>> Can I have a couple of volunteers to review the changes for this CR? 
>> The webrev can be found at: 
>> http://cr.openjdk.java.net/~johnc/7119908/webrev.0/
>> Summary:
>> As part of the code review comments for 7112743 (G1: Reduce overhead 
>> of marking closure during evacuation pauses) it was suggested that 
>> instead of recalculating the starting heap region for each worker 
>> thread, we reuse the values calculated during RSet scanning. These 
>> changes address that review comment. In these changes I maintain a 
>> cache that is used and updated by 
>> G1CollectedHeap::start_cset_region_for_worker(). The first time this 
>> routine is called by a worker thread during an evacuation pause 
>> (currently during RSet scanning) the cached value for the worker gets 
>> set; when the routine is called subsequently the region that was 
>> cached for the worker is returned. I employ a simple stamp mechanism 
>> based upon the number of GCs that ensures the validity of the regions 
>> in the cache and makes clearing the cache unnecessary.
>> Testing:
>> GC Test suite with a small marking threshold (10%) with and without 
>> verification; the configuration of SPECjbb used to test 7117423; 
>> Kitchensink.
>> Thanks,
>> JohnC

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