RFR(S): 8007036: G1: Too many old regions added to last mixed GC

John Cuthbertson john.cuthbertson at oracle.com
Tue Jan 29 21:35:28 UTC 2013

Hi Everyone,

Here's a new webrev based upon feedback from Vitaly: 


On 1/28/2013 1:59 PM, John Cuthbertson wrote:
> Hi Everyone,
> Can I have a couple of volunteers look over the changes for this CR? 
> The webrev is at: http://cr.openjdk.java.net/~johnc/8007036/webrev.0/
> Summary:
> When adding old regions to the collection set we don't take into 
> account whether the old regions added so far take us below the 
> G1HeapWastePercent. As a result we could end up adding (and 
> collecting) many more regions than we needed to. The actual number 
> added was the minimum between the number of candidate regions / 
> G1MixedGCCountTarget and 10% of the heap.
> Currently the calculation of the reclaimable bytes as a percentage of 
> the uses exact arithmetic. It might make sense, at some point in the 
> future, to use inexact arithmetic (rounding) in the decision on 
> whether to continue mixed GCs and use exact arithmetic when adding 
> regions.
> As part of this change I've also moved a couple routines from 
> CollectionSetChooser to G1CollectorPolicy. I think they "fit" better 
> in G1CollectorPolicy.
> Testing:
> GCOld with tenuring threshold = 1 and a marking threshold = 10.
> Many thanks to Monica for identifying the issue.
> Thanks,
> JohnC

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