JDK-8067336 : Allow that PLAB allocations at the end of regions are flexible
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 9
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2014-12-12
  • Updated: 2015-11-09
  • Resolved: 2015-08-25
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JDK 9
9 b81Fixed
Related Reports
Blocks :  
JDK-8067455 - [NEWTEST] Test flexible allocation at end of G1 region
Relates :  
JDK-8030849 - Investigate high fragmentation/waste in some situations during allocation during GC in G1
Relates :  
JDK-8067433 - Keep waste at end of regions for further allocation during GC
Description
Investigation in JDK-8030849 showed that there is a very large amount of wasted memory when allocating PLABs due to the region end being a hard boundary and the PLAB size not flexible.

E.g. if you want to allocate a PLAB of size 20k (to allocate regular sized objects), but there is only 19k left, the current allocation algorithm simply skips to the next region.

Fix this by allowing the allocator to return smaller (but large enough to fit the allocation) PLABs in this case.
Comments
Would be nice to test. RFE JDK-8067455 is filed for that case.
15-12-2014