JDK-8040162 : Avoid reallocating PLABs between GC phases in G1
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 9
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2014-04-14
  • Updated: 2015-11-09
  • Resolved: 2015-08-20
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JDK 9
9 b81Fixed
Related Reports
Blocks :  
JDK-8073146 - G1 merges thread local age tables too early with global age table
Relates :  
JDK-8030849 - Investigate high fragmentation/waste in some situations during allocation during GC in G1
Description
Currently G1 uses separate allocation buffers for the different gc phases (evacuation, soft reference processing): however, PLAB size is determined by overall allocation behavior. It is very expensive in terms of memory usage to size the buffers the same in phases that have a wildly different allocation behavior: evacuation typically allocates a lot, while other phases (soft reference processing) does not.

This often wastes a lot of memory at the end of the phases that do not allocate a lot. Particularly in the soft-reference processing phase, very often only a few objects are kept alive, although G1 sizes the buffer to the same size as for the actual evacuation phase which has a lot more survivors.

Fix this.

One suggestion is to use per-thread single instances of G1ParGCAllocBuffers during the whole GC phase.