JDK-8237500 : Increase the G1 eager reclaim candidate selection threshold
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 14,15
  • Priority: P4
  • Status: Open
  • Resolution: Unresolved
  • Submitted: 2020-01-18
  • Updated: 2021-05-28
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Other
tbdUnresolved
Related Reports
Relates :  
JDK-8233919 - Incrementally calculate the occupied cards in a heap region remembered set
Relates :  
JDK-8267836 - Separate eager reclaim remembered set threshold from G1RSetSparseRegionEntries
Relates :  
JDK-8223162 - Improve ergonomics for Sparse PRT entry sizing
Relates :  
JDK-8267933 - Opportunistically recalculate remembered sets for almost-eager reclaim candidates
Description
The selection of candidates for eager reclaim of humongous objects mostly depends on the number of remembered set entries for that humongous object.

Currently this threshold is ergonomically set to the sparse prt entry count as gathering the number of remembered set entries for the higher level PRTs is considered too costly. This sparse prt entry count is very low, although it has increased in JDK-8223162.

However, with JDK-8233919 this operation to get number of entries in the card set is a read of a single variable. JDK-8267836 also separates this threshold from the number of sparse prt entries. So the reason to keep the threshold low is gone.

Look into increasing this threshold to catch more eager reclaim candidates.