JDK-8079555 : REDO - Determining the desired PLAB size adjusts to the the number of threads at the wrong place
  • Type: Bug
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 9
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2015-05-07
  • Updated: 2020-11-27
  • Resolved: 2015-07-07
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JDK 9
9 b74Fixed
Related Reports
Cloners :  
JDK-8073204 - Determining the desired PLAB size adjusts to the the number of threads at the wrong place
Relates :  
JDK-8150952 - Net PLAB size is clipped to max PLAB size as a whole, not on a per thread basis
Relates :  
JDK-8257145 - Performance regression with -XX:-ResizePLAB after JDK-8079555
Relates :  
JDK-8030849 - Investigate high fragmentation/waste in some situations during allocation during GC in G1
Sub Tasks
JDK-8079556 :  
BACKOUT - Determining the desired PLAB size adjusts to the the number of threads at the wrong place - Resolved
Description
When calculating the size of the next PLAB we pass along the current number of gc workers, calculating an "optimal" PLAB size for that number of gc workers, that is fed directly to the desired PLAB size predictor.

When the number of workers changes dynamically (with -XX:+UseDynamicNumberOfGCThreads), the desired PLAB size value returned is still tuned to the number of threads that has been used previously, losing performance.

A better solution would be to feed the PLAB size predictor the value for a single thread, and calculate the actual desired size according to the current number of threads instead.