JDK-8242847 : G1 should not clear mark bitmaps with no marks
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 15
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2020-04-15
  • Updated: 2021-09-02
  • Resolved: 2021-08-25
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JDK 18
18 b13Fixed
Related Reports
Relates :  
JDK-8240556 - Abort concurrent mark after effective eager reclamation of humongous objects
Relates :  
JDK-8197850 - Calculate liveness in regions during marking
Description
Currently G1 clears the whole (next) mark bitmaps of the regions of the entire (committed) heap.

This is unnecessary, only bitmaps of regions that were dirtied need clearing. Since JDK-8197850 we track how much live data has been encountered during mark for every region, corresponding to the live objects we set a mark between bottom and ntams.

I.e. first, we only need to clear between [bottom, ntams[, and second only clear bitmaps which regions had at least one bit set (i.e. have some live data).

This is also a very interesting optimization for JDK-8240556
Comments
Changeset: e36cbd8e Author: Ivan Walulya <iwalulya@openjdk.org> Date: 2021-08-25 14:33:34 +0000 URL: https://git.openjdk.java.net/jdk/commit/e36cbd8e05774ea9847c69f9987a2242589acf7e
25-08-2021