Jon Masamitsu's Weblog
Monday Mar 05, 2007
GC Errata 1
Just some tid bits of information to fill some of the holes in the documentation.GC ergonomics is used by default on server class machines. Please see
http://java.sun.com/docs/hotspot/gc5.0/ergo5.html
for a description of GC ergonomics and server class machine. The flag "-server" causes the server VM to be used. "-server" does not cause the platform on which the VM is running to be considered a server class machine.
Prior to JDK6 the UseParallelGC collector (with GC ergonomics turned on) used only the flags InitialSurvivorRatio and MinSurvivorRatio to set the initial survivor ratio and the minimum survivor ratio, respectively. GC ergonomics dynamically resizes the survivor spaces, but these flags allow the user to specify initial and minimum sizes for the survivor spaces. The flag SurvivorRatio is used by the other collectors (that don't dynamically resize the survivor spaces) to set the size of the survivor spaces. SurvivorRatio was ignored by the UseParallelGC collector. Beginning with JDK6 if the SurvivorRatio is set on the command line for the UseParallelGC collector and InitialSurvivorRatio and MinSurvivorRatio are not explicitly set on the command line, then InitialSurvivorRatio and MinSurvivorRatio are set to SurvivorRatio + 2.
If UseAdaptiveSizePolicy is turned off when using the UseParallelGC collector, the tenured generation and the young generation stay at their initial sizes throughout the execution of the VM.
Starting with JDK5 update 06 the maximum tenuring threshold is limited to 15. The GC logging output does not correctly reflect this change. This is a bug that will be fixed under 6521376.
If a minimum heap size is set with -Xms and NewSize is not explicitly set, the minimum young generation size is calculated using NewRatio. The UseParallelGC collector is not correctly using the minimum value calculated from -Xms and NewRatio but is using the default value of NewSize. Explicitly setting NewSize does correctly set the minimum size for the generation. This is being fixed under bug 6524727.
Posted at 11:55AM Mar 05, 2007 by jonthecollector in Java | Comments[3]