Questions on bug JDK-8061611 "Remove deprecated command line flags"
derek.white at oracle.com
Mon Nov 10 22:43:47 UTC 2014
I'm tackling a "starter bug" JDK-8061611
<https://bugs.openjdk.java.net/browse/JDK-8061611> " Remove deprecated
command line flags" that sounds like it should be pretty trivial, but
some part of this reveal some unexpectedly twisted behavior. I guess
that makes it the perfect starter GC bug :-)
One of the command line flags to remove is
-XX:CMSParPromoteBlocksToClaim. It has been deprecated for a while.
Using it prints the message
"Please use -XX:OldPLABSize in place of
-XX:CMSParPromoteBlocksToClaim in the future"
Two little twists:
1) The variable CMSParPromoteBlocksToClaim is still used separately from
OldPLABSize when initializing the "_blocks_to_claim" array in
compactibleFreeListSpace.cpp. It looks like it uses it when calculating
the initial averages for AdaptiveWeightedAverage. Is it important to
retain the ability to set the variable CMSParPromoteBlocksToClaim
separately from OldPLABSize? And see last question***.
2) In the CMS case, there is code that attempts to synchronize the
OldPLABSize and CMSParPromoteBlocksToClaim variables in
Arguments::set_cms_and_parnew_gc_flags(). If OldPLABSize is set (via
command line or ergo), then CMSParPromoteBlocksToClaim is set to the
same value unless it is already set too. Then in any case OldPLABSize is
set to CMSParPromoteBlocksToClaim.
But note the default values:
OldPLABSize = 1024
CMSParPromoteBlocksToClaim = 16
So if both OldPLABSize and CMSParPromoteBlocksToClaim were left at
default values, then OldPLABSize just got set to 16, which seems way too
small to be useful. Not only is the OldPLABSize default 1024, but in
set_parnew_gc_flags() we just reset the value to 1024 (again). So this
seems pretty intentional, and setting to 16 seems accidental.This sounds
$ java -XX:+UseConcMarkSweepGC -XX:+PrintFlagsFinal -version | grep
uintx OldPLABSize := 16
In fact, I don't see why the "Size of old gen promotion LAB's (in
HeapWords)" should be equal to "Number of blocks to attempt to claim
when refilling CMS LAB". *** Aren't these different units?
Thanks for any thoughts,
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