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I'm tackling a "starter bug"
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<a id="key-val" rel="4751503"
href="https://bugs.openjdk.java.net/browse/JDK-8061611">JDK-8061611</a>
"
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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
:-)<br>
<br>
One of the command line flags to remove is
-XX:CMSParPromoteBlocksToClaim. It has been deprecated for a while.
Using it prints the message <br>
"Please use -XX:OldPLABSize in place of
-XX:CMSParPromoteBlocksToClaim in the future"<br>
<br>
Two little twists:<br>
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***.<br>
<br>
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.<br>
<br>
But note the default values:<br>
<blockquote>OldPLABSize = 1024<br>
CMSParPromoteBlocksToClaim = 16<br>
</blockquote>
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 bad???<br>
<br>
<tt>$ java -XX:+UseConcMarkSweepGC -XX:+PrintFlagsFinal -version |
grep OldPLABSize</tt><tt><br>
</tt><tt> uintx OldPLABSize :=
16 {product} </tt> <br>
<br>
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?<br>
<br>
Thanks for any thoughts,<br>
<br>
- Derek<br>
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