JDK-8068579 : Running with -XX:-UseParallelGC does not turn ParalleGC off
  • Type: Bug
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 8u40,8u45,9
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2015-01-07
  • Updated: 2018-06-21
  • Resolved: 2016-03-29
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9 b115Fixed
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Relates :  
Executing this command line with JDK 8u40 b03 (or JDK 7):

$ java -XX:-UseParallelGC -version

will start the JVM with the SerialGC. Using the same command line in JDK 8u40 b09 will start with ParallelGC:

$ /java  -XX:+PrintFlagsFinal -XX:-UseParallelGC -version | grep UseParallelGC 
     bool UseParallelGC                            := true                                {product}
java version "1.8.0_40-ea"
Java(TM) SE Runtime Environment (build 1.8.0_40-ea-b09)
Java HotSpot(TM) 64-Bit Server VM (build 25.40-b13, mixed mode)

While I think it is a bit odd to only say which GC you don't want I'm guessing that this is probably not an intended change.
Verified that this was introduced by JDK-8057531.

I think the likelihood is low, given that nearly all users select the gc they want, as opposed to disabling a gc they don't want. ILW = high, low, low -> p4.

Impact=High (regression) LIkelihood=High (always happens if the flag is set) Workaround=Low (specify -XX:+UseSerialGC instead) ILW=HHL -> P2 About the likelihood. Not sure if it should be high. It's high in the sense that the wrong GC will be selected every time if you set the flag on the command line. But it may not be such a common way to use the flag. So, my guess it that few customers will notice it but those who do will always notice it.

I haven't verified it completely, but I suspect that this change is an unintended side effect of JDK-8057531, which was pushed to JDK 8u40 b05. http://hg.openjdk.java.net/jdk8u/hs-dev/hotspot/rev/8ec8971f511a