Changing GC settings in EAP on Openshift gives error "Conflicting collector option used"

Environment

• Red Hat JBoss Enterprise Application Platform (EAP)
  • 7.x for OpenShift
  • 8.x for OpenShift
• JBoss Web Server
  • 3.x for OpenShift
  • 5.x for OpenShift

Issue

• Changing garbage collection option using command oc env dc/demoeap JAVA_OPTS_APPEND=-XX:+UseG1GC in JBoss EAP on Openshift, pods shows error Conflicting collector option used.
• How do I switch the default GC strategy from -XX:-UseParallelOldGC to -XX:+UseG1GC/-XX:+UseConcMarkSweepGC/-XX:+UseSerialGC?

Resolution

EAP containers:

Example jboss-eap-7/eap71-openshift:1.4-6 (upper):

From jboss-eap-7/eap71-openshift:1.4-6, environment variable GC_CONTAINER_OPTIONS is introduced to switch GC strategy from -XX:+UseParallelOldGC as default. This feature is implemented by Content from issues.jboss.org is not included.CLOUD-2286:

oc set env dc/demoeap GC_CONTAINER_OPTIONS="-XX:+UseG1GC"
deploymentconfig.apps.openshift.io/demoeap updated

Example jboss-eap-7/eap71-openshift:1.4-6 (lower):

Previous versions may require disabling the previously enabled GC option or default garbage collector, and enable the required GC option (below disable UseParallelOldGC and enable G1GC):

oc set env dc/eap-app JAVA_OPTS_APPEND="-XX:-UseParallelOldGC -XX:+UseG1GC"
deploymentconfig.apps.openshift.io/eap-app updated

In eap71-openshift:1.4-6 or later and subsequent minor versions of EAP such as 7.2/7.3/7.4, both JAVA_OPTS_APPEND and GC_CONTAINER_OPTIONS are env variables available to switch GC strategy. Using this option there is not need to disable parallel in fact.

EAP 8:

For EAP 8, the process will be relying on OpenJDK fully settings - not its own scripts and therefore ParallelGC comes by default, use GC_CONTAINER_OPTIONS for replacing it. Find more details in Differences in the default heap settings between JBoss EAP 7 and JBoss EAP 8 for OpenShift.

JWS containers:

From JWS versions below and later, environment variable GC_CONTAINER_OPTIONS is introduced to switch GC strategy from -XX:+UseParallelOldGC as default. This feature is implemented by This content is not included.CLOUD-2941. Other non-GC options can be set with CATALINA_OPTS_APPEND per How to append JVM options to JWS for OpenShift. Details:

• jboss-webserver-3/webserver31-tomcat7-openshift:1.3-9
• jboss-webserver-3/webserver31-tomcat8-openshift:1.3-9
• jboss-webserver-5/webserver50-tomcat9-openshift:1.1-2

Root Cause

The JVM can only use one garbage collector at time - meaning one cannot use Parallel+G1GC at the same time. Previous versions of java would allow multiple garbage collector one for each generation - that's not the case for OpenJDK 7/8/11/17.

Therefore one needs to remove other conflicting gc collectors in case multiple are being used: ParallelGC and G1GC for example. However, later images introduced specifically GC_CONTAINER_OPTIONS to be used for this matter.

Diagnostic Steps

1. oc set env dc/$dcname GC_CONTAINER_OPTIONS="-XX:+UseG1GC"
2. oc get dc/$dcname -o yaml | grep -B 5 -A 5 UseG1GC

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