If you're reading this article, you very likely know that Neo4j is a Java application. There are various garbage collectors (GC) available for Java:

• The Serial GC
• The Parallel GC
• The Concurrent Mark Sweep (CMS) Collector
• The G1 Garbage Collector
• And a few more

In Neo4j, these are set in the "neo4j.conf" file as:
 

server.jvm.additional=-XX:+UseG1GC # setting in 5.x

OR

dbms.jvm.additional=-XX:+UseG1GC # setting in 4.x

The G1 Garbage Collector (G1GC) is the default garbage collector already configured in the "neo4j.conf" file when installing Neo4j. 

It's crucial to understand that the Neo4j Ops Manager (NOM) can ONLY render GC metrics charts when the default garbage collector is used. NOM is designed to interpret metrics issued by the default garbage collector, not others. This understanding is key to troubleshooting any issues with the NOM GC metrics charts.

For example, a customer tried to use "The Parallel GC", which was activated by setting "server.jvm.additional=-XX:+UseParallelGC". They didn't know the impact of this change on NOM, so a few days later, they reported that they were seeing blank GC metrics charts in NOM.