G1 GC or Generational ZGC for AEM ON Prem LTS 6.5

​@sanjeevkumart45 

Java 21 is supported for AEM 6.5 LTS, but I haven't seen Adobe publish specific guidance recommending Generational ZGC over G1 for AEM prod deployments. Adobe's LTS release notes do mention overriding the default GC settings for optimal performance, but don't prescribe a particular collector.

From a JVM perspective, Generational ZGC can provide much lower pause times, while G1 is generally a safer choice when you want a balance between throughput and latency.

For AEM, I would not make the switch based on theoretical JVM benefits alone. I would benchmark both using a production-like workload and monitor GC pauses, CPU, throughput, heap usage and, importantly, Oak/indexing and workflow behavior.

Unless Adobe Support provides workload-specific guidance, G1 would probably be my conservative production choice today. Generational ZGC is definitely worth evaluating, but I would treat it as a performance test rather than a default recommendation.

Hello ​@sanjeevkumart45 

I haven't personally run AEM 6.5 LTS with Generational ZGC in production, so I wouldn't want to claim any real-world AEM-specific results.

From what I understand, I would still keep G1 as the safer baseline for AEM 6.5 LTS on JDK 21. It gives a good balance between throughput and pause times and is a well-established choice for AEM workloads.

Generational ZGC is definitely worth looking at if your main concern is GC pauses and application responsiveness. The biggest potential benefit would be much lower and more predictable pause times, rather than necessarily higher overall throughput. There can also be some additional CPU overhead.

For an AEM environment, I'd personally run a proper load test with the same repository and workload before switching. I'd compare GC pause times, CPU, request latency, Oak queries/indexing, workflows and replication.

Also, I haven't seen Adobe explicitly recommending Generational ZGC over G1 for AEM 6.5 LTS + JDK 21. If this is going into production, I'd probably confirm the supported/recommended GC configuration with Adobe Support as well.

Hi ​@sanjeevkumart45 

AEM 6.5 LTS supports Oracle Java 17 and Java 21. However, Adobe's public documentation does not recommend Generational ZGC over G1, nor does it publish a universal GC choice for all AEM workloads.

If the existing G1 configuration is stable, I would keep it as the production baseline. Generational ZGC is worth evaluating when GC pauses and tail latency are the actual bottleneck, but lower pauses do not necessarily mean higher throughput. ZGC can also introduce additional CPU overhead.

Before changing the collector, benchmark both configurations using the same AEM LTS service pack, JDK vendor/build, heap size, hardware, repository snapshot, indexes, traffic, workflows, indexing, replication, and asset-processing workload. 
Compare GC pause percentiles, CPU, request latency, throughput, heap usage, Oak query/indexing behavior, workflow backlog, replication queues, and error rates. Test the candidate in a lower environment first and retain a rollback plan.  Therefore, the safest recommendation is to retain a stable G1 configuration unless Generational ZGC demonstrates a measurable benefit for the specific AEM workload and Adobe Support confirms the production support position.