Gen ZGC EAB in K8S

[Evaristo José Camarero]

Hi there,
We are trying latest ZGC EAB for testing an Apache Geode Cluster (distributed Key Value store with similar use cases that Apache Cassandra)
We are using K8s, and we have PODs with 32 cores request (limit with 60 cores) per data node with 150GB heap per node.
          - -Xmx152000m          - -Xms152000m          - -XX:+UseZGC          - -XX:SoftMaxHeapSize=136000m   - -XX:ZAllocationSpikeTolerance=4.0   // We have some spiky workloads periodically           - -XX:+UseNUMA


ZGC is working great in regard GC pauses with no allocation stalls at all during almost all the time. We observe higher CPU utilization that G1 (Around 20% for a heavy workload using flag -XX:ZAllocationSpikeTolerance=4.0 that maybe is making ZGC more hungry than needed. We will play further with this)

BUT from time to time we see that Geode Clusters are missing heartbeats between and Geode logic shutdowns the JVM of the node with missing heartbeats. We believe that the main problem could be CPU starvation, because some seconds before this is happening we observe ZGC to use more workers for making the job done

[2023-03-12T18:29:38.980+0000] Adjusting Workers for Young Generation: 1 -> 2
[2023-03-12T18:29:39.781+0000] Adjusting Workers for Young Generation: 1 -> 3
[2023-03-12T18:29:40.181+0000] Adjusting Workers for Young Generation: 1 -> 4
[2023-03-12T18:29:40.382+0000] Adjusting Workers for Young Generation: 1 -> 5
[2023-03-12T18:29:40.582+0000] Adjusting Workers for Young Generation: 1 -> 6
[2023-03-12T18:29:40.782+0000] Adjusting Workers for Young Generation: 1 -> 7
[2023-03-12T18:29:40.882+0000] Adjusting Workers for Young Generation: 1 -> 8
[2023-03-12T18:29:40.982+0000] Adjusting Workers for Young Generation: 1 -> 10
[2023-03-12T18:29:41.083+0000] Adjusting Workers for Young Generation: 1 -> 13
[2023-03-12T18:29:41.183+0000] Adjusting Workers for Young Generation: 1 -> 16

As commented we are using K8S with PODs that have 32 cores for request and 60 cores for limit (and it also true that our K8s workers are close to the limit in CPU utilization). ZGC is assuming on booting that machine has 60 cores (as logged). What is the best way to configure the ZGC to provide a hint to be tuned for a host with 32 cores (basically the 60 cores limit is just to avoid K8s produced throttling)? Is it using ParallelGCThreads flag? Any other thoughts?
Regards,
Evaristo
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