RAM usage of Kafka

[saloni-2404]

Hello Jakub, happy new year.
Here is a question, why Kafka could be using too much RAM, and what will happen if the RAM limit is breached?

[scholzj]

TBH, I'm not sure I have an answer. I do not really understand how these things work. But for a start you would probably need to clarify what do you mean with RAM and limits. Do you mean the Kubernetes requests and limits? Or do you mean more things like the Java heap etc.?

[scholzj]

I don't think that can be answered like this. That depends on many factors.

[Gulnaz2021]

Hello Jacob,
Can you please help with the information that, the resource limit the we define in kafka.yaml, suppose for memory. Then is it that the Strimzi operator would inform Kafka internally the limit of the available resource and that it should not exceed the limit, example for memory. So, that Kafka would try to consume all the available memory, but would not cross the limit.

[scholzj]

Not really. Java has its own container awareness and is able to detect this on its own from the container it self. So there is nothing like passing to Kafka some option --memory=8Gi or something like that. We set it in the Pod / container and Java detects it from there. There are some additional options like the -Xmx etc., which tell the Java VM about how the memory should be used as well.

[Gulnaz2021]

So, suppose for the below example, that we would configure in kafka.yaml file:
resources:
requests:
memory: 18Gi
cpu: "4"
limits:
memory: 20Gi
cpu: "8",
Then how would Kafka know, that memory limit is 20 Gi.
And what we've observed, is that for the Kafka cluster, memory utilization has always been near about the memory limit, but has never been crossed. Even with the reduced memory, similar pattern has been observed. So, is it that expected behavior of Kafka to try to use as much of the available memory.

[scholzj]

The available memory can be queried from inside the container. It is just a system information as any other. So when Java starts with your configuration, it sees that it has 20Gi of available memory.