When a container in a Pod is created in a Kubernetes cluster with a limit and request set, how aware can that container be of those limits and requests? Would an application running inside the container be able to get these limits and requests to, for example, reduce the amount of resources it uses if the limits and requests were particularly low?
When you create a Pod, the Kubernetes scheduler selects a node for the Pod to run on. Each node has a maximum capacity for each of the resource types: the amount of CPU and memory it can provide for Pods. The scheduler ensures that, for each resource type, the sum of the resource requests of the scheduled Containers is less than the capacity of the node. Note that although actual memory or CPU resource usage on nodes is very low, the scheduler still refuses to place a Pod on a node if the capacity check fails. This protects against a resource shortage on a node when resource usage later increases, for example, during a daily peak in request rate.
If a Container exceeds its memory limit, it might be terminated. If it is restartable, the kubelet will restart it, as with any other type of runtime failure.
If a Container exceeds its memory request, it is likely that its Pod will be evicted whenever the node runs out of memory.
Container might or might not be allowed to exceed its CPU limit for extended periods of time. However, it will not be killed for excessive CPU usage
To get the information about resource usage, you need a monitoring system, configured for your cluster (heapster, prometheus, etc). Requests and limits can be adjusted according to this data manually or automatically.
One of possible ways to automate this process is to create a dedicated microservice, that will watch resources usage (by collecting and analyzing data from monitors), generate manifests with new limits and send requests kube api to recreate pods.
Check mem_limit within a docker container with the tl;dr of
cat /sys/fs/cgroup/memory/memory.limit_in_byteswill show the limit, and then presumably the requests value is the allocated memory the container started with, but I would need to verify that assumption
I personally don't even understand the unit when trying to apply limits: cpu: so I for sure wouldn't know how to verify that value
The Downwards API can be used to pass the requests and limits to the container process as environment variables