LowRiskOverCommitment Plugin

The LowRiskOverCommitment plugin is one of the Trimaran scheduler plugins, described in Trimaran: Real Load Aware Scheduling. The Trimaran plugins employ the load-watcher in order to collect measurements from the nodes as described here.

Though containers are allowed to specify resource limit values beyond the requested (guaranteed) values, most schedulers are only concerned with the requested values. This may result in a cluster where some (or all) nodes are overcommitted, thus leading to potential CPU congestion and performance degradation and/or memory OOM situations and pod evictions. The LowRiskOverCommitment plugin takes into consideration (1) the resource limit values of pods (limit-aware) and (2) the actual load (utilization) on the nodes (load-aware) could provide a low risk environment for pods and alleviate issues with overcommitment, while allowing pods to use their limits.

The LowRiskOverCommitment plugin evaluates the performance risk of overcommitment and selects the node with lowest risk. It achieves this goal by combining two risk factors: limit risk and load risk. The limit risk is based on requests and limits values. And, the load risk is based on observed load. LowRiskOverCommitment is risk-aware as well as load-aware. The outcome is that burstable and best effort pods are placed on nodes where the chance of being impacted by overcommitment is minimized, while providing them a chance to burst up to their full limits.

The LowRiskOverCommitment plugin has the following configuration parameters:

• smoothingWindowSize : The number of windows over which metrics are smoothed. (Default 5)
• riskLimitWeights : A map resource weights (between 0 and 1) of risk due to limit specifications (as opposed to risk due to load utilization). (Default [cpu: 0.5, memory: 0.5])

In addition, we have the metricProviderconfiguration parameters, depending on whether the load-watcher is in service or library mode, respectively.

Following is an example scheduler configuration with the LowRiskOverCommitment plugin enabled, and using the load-watcher in library mode, collecting measurements from the Prometheus server.

apiVersion: kubescheduler.config.k8s.io/v1
kind: KubeSchedulerConfiguration
leaderElection:
  leaderElect: false
profiles:
- schedulerName: trimaran
  plugins:
    score:
      enabled:
       - name: LowRiskOverCommitment
  pluginConfig:
  - name: LowRiskOverCommitment
    args:
      smoothingWindowSize: 5
      riskLimitWeights:
        cpu: 0.5
        memory: 0.5
      metricProvider:
        type: Prometheus
        address: http://prometheus-k8s.monitoring.svc.cluster.local:9090