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diff --git a/scheduler.md b/scheduler.md new file mode 100644 index 00000000..ac01e6db --- /dev/null +++ b/scheduler.md @@ -0,0 +1,50 @@ + +# The Kubernetes Scheduler + +The Kubernetes scheduler runs as a process alongside the other master +components such as the API server. Its interface to the API server is to watch +for Pods with an empty PodSpec.NodeName, and for each Pod, it posts a Binding +indicating where the Pod should be scheduled. + +## The scheduling process + +The scheduler tries to find a node for each Pod, one at a time, as it notices +these Pods via watch. There are three steps. First it applies a set of "predicates" that filter out +inappropriate nodes. For example, if the PodSpec specifies resource limits, then the scheduler +will filter out nodes that don't have at least that much resources available (computed +as the capacity of the node minus the sum of the resource limits of the containers that +are already running on the node). Second, it applies a set of "priority functions" +that rank the nodes that weren't filtered out by the predicate check. For example, +it tries to spread Pods across nodes while at the same time favoring the least-loaded +nodes (where "load" here is sum of the resource limits of the containers running on the node, +divided by the node's capacity). +Finally, the node with the highest priority is chosen +(or, if there are multiple such nodes, then one of them is chosen at random). The code +for this main scheduling loop is in the function `Schedule()` in +[plugin/pkg/scheduler/generic_scheduler.go](../../plugin/pkg/scheduler/generic_scheduler.go) + +## Scheduler extensibility + +The scheduler is extensible: the cluster administrator can choose which of the pre-defined +scheduling policies to apply, and can add new ones. The built-in predicates and priorities are +defined in [plugin/pkg/scheduler/algorithm/predicates/predicates.go](../../plugin/pkg/scheduler/algorithm/predicates/predicates.go) and +[plugin/pkg/scheduler/algorithm/priorities/priorities.go](../../plugin/pkg/scheduler/algorithm/priorities/priorities.go), respectively. +The policies that are applied when scheduling can be chosen in one of two ways. Normally, +the policies used are selected by the functions `defaultPredicates()` and `defaultPriorities()` in +[plugin/pkg/scheduler/algorithmprovider/defaults/defaults.go](../../plugin/pkg/scheduler/algorithmprovider/defaults/defaults.go). +However, the choice of policies +can be overridden by passing the command-line flag `--policy-config-file` to the scheduler, pointing to a JSON +file specifying which scheduling policies to use. See +[examples/scheduler-policy-config.json](../../examples/scheduler-policy-config.json) for an example +config file. (Note that the config file format is versioned; the API is defined in +[plugin/pkg/scheduler/api/](../../plugin/pkg/scheduler/api/)). +Thus to add a new scheduling policy, you should modify predicates.go or priorities.go, +and either register the policy in `defaultPredicates()` or `defaultPriorities()`, or use a policy config file. + +## Exploring the code + +If you want to get a global picture of how the scheduler works, you can start in +[plugin/cmd/kube-scheduler/app/server.go](../../plugin/cmd/kube-scheduler/app/server.go) + + +[]() |