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Dec 14, 2016
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devel/scheduler: update intro and scheduling algo #164

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90 changes: 54 additions & 36 deletions contributors/devel/scheduler.md
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Original file line number Diff line number Diff line change
@@ -1,39 +1,63 @@
# 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 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,
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@jayunit100 jayunit100 Dec 12, 2016

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it's (not your fault, it existed before :) but might as well fix now right :)

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@hongchaodeng hongchaodeng Dec 12, 2016

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Can you read again? It should be "Its interface to the ... is to".

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@hongchaodeng hongchaodeng Dec 12, 2016

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@jayunit100
I see no problem in this sentence:

"Its interface to the API server is to watch..."

Can you clarify what it should be?

and for each Pod, it posts a binding indicating where the Pod should be scheduled.

## The scheduling process
## Exploring the code

We are dividng scheduler into three layers from high level:
- [plugin/cmd/kube-scheduler/scheduler.go](http://releases.k8s.io/HEAD/plugin/cmd/kube-scheduler/scheduler.go):
This is the main() entry that does initialization before calling the scheduler framework.
- [pkg/scheduler/scheduler.go](http://releases.k8s.io/HEAD/pkg/scheduler/scheduler.go):
This is the scheduler framework that handles stuff (e.g. binding) beyond the scheduling algorithm.
- [pkg/scheduler/generic_scheduler.go](http://releases.k8s.io/HEAD/pkg/scheduler/generic_scheduler.go):
The scheduling algorithm that assigns nodes for pods.

## The scheduling algorithm

```
+-------+
+---------------+ node 1|
| +-------+
|
+----> | Apply pred. filters
| |
| | +-------+
| +----+---------->+node 2 |
| | +--+----+
| watch | |
| | | +------+
| +---------------------->+node 3|
+--+---------------+ | +--+---+
| Pods in apiserver| | |
+------------------+ | |
| |
| |
+------------V------v--------+
| Priority function |
+-------------+--------------+
|
| node 1: p=2
| node 2: p=5
v
select max{node priority} = node 2
For given pod:

+---------------------------------------------+
| Schedulable nodes: |
| |
| +--------+ +--------+ +--------+ |
| | node 1 | | node 2 | | node 3 | |
| +--------+ +--------+ +--------+ |
| |
+-------------------+-------------------------+
|
|
v
+-------------------+-------------------------+

Pred. filters: node 3 doesn't have enough resource

+-------------------+-------------------------+
|
|
v
+-------------------+-------------------------+
| remaining nodes: |
| +--------+ +--------+ |
| | node 1 | | node 2 | |
| +--------+ +--------+ |
| |
+-------------------+-------------------------+
|
|
v
+-------------------+-------------------------+

Priority function: node 1: p=2
node 2: p=5

+-------------------+-------------------------+
|
|
v
select max{node priority} = node 2
```

The Scheduler tries to find a node for each Pod, one at a time.
Expand All @@ -60,12 +84,6 @@ the policies used are selected by the functions `defaultPredicates()` and `defau
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](http://releases.k8s.io/HEAD/plugin/pkg/scheduler/api/)).
Thus to add a new scheduling policy, you should modify [plugin/pkg/scheduler/algorithm/predicates/predicates.go] (http://releases.k8s.io/HEAD/plugin/pkg/scheduler/algorithm/predicates/predicates.go) or add to the directory [plugin/pkg/scheduler/algorithm/priorities](http://releases.k8s.io/HEAD/plugin/pkg/scheduler/algorithm/priorities/), 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](http://releases.k8s.io/HEAD/plugin/cmd/kube-scheduler/app/server.go)

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