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Merge pull request #164 from hongchaodeng/s 

devel/scheduler: update intro and scheduling algo
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Hongchao Deng 2016-12-14 08:34:50 -08:00 committed by GitHub
parent 524fd95a13 7d5d4f6e76
commit e3871da9ff
1 changed files with 54 additions and 36 deletions
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contributors/devel/scheduler.md
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@ -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,
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.
@ -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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