Sorting Policies

The scheduler uses policies allow changing the scheduling behaviour without code changes. Policies can be set for:

• Applications
• Nodes
• Requests

Application sorting

The application sorting policy is set for each queue via the config. A sorting policy setting is only effective on a leaf queue. Each leaf queue can use a different policy.

A sorting policy only specifies the order in which the applications are sorted within a queue. That order is crucial in specifying which application is considered first when assigning resources. Sorting policies do not affect the number of applications that are scheduled or active in the queue at the same time. All applications that have pending resource requests can and will be scheduled in a queue unless specifically filtered out. Even when applications are sorted using a first in first out policy multiple applications will run in a queue in parallel.

A parent queue will always use the fair policy to sort the child queues.

The following configuration entry sets the application sorting policy to fifo for the queue root.sandbox:

partitions:
  - name: default
    queues:
    - name: root
      queues:
      - name: sandbox
        properties:
          application.sort.policy: fifo

The only applications that are considered during scheduling must have outstanding requests. A filter is applied while sorting the applications to remove all that do not have outstanding requests.

FifoSortPolicy

Short description: first in first out, based on application create time
Config value: fifo (default)
Behaviour:
Before sorting the applications are filtered and must have pending resource requests.

After filtering the applications left are sorted based on the application create time stamp only, no other filtering is applied. Since applications can only be added while the system is locked there can never be two applications with the exact same time stamp.

The result is that the oldest application that requests resources gets resources. Younger applications will be given resources when all the current requests of older applications have been fulfilled.

FairSortPolicy

Short description: fair based on usage
Config value: fair
Behaviour:
Before sorting the applications are filtered and must have pending resource requests.

After filtering the applications left are sorted based on the application usage. The usage of the application is defined as all confirmed and unconfirmed allocations for the applications. All resources defined on the application will be taken into account when calculating the usage.

The result is that the resources available are spread equally over all applications that request resources.

StateAwarePolicy

Short description: limit of one (1) application in Starting or Accepted state
Config value: stateaware
Behaviour:
This sorting policy requires an understanding of the application states. Applications states are described in the application states documentation.

Before sorting applications the following filters are applied to all applications in the queue: The first filter is based on the application state. The following applications pass through the filter and generate the first intermediate list:

• all applications in the state running
• one (1) application in the starting state
• if there are no applications in the starting state one (1) application in the accepted state is added

The second filter takes the result of the first filter as an input. The preliminary list is filtered again: all applications without a pending request are removed.

After filtering based on status and pending requests the applications that remain are sorted. The final list is thus filtered twice with the remaining applications sorted on create time.

To recap the staring and accepted state interactions: The application in the accepted state is only added if there is no application in the starting state. The application in the starting state does not have to have pending requests. Any application in the starting state will prevent accepted applications from being added to the filtered list.

For further details see the Example run in the design document.

The result is that already running applications that request resources will get resources first. A drip feed of one new applications is added to the list of running applications to be allocated after all running applications.

Node sorting

The node sorting policy is set for a partition via the config. Each partition can use a different policy.

The following configuration entry sets the node sorting policy to fair for the partition default:

partitions:
  - name: default
    nodesortpolicy:
        type: fair

FairnessPolicy

Short description: available resource, descending order
Config value: fair (default)
Behaviour:
Sort the list of nodes by the amount of available resources so that the node with the highest amount of available resource is the first in the list. All resources defined on a node will be taken into account when calculating the usage. Resources of the same type are compared for the nodes.

This results in a node with the lowest utilisation to be considered first for assigning new allocation. Resulting in a spread of allocations over all available nodes. Leading to an overall lower utilisation of the individual available nodes, unless the whole environment is highly utilised. Keeping the load on all nodes at a similar level does help In an environment that auto scales by adding new nodes this could trigger unexpected auto scale requests.

BinPackingPolicy

Short description: available resource, ascending order
Config value: binpacking
Behaviour:
Sort the list of nodes by the amount of available resources so that the node with the lowest amount of available resource is the first in the list. All resources defined on a node will be taken into account when calculating the usage. Resources of the same type are compared for the nodes.

This results in a node with the highest utilisation to be considered first for assigning new allocation. Resulting in a high(er) utilisation of a small(er) number of nodes, better suited for cloud deployments.

Request sorting

There is currently one policy for sorting requests within an application. This policy is not configurable. Sorting requests is only possible based on the priority of the request. If there are multiple requests within an application that have the same priority the order of the requests is undetermined. This means that the order of requests with the same priority can, and most likely will, change between runs.