Rank Scheduler

The OpenNebula Rank Scheduler is responsible for allocating pending Virtual Machines to available hypervisor nodes. It’s a dedicated daemon installed alongside the OpenNebula Daemon (oned), but can be deployed independently on a different machine. The Scheduler is distributed as part of the operating system package opennebula with the system service opennebula-scheduler.

Overview

The Rank Scheduler implements a match-making algorithm for VM allocation. The goal of this policy is to prioritize the resources that are more suitable for the VM.

The match-making algorithm works as follows:

• Each disk of a running VM uses storage from an Image Datastore. VMs that require more storage than is currently available are filtered out, and will remain in the pending state.
• Hosts that do not meet VM requirements (see the SCHED_REQUIREMENTS attribute) or do not have enough resources (available CPU and memory) to run the VM are filtered out (see below for more information).
• The same happens for System Datastores: those that do not meet the DS requirements (see the SCHED_DS_REQUIREMENTS attribute) or do not have enough free storage are filtered out.
• Finally, if the VM uses automatic network selection, the Virtual Networks that do not meet the NIC requirements (see the SCHED_REQUIREMENTS attribute for NICs) or do not have enough free leases are filtered out.
• The SCHED_RANK and SCHED_DS_RANK expressions are evaluated upon the Host and Datastore list using the information gathered by the monitor drivers. Also, the NIC/SCHED_RANK expressions are evaluated upon the Network list using the information in the Virtual Network template. Any variable reported by the monitor driver (or manually set in the Host, Datastore, or Network template) can be included in the rank expressions.
• Resources with a higher rank are used first to allocate VMs.

The Scheduling algorithm allows us to easily implement several placement heuristics (see below) depending on the selected RANK expressions.

You can define the policy to place a VM in one of two places:

• Globally, for all VMs: in the configuration file /etc/one/schedulers/rank.conf for the scheduler.
• For each VM: as defined by the general SCHED_RANK and SCHED_DS_RANK, and the NIC-specific SCHED_RANK in the VM template.

Configuration

The Scheduler configuration file is /etc/one/schedulers/rank.conf on the Front-end, and can be customized with the parameters listed in the table below.

• MAX_HOST: Maximum number of Virtual Machines dispatched to a given Host in each scheduling action (Default: 1)
• MEMORY_SYSTEM_DS_SCALE: This factor scales the VM’s usage of the system DS according to memory size. This factor can be used to make the scheduler consider the overhead caused by checkpoint files (system_ds_usage = system_ds_usage + memory_system_ds_scale * memory).
• DIFFERENT_VNETS: When set (YES) the NICs of a VM will be assigned to different Virtual Networks.

The default scheduling policies for Hosts, Datastores and Virtual Networks are defined as follows:

• DEFAULT_SCHED: Definition of the default scheduling algorithm.

  • RANK: Arithmetic expression to rank suitable Hosts based on their attributes.
  • POLICY: A predefined policy, can be set to:

• DEFAULT_DS_SCHED: Definition of the default storage scheduling algorithm. IMPORTANT: storage policies work only for shared datastores.
  • RANK: Arithmetic expression to rank suitable datastores based on their attributes.
  • POLICY: A predefined policy, can be set to:

• DEFAULT_NIC_SCHED: Definition of the default Virtual Network scheduling algorithm.
  • RANK: Arithmetic expression to rank suitable networks based on their attributes.
  • POLICY: A predefined policy, can be set to:

• EXTERNAL_SCHEDULER: Configuration to contact an external scheduler module:

  • SERVER, the http URL to perform the POST operation
  • PROXY, if needed to contact the external scheduler
  • TIMEOUT, how long to wait for a response

• VM_ATTRIBUTE: Attributes serialized to External Scheduler. The format is XPATH:<NAME> where:

  • XPATH is the xpath of the attribute
  • NAME (optional) is the name of the attribute used in the JSON doc sent to the external scheduler (if not set, the original name will be used).
  • Examples:

    • VM_ATTRIBUTE = "/VM/TEMPLATE/CPU"
    • VM_ATTRIBUTE = "//CPU"
    • VM_ATTRIBUTE = "/VM/TEMPLATE/VMGROUP/ROLE:GROUP_ROLE"

• LOG: Configuration for the logging system.
  • SYSTEM: Defines the logging system. Use file to log to the sched.log file, syslog to use syslog, std to use the default log stream (stderr).
  • DEBUG_LEVEL: Logging level. Use 0 for ERROR, 1 for WARNING, 2 for INFO, 3 for DEBUG, 4 for DDEBUG, 5 for DDDEBUG.

The optimal values of the scheduler parameters depend on the hypervisor, storage subsystem, and a number of physical Hosts. The values can be derived by finding out the max. number of VMs that can be started in your setup without getting hypervisor-related errors.

User Policies

VMs are dispatched to Hosts in a FIFO fashion. You can alter this behavior by giving each VM (or the base template) a priority. Just set the attribute USER_PRIORITY to sort the VMs based on this attribute and so alter the dispatch order. For example, the USER_PRIORITY can be set in the VM templates for a specific user group if you want to prioritize the templates in that group. Note that this priority is also used for rescheduling.

Pre-defined Placement Policies

The following list describes the predefined policies for DEFAULT_SCHED configuration parameter:

Packing Policy

• Target: Minimize the number of cluster nodes in use
• Heuristic: Pack the VMs in the cluster nodes to reduce VM fragmentation
• Implementation: Use those nodes with more VMs running first

RANK = RUNNING_VMS

Striping Policy

• Target: Maximize the resources available to VMs in a node
• Heuristic: Spread the VMs in the cluster nodes
• Implementation: Use those nodes with less VMs running first

RANK = "- RUNNING_VMS"

Load-aware Policy

• Target: Maximize the resources available to VMs in a node
• Heuristic: Use those nodes with less load
• Implementation: Use those nodes with more FREE_CPU first

RANK = FREE_CPU

Fixed Policy

• Target: Sort the Hosts manually
• Heuristic: Use the PRIORITY attribute
• Implementation: Use those nodes with more PRIORITY first

RANK = PRIORITY

Pre-defined Storage Policies

The following list describes the pre-defined storage policies for DEFAULT_DS_SCHED configuration parameter:

Packing Policy

Tries to optimize storage usage by selecting the DS with less free space

• Target: Minimize the number of system datastores in use
• Heuristic: Pack the VMs in the system datastores to reduce VM fragmentation
• Implementation: Use those datastores with less free space first

RANK = "- FREE_MB"

Striping Policy

• Target: Maximize the I/O available to VMs
• Heuristic: Spread the VMs in the system datastores
• Implementation: Use those datastores with more free space first

RANK = "FREE_MB"

Fixed Policy

• Target: Sort the datastores manually
• Heuristic: Use the PRIORITY attribute
• Implementation: Use those datastores with more PRIORITY first

RANK = PRIORITY

Logging

The Rank Scheduler logs are located in /var/log/one/rank_sched.log. This file is truncated on each scheduler invocation.

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