Solver Scheduler is an Openstack Nova Scheduler driver that provides a smarter, complex constraints optimization based resource scheduling in Nova. It is a pluggable scheduler driver, that can leverage existing complex constraint solvers available in open source such as PULP, CVXOPT, Google OR-TOOLS, etc. It can be easily extended to add complex constraint models for various use cases, written using any of the available open source constraint solving frameworks.

The project code has to be patched onto an existing installation of Openstack Nova, as it is a pluggable Nova scheduler driver.

• The new scheduler driver module:

  nova/scheduler/solver_scheduler.py

• A patched version of host_manager module from the master Nova project, with a new method:

  nova/scheduler/host_manager.py

• The code includes a reference implementation of a solver that models the scheduling problem as a Linear Programming model, written using the PULP LP modeling language. It uses a PULP_CBC_CMD, which is a packaged constraint solver, included in the coinor-pulp python package.

  nova/scheduler/solvers/hosts_pulp_solver.py

• There are two examples of pluggable solvers using coinor-pulp or or-tools package, where costs functions and linear constraints can be plugged into the solver.

  nova/scheduler/solvers/hosts_pulp_solver_v2.py
  nova/scheduler/solvers/hosts_ortools_linear_solver.py

• The cost functions pluggable to solver:

  nova/scheduler/solvers/costs/ram_cost.py
  nova/scheduler/solvers/costs/ip_distance_cost.py

• The linear constraints that are pluggable to solver:

  nova/scheduler/solvers/linearconstraints/active_host_constraint.py
  nova/scheduler/solvers/linearconstraints/affinity_constraint.py
  nova/scheduler/solvers/linearconstraints/num_hosts_per_instance_constraint.py
  nova/scheduler/solvers/linearconstraints/max_instances_per_host_constraint.py
  nova/scheduler/solvers/linearconstraints/resource_allocation_constraint.py

• coinor.pulp>=1.0.4
• or-tools>=1.0.2902 (Alternative. There is a known issue with or-tools package. See below.)

• In some cases, the installation of or-tools package may cause unexpected crash of multiple OpenStack services due to a dependency problem. There has not been report of this issue for usage in Devstack environment.

The Solver Scheduler Manger will allow you to manage the solver scheduler in your openstack installation.

• Note:
  • This is an alpha version, which was tested on Ubuntu 12.04 and OpenStack Havana only.
  • It is recommended that a backup of the following files be kept before using this manager:
    /etc/nova/nova.conf
    nova/scheduler/host_manager.py

To install the manager, run:
curl https://raw.github.com/CiscoSystems/nova-solver-scheduler/master/install_manager | sudo bash

To install solver scheduler with this manager, use the following command as root:

solver-scheduler install

To manage the solver scheduler, use one of the following commands as root:

solver-scheduler activate
solver-scheduler deactivate
solver-scheduler remove
solver-scheduler update
solver-scheduler help

• This is a configuration sample for the solver-scheduler. Please add these options to nova.conf.
• Note:
  • Instead of being added, the following existing options should be updated with new values: scheduler_driver
  • The module 'nova.scheduler.solvers.hosts_pulp_solver' is self-inclusive and non-pluggable for costs and constraints. Therefore, if the option 'scheduler_host_solver' is set to use this module, there is no need for additional costs/constraints configurations.
  • Please refer to the 'Configuration Details' section below for proper configuration of costs and constraints.

#
# Solver Scheduler Options
#

# Default driver to use for the scheduler
scheduler_driver = nova.scheduler.solver_scheduler.ConstraintSolverScheduler

# Default solver to use for the solver scheduler
scheduler_host_solver = nova.scheduler.solvers.hosts_pulp_solver_v2.HostsPulpSolver

# Cost functions to use in the linear solver
scheduler_solver_costs = RamCost, IpDistanceCost

# Weight of each cost (every cost function used should be given a weight.)
scheduler_solver_cost_weights = RamCost:0.25, IpDistanceCost:0.75

# Constraints used in the solver
scheduler_solver_constraints = ActiveHostConstraint, NumHostsPerInstanceConstraint, MaxDiskAllocationPerHostConstraint, MaxRamAllocationPerHostConstraint

# Way of ram usage
# set negative for balancing
# set positive for stacking
ram_cost_optimization_multiplier = -1

# Virtual-to-physical disk allocation ratio
linearconstraint_disk_allocation_ratio = 1.0

# Virtual-to-physical ram allocation ratio
linearconstraint_ram_allocation_ratio = 1.0

• Available costs

  • RamCost
    Help to balance (or stack) ram usage of hosts.
    The following option should be set in configuration when using this cost:
    ram_cost_optimization_multiplier = <a real number>
    Set the multiplier to negative number for balanced ram usage,
    set the multiplier to positive number for stacked ram usage.

  • IpDistanceCost
    Help to place instances close to a set of volumes.
    The distance between instances and volumes are evaluated by using ip address.
    The following scheuler hint is expected when using this cost:
    ip_distance_cost_volume_id_list = <a list of volume ids>

• Available linear constraints

  • ActiveHostConstraint
    Only enabled and operational hosts are allowed in solution.
    Normally this constraint should always be enabled.

  • AffinityConstraint
    Force instances to be placed at either different or same hosts as a given set of instances.
    The following scheduler hint is expected when using this constraint:
    different_host = <a list of instance uuids> or same_host= <a list of instance uuids>

  • NumHostsPerInstanceConstraint
    The purpose of this constraint is to avoid trivial solution (i.e. instances placed nowhere).
    Normally this constraint should always be enabled.

  • MaxInstancesPerHostConstraint
    Specify the maximum number of instances placed in each host in each scheduling process.
    The following scheduler hint is expected when using this constraint:
    max_instances_per_host = <a positive integer>
    By default, max_instances_per_host = 1, resulting in an anti-affinity placement solution.

  • MaxDiskAllocationPerHostConstraint
    Cap the virtual disk allocation of hosts.
    The following option should be set in configuration when using this constraint:
    linearconstraint_disk_allocation_ratio = <a positive real number> (virtual-to-physical disk allocation ratio, if >1.0 then over-allocation is allowed.)

  • MaxRamAllocationPerHostConstraint
    Cap the virtual ram allocation of hosts.
    The following option should be set in configuration when using this constraint:
    linearconstraint_ram_allocation_ratio = <a positive real number> (virtual-to-physical ram allocation ratio, if >1.0 then over-allocation is allowed.)

  • MaxVcpuAllocationPerHostConstraint
    Cap the vcpu allocation of hosts.

This is an example usage for creating VMs with volume affinity using the solver scheduler.

• Install the solver scheduler.

• Update the nova.conf with following options:

# Default driver to use for the scheduler
scheduler_driver = nova.scheduler.solver_scheduler.ConstraintSolverScheduler

# Default solver to use for the solver scheduler
scheduler_host_solver = nova.scheduler.solvers.hosts_pulp_solver_v2.HostsPulpSolver

# Cost functions to use in the linear solver
scheduler_solver_costs = IpDistanceCost

# Weight of each cost (every cost function used should be given a weight.)
scheduler_solver_cost_weights = IpDistanceCost:1.0

# Constraints used in the solver
scheduler_solver_constraints = ActiveHostConstraint, NumHostsPerInstanceConstraint, MaxDiskAllocationPerHostConstraint, MaxRamAllocationPerHostConstraint

# Virtual-to-physical disk allocation ratio
linearconstraint_disk_allocation_ratio = 1.5

# Virtual-to-physical ram allocation ratio
linearconstraint_ram_allocation_ratio = 1.5

• Restart nova-scheduler and then do the followings as admin:

• Create multiple volumes at different hosts

• Run the following command to boot a new instance. (The id of a volume you want to use should be provided as scheduler hint.)

nova boot --image=<image-id> --flavor=<flavor-id> --hint ip_distance_cost_volume_id_list=<volume-id> <server-name>

• The instance should be created at the same host as the chosen volume as long as the host is active and has enough resources.