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New policy language for MOM

Currently, MOM policies are written in a lisp-like scripting language that can be confusing to use. We want to change the model and the language, to make it easy for users to write their own policies.

Owner

  • Owner: Andrej Krejcir
  • Email: akrejcir@redhat.com

Declarative policy representation

The rule representation uses YAML format. The main idea can be explained on a simple example:

scope: VM

conditions:
  io_or_net_overutilized:
    any:
      - io.read_bytes_per_s  > policy.io_threshold
      - io.write_bytes_per_s > policy.io_threshold
      - net.throughput > policy.net_threshold

rules:
    cpu_limit:
        output: cpu.max_load
        min: 10
        target: 0
        function:
            name: linear
            change: 1
            time: 1 sec

        when: io_or_net_overutilized

    cpu_release:
        output: cpu.max_load
        max: 100
        function:
            name: exponential
            factor: 2
            time: 30 sec

        when: not io_or_net_overutilized

The first line sets the scope of the rules in the file:

scope: VM

Scope can be:

  • Host - Rules modify global properties of a host (like KSM). They are executed once per rule evaluation cycle.
  • VM - Rules modify properties of VMs. They are executed for all VMs every rule evaluation cycle.

Next is optional definitions of conditions. They are for convenience, so the same expression is not repeated in multiple rules.

conditions:
  io_or_net_overutilized:
    any:
      - io.read_bytes_per_s  > policy.io_threshold
      - io.write_bytes_per_s > policy.io_threshold
      - net.throughput > policy.net_threshold

The condition io_or_net_overutilized is defined as the logical or of three conditions. The objects io, net and policy are VM property objects and contain the properties the rules have access to. Other property objects can be cpu, memory or anything else.

The next section defines rules.

rules:
    cpu_limit:
        output: cpu.max_load
        min: 10
        target: 0
        function:
            name: linear
            change: 1
            time: 1 sec

        when: io_or_net_overutilized

This rule is triggered when the condition io_or_net_overutilized is satisfied. The property cpu.max_load of the VM is linearly decreased by 1 percent per second. Until the minimum value 10 is reached.

Meaning of the rule attributes:

  • output - The property that is modified by the rule.
  • min - The minimum allowed value of the output property. It can be any expression, not only a constant.
  • target - The ideal value we are trying to reach (can be any expression).
  • function - Defines how the output is changed in response to computed target.
    • name - Name of the function
    • change, time - Parameters of the function (described below).
  • when - Condition, that has to be satisfied. It can be any boolean expression and use the predefined conditions.

The second rule is:

cpu_release:
    output: cpu.max_load
    max: 100
    target: 100
    function:
        name: exponential
        factor: 2
        time: 30 sec

    when: not io_or_net_overutilized

It is triggered when io_or_net_overutilized is not satisfied. The maximum CPU load of the VM is exponentially increasing. It doubles every 30 seconds, until the maximum 100 is reached.

Details of the representation

Functions

At least three function types are available in the rules:

  • constant - The output is immediately set to the target. This function has no parameters and it is the default used, when no function attribute is present in the rule.

  • linear - The output is changed linearly by a given change over a period of time. Parameters:
    • change - The absolute change
    • time - The interval
  • exponential - The output is changed exponentially by factor over a period of time. Parameters:
    • factor - How big is the relative change
    • time - The interval

Vars block

The block contains values used in the rules. An example from the KSM rule:

vars:
    ksm_pages_boost: 300
    ksm_pages_decay: -50
    ksm_npages_min: 64
    ksm_npages_max: 1250
    ksm_sleep_ms_baseline: 10
    ksm_free_percent: 0.20

An example from the ballooning rule:

vars:
    host_free_percent: host.mem_free_avg / host.mem_available
    vm_used_mem: memory.balloon_cur - memory.unused

Minimum needed change and maximum allowed change

In some cases it may not be practical to modify a value if the change would be too small. The output will be modified once enough time has passed and the change is big enough.

  • min_absolute_change - Will not modify the output if the absolute change is smaller.
  • min_relative_change - Will not modify the output if the relative change is smaller.
rules:
  balloon_shrink:
    output: memory.balloon
    min: policy.balloon_min
    min_relative_change: min_balloon_change

    target: 0

    function:
      type: exponential
      factor: 0.5
      time: 1 min

    when: shrink_balloon

Maximum change can be limited to a specified amount over a period of time.

  • max_absolute_change - Limits the absolute change.
  • max_relative_change - Limits the relative change.
rules:
  balloon_grow:
    output: memory.balloon
    target: policy.balloon_max
    max: policy.balloon_ma
    max_absolute_change:
      value: 100
      time: 1 sec

    function:
      name: exponential
      factor: 2
      time: 1 min

    when: not shrink_balloon

Combining change limits with constant function has the same effect as using linear or exponential function.

Rule influence

When multiple rules modify the same output, the final value is a linear combination of their outputs. Weight of each rule is set by the influence parameter that can be any expression:

rules:
    cpu_rule_1:
        output: cpu.max_load
        target: 100
        influence: 2 # <-------------

        function:
            name: linear
            change: 1
            time: 1 sec


    cpu_rule_2:
        output: cpu.max_load
        target: 0
        influence: 1 # <-------------

        function:
            name: linear
            change: 1
            time: 1 sec

Attribute with a list of values

Some attributes of the rule can take a list of values:

# Logical AND
all:
    - condition1
    - condition2

# Logical OR
any:
    - condition1
    - condition2

# Similar
when_any: ...
when_all: ...

# This is the minimum allowed output,
# so the maximum of value1, value2 is used as minimum
min:
    - value1
    - value2

# Minimum of the 2 values is used as maximum allowed output
max:
    - value1
    - value2

Behavior implementation

The general behavior can be expressed in pseudocode:

if rule.when:
  # sets the limits
  target = min(max(rule.target, rule.min), rule.max)

  # time between rule executions
  elapsed_time = ...

  # limits the slope and approach speed
  immediate_target = function(output, target, elapsed_time, ...)

  # changes the output
  delta = immediate_target - output
  if (abs(delta) > minimum_absolute_change and
      abs(delta) > min_relative_change * output):

    abs_max = rule.max_absolute_change * elapsed_time
    delta = min(delta, abs_max)
    delta = max(delta, -abs_max)

    rel_max = output * rule.max_relative_change^elapsed_time
    delta = min(delta, rel_max)
    delta = max(delta, -rel_max)

    output += delta