Most of them are outdated, but provide historical design context.
They are not user documentation and should not be treated as such.
Documentation is available here.
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: Andrej Krejcir
- Email: email@example.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 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
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
io_or_net_overutilized is defined as the logical
or of three conditions.
policy are VM property objects and
contain the properties the rules have access to.
Other property objects can be
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.
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
outputis changed in response to computed target.
name- Name of the function
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
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
functionattribute is present in the rule.
linear- The output is changed linearly by a given
changeover a period of
change- The absolute change
time- The interval
exponential- The output is changed exponentially by
factorover a period of
factor- How big is the relative change
time- The interval
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
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
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