Policy

MOM is designed to enable dynamic tuning of a KVM host in response to statistics that are continuously collected from the host and its running guests.
This tuning policy is described using a LISP-like mini-language as described here.

Comments

Comments are expressed with the ‘#’ symbol. When ‘#’ is encountered, it and all remaining characters on the line are ignored.

Example:

Code	Result
# This is a full line comment (+ 1 1) # This is a partial-line comment	2

Strings

Strings are created by placing text between double-quotes. Variables can be assigned string values. Sting concatenation and repetition can be performed by using the ‘+’ and ‘*’ operators.

Example:

Code	Result
"foo" "bar" # Operators on strings have the same effect as for Python (+ "Hello " "World!") (+ (* 3 "Hey ") "!") # Multi-line string "multi- line"	foo bar Hello World! Hey Hey Hey ! multi- line

Numbers

Numbers can be expressed as a floating point, integer, integer in scientific notation, hexidecimal, and octal. However, numerical results are always base 10 integers or floats. The basic arithmetic operators +, -, *, /, <<. and >> are supported and have their usual meanings.

Example:

Code	Result
10 00 # Octal .3 # The leading 0 on a float is not required (* 0 1) (+ 1 2) (/ 11 2) # Integer division (/ 11 2.0) # Floating point division (* 3 6) (- 1 9) # Negative result (* (- 8 6) 9) (>> (<< 1 4) 2) (+ 0xFF 0x1) # Hex numbers (* 011 02) (+ 0xa 10) # Numeric type mixing (+ 10.0e3 100e-2) # Scientific notation for integers and floats	10 0 0.3 0 3 5 5.5 18 -8 18 4 256 18 20 10001.0

Logic and Comparison

The policy language supports 3 logic operators: and, or, not. These operators use short-circuit logic behavior equivalent to the Python language. See the example below for more details. In addition to the logic operators, comparisons are supported using: <, >, <=, >=, ==, and !=.

Example:

Code

Result

(and 1 "") # The values 0 and "" evaluate to False # Everything else evaluates to True (and 0 1) # 'and' returns the first False value (and 1 2) # if all values are True, the last value is returned (or "" 17) # 'or' returns the first True value encountered (or "" "") # if all values are False, 'or' returns the last one (not "") (not -0) (< 5 4) (> 1 0) (<= 10 10) (>= 2 (/ 10 2)) (== (+ 1 2) (/ 9 3)) (!= "foo" "foo") (== 0x0 0)

"" 0 2 17 "" True True False True True False True False True

Macros

Macros enhance readability of a policy by grouping related and reusable statements into a single named block. While very similar to functions, macros have at least one key practical difference. They cannot be used for recursion. Attempting to call a macro from within itself will trigger an error. However, nesting of macros is allowed.

Example:

Code	Result
(def foo () 10) (def bar (a) (* 2 a)) (/ (foo) (bar 5)) (def baz (b) (- 2 (bar b))) (baz 12) (def foo (a) { (def bar (b) (+ b 1)) # Nested function (bar a) }) (foo 9)	foo bar 1 baz -22 foo 10

Variables and Scope

Compound Statements

Remember to state that the result of a compound statement is the last value in the block.

The let Statement

The if Statement

Entities

The Input Entities: Host and Guests

Input Entity Methods

Prop, Stat, StatAvg, SetVar, GetVar

Controller Entities

Control()

The with Guests Construct

External Functions

• (abs x): Return the absolute value of ‘x’