launchpad development
Dispatch time estimation for build farm jobs
Introduction
Due to technical limitations (the art of writing psychic software is not very well established yet 
job dispatch times are estimations only.
For the purpose of this description a 'platform' is considered to be the combination of a
- processor (e.g. i386 or amd64) and a
- virtualization setting (one of: true, false or null)
Build farm jobs can either target a specific platform (e.g. binary builds) or be platform-independent (e.g. "generate a source package from a recipe" builds). The former can only make use of build machines (or "builders" in Soyuz parlance) of the given platform while platform-independent jobs may run on any available builder.
Jobs with an unspecified virtualization setting will be dispatched to virtual builders only.
Builders can -- roughly speaking -- either be idle or building. For any job running on a particular builder its estimated duration and its start time are available allowing us to estimate the job's remaining execution time.
By the way, did I already mention that job dispatch times are an estimation only?
Problem definition
Given:
- a queue of pending (i.e. ready to build) build farm jobs sorted in descending order according to their score
- a pool of builders
Wanted: the estimated dispatch time for a specific job in the pending queue i.e. an estimation for the job of interest (aka JOI).
Solution overview
There are two questions we need to answer in order to come up with a dispatch time estimation:
how long will the jobs ahead of the JOI (in the pending queue) take to run? This is the predecessor lead time (PLT).
how long will it take until the job at the head of the pending queue is dispatched to a builder? This is the time to next builder (TNB).
The dispatch time estimation for the JOI is then calculated as follows: now() + PLT + TNB
