Nova Compute Load Averages

Introduction
The system load averages for a compute could be used to define policies during the scheduler of an instance. Unix based system provides a text file '/proc/loadavg' that allow to get information of the system load averages for the last 1, 5, and 15 minutes.

Overview
Currently the base class `virt.driver.ComputeDriver` defines a method "get_host_uptime". The method asks the driver to call the command 'uptime'. This command provides the system load averages and some other information, the problem is that, this method does not define a correct output format and a reflexion is necessary about the really necessity to keep the API.

libvirt
get_host_uptime: 13:37:30 up 7 days, 4:29, 19 users,  load average: 0.04, 0.04, 0.0

vmware
get_host_uptime: Please refer to {HOST_IP} for the uptime

xenapi
get_host_uptime: 13:37:30 up 7 days, 4:29, 19 users,  load average: 0.04, 0.04, 0.0

Proposal
The proposal is to define a new API 'loadavg' that return a tuple for the last 1, 5 and 15 minutes of the system load averages. The load average is a value between 0 and 1.

{       "hypervisor": { "hypervisor_hostname": "fake-mini", "id": %(hypervisor_id)s, "loadavg": [0.20, 0.12, 0.14] }   }

Why

 * Operator can get information about the load system from the past 1, 5, 15 minutes
 * Monitoring system like (Munin) could use this information to product charts
 * The scheduler could use this information for a scheduling

Implementation

 * A new method will be added to 'nova.virt.driver' called 'get_host_loadavg'
 * A new API will be added to 'nova.api.openstack.hypervisors' called 'loadavg'

Unix based system
For Unix based system the load average could be found from 'proc/loadavg'

Windows based system
TODO(sahid): It looks there are some equivalences.

Usage

 * V2 REST API: not implemented.
 * V3 REST API: /v3/os-hypervisors/{ID}/loadavg
 * If a driver does not implement the load average an exception will be raised 'NotImplmentedError'

The blueprint can be seen at: https://blueprints.launchpad.net/nova/+spec/compute-loadavg Code in review: https://review.openstack.org/#/c/74109/