Hi Stackers,

tl;dr
=====

Move CPU and RAM allocation ratio definition out of the Nova scheduler and into the resource tracker. Remove the calculations for overcommit out of the core_filter and ram_filter scheduler pieces.

Details
=======

Currently, in the Nova code base, the thing that controls whether or not the scheduler places an instance on a compute host that is already "full" (in terms of memory or vCPU usage) is a pair of configuration options* called cpu_allocation_ratio and ram_allocation_ratio.

These configuration options are defined in, respectively, nova/scheduler/filters/core_filter.py and nova/scheduler/filters/ram_filter.py.

Every time an instance is launched, the scheduler loops through a collection of host state structures that contain resource consumption figures for each compute node. For each compute host, the core_filter and ram_filter's host_passes() method is called. In the host_passes() method, the host's reported total amount of CPU or RAM is multiplied by this configuration option, and the product is then subtracted from the reported used amount of CPU or RAM. If the result is greater than or equal to the number of vCPUs needed by the instance being launched, True is returned and the host continues to be considered during scheduling decisions.

I propose we move the definition of the allocation ratios out of the scheduler entirely, as well as the calculation of the total amount of resources each compute node contains. The resource tracker is the most appropriate place to define these configuration options, as the resource tracker is what is responsible for keeping track of total and used resource amounts for all compute nodes.

Benefits:

* Allocation ratios determine the amount of resources that a compute node advertises. The resource tracker is what determines the amount of resources that each compute node has, and how much of a particular type of resource have been used on a compute node. It therefore makes sense to put calculations and definition of allocation ratios where they naturally belong. * The scheduler currently needlessly re-calculates total resource amounts on every call to the scheduler. This isn't necessary. The total resource amounts don't change unless either a configuration option is changed on a compute node (or host aggregate), and this calculation can be done more efficiently once in the resource tracker.
 * Move more logic out of the scheduler
* With the move to an extensible resource tracker, we can more easily evolve to defining all resource-related options in the same place (instead of in different filter files in the scheduler...)

Thoughts?

Best,
-jay

* Host aggregates may also have a separate allocation ratio that overrides any configuration setting that a particular host may have

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