Sobchak

You mark that frame an eight, you're entering a world of pain. A world of pain.

Sobchak is your friendly neighbourhood OpenStack instance scheduling optimizer, which generates a list of instance migrations to optimally make use of hypervisor resources.

Installation

Installation is as simple as it gets; clone this repository, cd into it, setup a virtual environment to prevent dependency hell on your system and install with pip.

$ git clone http://curlba.sh/jhartog/sobchak.git
$ cd sobchak
$ virtualenv -p python3 venv
$ source venv/bin/activate
$ pip install .

Usage

Make sure you've sourced your OpenStack openrc file, adjust the values set in config.yaml and run sobchak!

$ sobchak

To generate a more human-readable report containing management-pleasing graphs and information about the improvements made, just add the --generate-report or -R argument.

$ sobchak -R

Generating a list of migrations

Forming a strategy

The problem we want to solve arises when compute nodes are running out of assignable memory, but are left with a large number of spare Virtual CPU's or vice versa.

Sobchak tries to solve this problem by leaving compute nodes with available memory and VCPU resources in a certain ratio so that VMs with a common VCPU/RAM ratio can fill the remaining gap.

Fig. 1: A hypervisor after naive instance scheduling.

A great way to look at the problem is by looking at VCPU/memory pairs as two-dimensional vectors; we must try to get the sum of VM vectors to match the hypervisor vector as close as possible to distribute all available resources. This can be achieved by predicting the slope/direction of the end of the VM vector route and steering towards an extension of that slope.

Fig. 2: The yellow line represents the direction of the most common RAM/VCPUs ratio among smaller VMs.

A way to determine if a hypervisor is on its way to use all of its resources, is to compare the slope of the difference vector of the hypervisor and the sum of all its VMs with the slope of the most common ratio within the small VMs. The difference of the angles - let's call it the relative angle - will tell us how well the hypervisor is on its way to use all resources (i.e. its absolute value) and the direction it should change to (i.e. its sign).

Fig. 3: The yellow angle represents the "relative angle".

The bigger the relative angle, the more difficult it is to reach the angle towards an optimal solution. We can use this number to find out which hypervisor pairs can be used to "cancel each other out" by exchanging VMs.

Note that there is one scenario where a high absolute value of a relative angle does not represent badly distributed resources; it's when both the assignable memory and VCPU's are close to zero. This means that a relative angle should be weighed with the sum of two reversed Sigmoid functions on both resources to create a score which is also small when the hypervisor resources are almost fully distributed.

Finding out where improvements can be made

So to summarize: we're able to give hypervisors a score which tells us how well the resources have been distributed.

Besides its score, a hypervisor also has a certain potential (or lack of it) to improve the score of other hypervisors.

If a hypervisor only contains VMs which use memory and processor resources in the same ratio as the most common ratio, then those VMs cannot help another hypervisor to "steer" towards the ideal path to perfect resource distribution. On the other hand, a hypervisor containing VMs with uncommon resource ratio's has the ability to improve another hypervisor's score by exchanging VMs.

Let's set some more definitions: a VM resource ratio which steers left or right compared to the most common ratio gives its hypervisor left-/right-handed divergence.

Fig. 4: The blue dashed line represents right-handed divergence, the green dashed line represents left-handed divergence.

All these properties give us enough information to determine which hypervisors should be paired to improve the overall resource distribution.

Executing the migrations

In theory, there is no difference between theory and practice. But, in practice, there is.

Executing the list of migrations will work if they're performed right now, but what if another instance has been provisioned during our migrations? There's always a chance that a hypervisor which should have enough resources left at some point in the process suddenly doesn't.

If only we could disable all affected hypervisors and overrule this disabled state for our own list of migrations.

TODO: try to migrate to a disabled hypervisor

TBD :o