David Mytton

Carbon aware workloads – current status, limitations, and opportunities

Published in Cloud, Environment, IT Energy, Product Engineering.

Carbon aware workloads – current status, limitations, and opportunities

Moving compute workloads to regions with lower-carbon electricity is one way to reduce the impact of their associated data center energy consumption. The problem is that carbon intensity changes throughout the day as the grid fuel mix changes.

As a result, I’ve seen lots of papers proposing algorithms for carbon aware scheduling. They typically involve analyzing grid emissions data for each data center region and then migrate workloads to regions with lower carbon intensity. This can be achieved using emissions data APIs from organizations like WattTime, particularly for forecasting near-future emissions so that migrations can occur in-advance of immediate need.

A number of problems stand out from my initial research:

  1. Data. Reliable and consistent data is crucial for these systems to work. However, there are a large number of grid operators, even within countries, which means a large number of data sources. These don’t always match with the region a data center is in, may not provide forecasted carbon emissions, and may be private. This is why organizations like WattTime and ElectricityMap have developed commercial API products.
  2. Workloads are different. Data center level scheduling and migration is not realistic for most environments. Google has blogged about how it migrates some workloads, such as YouTube video encoding, based on carbon intensity, but few organizations have complete control over the entire technology stack from software through to data center energy contracts. Most of the papers I’ve read focus too much on whole data centers rather than more granular workloads. They assume IT users can just move everything from one data center to another, which isn’t really feasible.
  3. Multiple IT considerations. Carbon intensity is just one of many factors infrastructure teams need to think about. Latency, availability of cloud services (cloud regions are not equal, even from the same vendor), and data protection are just a few reasons why you might pick one cloud region over another.

These reasons may be why we’ve seen limited uptake of carbon aware scheduling. They are often too high-level or think that reducing carbon intensity is the only priority. SDKs need to be written to make it easy for developers and infrastructure operators to implement at a granular level where it makes sense in their applications.

An example of one such approach is a low carbon Kubernetes scheduler described in a 2019 paper by James & Schien. This describes an approach where the Kubernetes scheduler is adapted to prioritize new node placement based on carbon intensity data from solar energy (and could be extended to cover other renewable energy sources). Unfortunately, I could not find the actual code, and it still assumes that the cluster can be re-deployed in a new, low-carbon zone and the old cluster is deleted. This would be fine for stateless workloads, but isn’t suitable for those using persistent data where large volumes need to be migrated (or replicated).

I’m spending more time looking into what more can be done here. For example:

  • Can a carbon aware load balancer route requests to lower-carbon backends? Would this incur bandwidth overhead that mitigates the benefits?
  • Can low-carbon be indicated as a preference in cloud environments? The platform operator is going to have the best data from energy contracts and location-specific grid mix data. For example, the Green Compute option for Cloudflare Workers.
  • Can job queues consider carbon intensity when they route tasks to workers? This depends on whether the job needs to be executed immediately or can handle some delay.
  • What can user devices do to help the user reduce energy consumption at times of higher carbon intensity. For example, the new update scheduling functionality in Windows.

Software developers play an important role in improving the energy impact of the products they work on because they have more impact than behavior change from individuals. Microsoft changing when Windows runs its update processes will impact billions of users. Encoding video at less carbon-intensive times of day will impact the many thousands of hours of video. Making it easier for developers to build this functionality seems like an interesting place to focus.