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Is carbon the right metric for developers to optimize?
Developers need metrics that are consistent and easy to optimize, but carbon intensity varies in time and space. So when is it a useful metric for developers to consider?
Carbon dioxide and its equivalents are the key to national and international net-zero emissions targets. This is the right measure because we know that CO2e is the main driver of climate change. Governments can monitor progress by sector and programs such as transitioning electricity generation to clean energy have a large impact. There is also a direct relationship with the cause of those emissions. For electricity, burning fossil fuels causes CO2e emissions, so stopping that makes sense.
However, computing systems and software do not generate carbon emissions. Generating the electricity they use is where those emissions come from. There is a relationship between using computing systems and energy consumption, but that is not necessarily linear, and might not emit carbon. So it’s more complicated.
Relevant system boundaries
Modern computer systems are rarely isolated. Native, local-only software is almost a thing of the past. Whether it’s email, video streaming, conferencing, or gaming, your device - a laptop or phone - is typically connected via a network to software being delivered through the cloud. This means the system boundary has multiple components.
Carbon emissions vary in time and space. The emissions of the grid powering a data center in the US will be different from the emissions where your laptop is connected to in the UK. Which part of this does the software developer have control over?
Optimizing energy on the client
Answering this question requires considering where your code runs. The web browser on the user device is one place. Whether the code is using the CPU, GPU, or maybe even a specialist processor, energy is required.
Client power consumption metrics are useful for developers because they can be optimized. The same operation running on the same computer is likely to consume the same amount of power each time it is run. As I wrote about last week, Firefox and Safari are now exposing power metrics alongside other performance profiling to help developers optimize their code. Optimizing performance is often a proxy for optimizing power consumption, but not always.
Client carbon metrics are not as useful. If there are thousands of devices all connected to different electricity grids at different times and locations, and so having varying carbon intensity, how can a developer optimize them?
Certain tasks can be scheduled differently, such as how Windows uses local grid carbon intensity to decide when to install updates. Computers often perform background tasks, so this is a good example of intelligent scheduling, but it is limited to tasks which run periodically. The carbon emissions of that same task will vary every time, making it a lot more difficult to optimize, especially over many clients. But, if you need to have a meeting now, it’s not very helpful if Zoom refuses to connect until the carbon intensity falls.
This means that energy is where developers have the most leverage. If you have to use the application right now, reducing its carbon footprint by optimizing its power consumption is the only option.
Optimizing carbon on the server
In contrast to the client, software runs on a server in fewer, known locations. When picking those locations, carbon can be part of the decision. For example, Google Cloud makes it easy to see which region has a lower carbon intensity. Without any code changes, developers can reduce the carbon emissions from the energy used to power the data center where their code runs just by picking one region over another.
At a larger scale, the owners of the data center have more control over the carbon emissions of their facilities. There are many issues with Renewable Energy Credits and Power Purchase Agreements, but they are part of the journey towards 100% renewable energy 24/7. Developers are probably not making these decisions, but they may influence which cloud provider or colo data center their company uses. Even just deciding to use Google over Amazon because of their environmental efforts, or racking your equipment in a data center that sources its primary power from hydro, has a real impact.
And even if they don’t, knowing the location and having flexibility over where (and when) the code runs means that carbon aware scheduling can become useful. This is easier with tasks that can be scheduled rather than those that are real-time and latency sensitive. Introducing carbon scheduling into batch processing is easier to do on the server, but there are still considerable barriers to achieving it.
The more metrics the better
Regardless of which metric is better, it’s great to see systems become more transparent. Several years ago it was difficult to get even basic energy and carbon data out of cloud providers, network operators, browsers, and operating systems. Measuring software power consumption meant buying a power meter and attempting to separate your code from all the other things the OS is doing. Now we have carbon calculators, power reporting, and APIs to return the precise power consumption of a specific task.
All this new data means we can start to be a bit more critical about what metrics are useful for which purpose. The long term goal is to measure and optimize both energy and carbon, but for now my recommendation is energy is more useful on the client whereas carbon is easier to optimize on the server.
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