Green software use on HPC: Key Points

Introduction

This course covers green HPC system use, in particular
Carbon Efficiency: Emit the least amount of carbon possible.
Energy Efficiency: Use the least amount of energy possible.
Carbon Awareness: Do more when the electricity is cleaner and do less when the electricity is dirtier.
Hardware Efficiency: Use the least amount of embodied carbon possible.
Measurement: What you cannot measure, you cannot improve.
Reducing Emissions: Understand mechanisms of carbon reduction.

CO₂e is a measurement term used to measure the impact of all greenhouse gases.
Some activities we undertake on HPC systems can potentially have positive carbon impacts but these can be very difficult to quantify.
Everything we do emits carbon into the atmosphere, and our goal is to emit the least amount of carbon possible. This constitutes the first principle of green HPC system use: carbon efficiency, emitting the least amount of carbon possible per unit of work.

In regions where electricity production is dominated by burning fossil fuels, electricity is a good proxy for carbon, so using HPC in an energy efficient way is equivalent to using HPC in a way that is carbon efficient.
In regions of where electricity production is dominated by low carbon energy sources, electricity is not a good proxy for carbon.
Green HPC use takes responsibility for its electricity consumption and considers how this relates to carbon emissions.
Quantifying the energy consumption of your HPC use is a step in the right direction to start thinking about how you can operate more efficiently. However, understanding the energy consumption of your use of HPC is not the only story. The hardware your software is running on uses some of the electricity for operational overhead. This is called power usage efficiency (PUE) for HPC systems (and for computing resources hosted in data centres more generally).
How you go about improving the energy efficiency of use of an HPC system depends on the software you are using and the input parameters as well as the hardware.
Often, the only practical way forward in the face of this complexity is to perform some benchmarking to assess how energy efficiency can be improved.

Carbon awareness means understanding that the energy you consume does not always have the same impact in terms of carbon intensity.
Carbon intensity varies depending on the time and place it is consumed.
The nature of fossil fuels and renewable energy sources means that consuming energy when carbon intensity is low increases the demand for renewable energy sources and increases the percentage of renewable energy in the supply.
Demand shifting means moving your energy consumption to different locations or times of days where the carbon intensity is lower.
Demand shaping means adapting your energy consumption around carbon intensity variability in order to consume more in periods of low intensity and less in periods of high intensity.

Embodied carbon is the amount of carbon pollution emitted during the creation and disposal of an HPC system.
When calculating your total carbon pollution, you must consider both that which is emitted when running the on the HPC system as well as the embodied carbon associated with its creation and disposal.
Extending the lifetime of an HPC system has the effect of amortising the carbon emitted so that its embodied CO₂e/year is reduced.
Increasing utilisation and performance also improve the embodied carbon efficiency from HPC system use.

The GHG protocol is a metric for measuring an organisation’s total carbon emissions and is used by organisations all over the world.
The GHG protocol puts carbon emissions into three scopes. Scope 3, also known as value chain emissions, refers to the emissions from organisations that supply others in a chain. In this way, one organisation’s scope 1 and 2 will sum up into another organization’s scope 3.
You can use the GHG protocol to estimate your emissions from HPC system use but it requires access to reasonable quality information from the HPC systems you are using.
The HPC-CI is a metric designed specifically to calculate emissions from HPC systems and is a rate rather than a total. This can be used to measure improvements in emissions efficiency and drive reductions in emissions.

There are a number of methodologies commonly applied to help in the overall fight against climate change. These fall into the general categories of carbon elimination (also known as “abatement”), carbon avoidance (a.k.a. “compensating”), or carbon removal (a.k.a. “neutralising”).
Abatement is the most effective way to fight climate change although complete carbon elimination is not possible.
Compensating includes the adoption of renewable energy sources, sustainable living practices, recycling, planting trees etc.
Neutralisations refer to the removal and permanent storage of atmospheric carbon to counterbalance the effect of releasing CO₂ into the atmosphere. Neutralisations tend to remove the carbon from the atmosphere in the short and medium-term.
Net zero aims to eliminate emissions and only offset the residual emissions that you cannot eliminate to reach the 1.5°C target set by the Paris Climate Agreement.
Which strategies users or HPC system operators prioritise to reduce emissions depends on if the operational or embodied emissions dominate.
A key part of reducing emissions from HPC use is reducing our consumption of HPC resources.