Updates hardware efficiency episode

episodes/hardware-efficiency.md

@@ -6,19 +6,23 @@ exercises: 5
 
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-- TBC
+- Understand what is meant by "embodied carbon" in the contect of HPC systems
+- Learn how embodied carbon efficiency can be improved by extending the lifespan of HPC systems and improving performance of applications on HPC systems
+- Appreciate that the balance between embodied and operational carbon emissions on a particular HPC system influences how we go about maximising carbon efficiency
 
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 :::::::::::::::::::::::::::::::::::::::: questions
 
-- TBC
+- What is embodied carbon on HPC systems?
+- How can embodied carbon efficiency be improved on HPC systems?
+- What can I do to improve the embodied carbon efficiency of my use of HPC systems?
 
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 ## Introduction
 
-The hardware used in the process of creating your software is an important element to consider for a Green Software practitioner.
+The hardware used that makes up the HPC systems you are using is an important element to consider when looking to be greener users of HPC.
 
 You will see how embodied carbon is a hidden cost when it comes to hardware and the different measures you can take to reduce the impact that the creation, destruction and running of this hardware involves. For example, extending its lifetime or switching to cloud servers.
 
@@ -28,56 +32,64 @@ You will see how embodied carbon is a hidden cost when it comes to hardware and
 
 The device you are using to read this on produced carbon when it was manufactured and, once it reaches the end of life, disposing of it may release more. Embodied carbon (also referred to as "embedded carbon") is the amount of carbon pollution emitted during the creation and disposal of a device.
 
-When calculating the total carbon pollution for computers running software, both the carbon pollution associated with running the computer as well as the embodied carbon of the computer must be accounted for.
+When calculating the total carbon pollution for HPC services, both the carbon pollution associated with running the system as well as the embodied carbon of the system must be accounted for.
 
-![alt_text](./fig/17_embodioed_carbon.png "image_tooltip")
+![Bar chart illustrating embodied carbon from various end-user devices](./fig/17_embodioed_carbon.png "Bar chart illustrating embodied carbon from various end-user devices")
 
 Embodied carbon varies significantly between end-user devices. For some devices, the carbon emitted during manufacturing is much higher than that emitted during usage, as illustrated by a [study](https://www.ifi.uzh.ch/dam/jcr:fa4e956e-7a53-4038-98a5-00e09e2f4303/Study_Digitalization_Climate_Protection_Summary_Oct2017.pdf) from University of Zurich. As a result, the embodied carbon cost can sometimes be much higher than the carbon cost of the electricity powering it.
 
 By thinking in terms of embodied carbon, any device, even one not consuming electricity, is responsible for the release of carbon over its lifetime.
 
-### Amortization
+### Amortisation
 
-A way to account for embodied carbon is to amortize the carbon over the expected life span of a device. For example, suppose it took 4000kg CO2eq to build a server, and we expect it to last four years. Amortization means that we can say the server emits 1000kg CO2eq/year.
+A way to account for embodied carbon is to amortise the carbon over the expected life span of a device. For example, suppose it took 4000&nbsp;kgCO<sub>2</sub>e to build an HPC system, and we expect it to last four years. Amortisation means that we can say the HPC system emits 1000&nbsp;kgCO2<sub>e</sub>/year.
 
-![alt_text](./fig/18_amortization.png "image_tooltip")
+![Diagram illustrating amortisation](./fig/18_amortization.png "Diagram illustrating amortisation")
 
 ## How to improve hardware efficiency
 
-If we take into account the embodied carbon, it is clear that by the time we come to buy a computer, it's already emitted a good deal of carbon. Computers also have a limited lifespan, which means they eventually are unable to handle modern workloads and need to be replaced. In these terms, hardware is a proxy for carbon, and since our goal is to be carbon efficient, we must also be hardware efficient.
+If we take into account the embodied carbon, it is clear that by the time we come to install an HPC system, it's already emitted a good deal of carbon. Computers also have a limited lifespan, which means they eventually are unable to handle modern workloads and need to be replaced. In these terms, hardware is a proxy for carbon, and since our goal is to be carbon efficient, we must also be hardware efficient.
 
 There are two main approaches to hardware efficiency:
 
-- For end-user devices, it's **extending the lifespan** of the hardware.
-- For cloud computing, it's **increasing the utilization** of the device.
+- **Extending the lifespan** of the hardware - which reduces the carbon emisison rate per unit of time due to amortisation
+- **Increasing the utilisation and performance** of the hardware - getting more useful work out of the hardware per unit of time
 
 ### Extending the lifespan of hardware
 
-In the example we saw previously, if we can add just one more year to the lifespan of our server, then the amortized carbon drops from 1000kg CO2eq/year to 800kg CO2eq/year.
+In the example we saw previously, if we can add just one more year to the lifespan of our HPC system, then the amortised carbon emissions rate drops from 1000&nbsp;kgCO<sub>2</sub>e/year to 800&nbsp;kgCO<sub>2</sub>e/year.
 
-![alt_text](./fig/19_lifespan.png "image_tooltip")
+![Diagram illustrating amortisation](./fig/19_lifespan.png "Diagram illustrating amortisation")
 
-Hardware is retired when it breaks down or struggles to handle modern workloads. Of course, hardware will always break down eventually but, as developers, we can use software to build applications that run on older hardware and extend their lifetime.
+HPC systems have historically had lifetimes of around 5-7 years at which point they are replaced by newer systems that provide improved performance and functionality and that are typically more energy efficient. However, extending the lifetime of HPC systems for longer periods may lead to improved carbon efficiency and using older HPC systems for our research may lead to more carbon efficient use of HPC.
 
-### Increasing device utilization
+:::::::::::::::::::::::::::::::::::::::: callout
 
-In the cloud space, hardware efficiency most often translates to an increase in the utilization of servers. It’s better to use one server at 100% utilization than 5 servers at 20% utilization because of the cost of embodied carbon. In the same way that owning one car and using it every day of the week is much better than owning five and using a different one each day of the week, it is much more efficient to use servers at their full capacity rather than employing several at below capacity. Although emissions are the same, the embodied carbon that is used is much lower.
+## Carbon efficiency can be complex - flexibility is key
 
-![alt_text](./fig/20_increasing_utilization.png "image_tooltip")
+Understanding what is the most carbon efficient way to make use of HPC systems (and the most carbon efficient way to operate then, including choosing their lifetime) can be complex as it depends on many factors: the embodied carbon of the hardware, the lifetime, the carbon intensity of the electricity supply and how that changes over the service lifetime, how efficiently the workload you are running runs on the type of hardware provided by the system. However, one key aspect of being able to use HPC in a carbon efficient way is for your workflow to have the flexibility to run on different system types in an efficient manner.
 
-The most common reason for having under-utilized servers is so that peak capacity is accounted for. Running servers at 20% means that you know you will be able to handle peaks in demand without impacting performance. However, in the meantime, all that spare capacity just sitting there idle represents wasted embodied carbon. Being hardware efficient means making sure that every hardware device is being utilized as much as possible for as long as possible.
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+### Increasing utilisation and performance
+
+As well as increasing the lifespan to improve the embodied carbon efficiency, we can also get more out of the HPC hardware while we have it.
 
-This is one of the main advantages of the public cloud; you know that when you do need to scale up, the space will be there to take up the slack. With multiple organizations making use of the public cloud, spare capacity can always be made available to whoever needs it, so that no servers are sitting idle.
+At a service/system level this often corresponds to maximising the usage of the service - it’s better to have 100% utilisation than 20% utilisation because of the cost of embodied carbon (and also because of the fact that even idle components in HPC systems consume some electricity).
 
-It’s important to note that simply moving operations to the public cloud will not automatically reduce your emissions. It simply gives you the space to be able to re-architect your software so that a reduction is possible.
+For individual users and groups on HPC systems, improving the carbon efficiency with respect to embodied carbon typically corresponds to increasing the performance of their use of the system so you get more output per unit of time. Of course, increasing the performance of your use may lead to higher power draw and higher electricity use increasing the emissions from the use of electricity. This means that you need to know what the balance between embodied emissions and emissions from electricity use are for the HPC system you are using to make useful choices about improving your carbon efficiency. There are three potential scenarios:
 
-![alt_text](./fig/21_private_vs_public_cloud.png "image_tooltip")
+- **Embodied carbon dominates:** run as high performance as possible irrespective of electricity use to maximise carbon efficiency
+- **Embodied carbon and carbon from electricity use are evenly balanced:** you need to find a balance of performance and energy efficiency to maximise carbon efficiency
+- **Carbon from electricity use dominates:** run in as energy efficient manner as possible to maximise carbon efficiency
 
 :::::::::::::::::::::::::::::::::::::::: keypoints
 
-- Embodied carbon is the amount of carbon pollution emitted during the creation and disposal of a device.
-- When calculating your total carbon pollution, you must consider both that which is emitted when running the computer as well as the embodied carbon associated with its creation and disposal.
-- Extending the lifetime of a device has the effect of amortizing the carbon emitted so that its CO2eq/year is reduced.
+- 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 CO<sub>2</sub>e/year is reduced.
+- Increasing utilisation and performance also improve the embodied carbon efficiency from HPC system use.
+- You need to know what the balance between embodied carbon and operational carbon is on the HPC systems you are using to be able to decide how to maximise your carbon efficiency.
 
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