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---
title: "Session 1: What is HPC?"
subtitle: "Introduction to High Performance Computing Concepts"
format: html
---
# Session Content

## Session aims

By the end of this session, you will be able to:

- Understand key HPC terminology and concepts
- Explain the difference between serial and parallel programs
- Recognize when HPC might benefit your research
- Describe the basic architecture of HPC cluster systems

Please scroll to the bottom of the page to see the exercises for this session.

## Some commonly used terms

Let's define some frequently used jargon. Don't worry, we will delve into each of these in more depth later.

[**View Interactive Slides: HPC Terminology**](what-is-hpc-terminology-slides.qmd){.btn .btn-primary target="_blank"}

::: {.callout-note}
## Key takeaway

HPC cluster systems are formed of a large number of separate computers called nodes.

Each node is about equivalent to a high-end workstation or server. They are linked together by a high-speed network which allows very rapid communications between the nodes.

**Read More**: [Getting started on HPC](https://arcdocs.leeds.ac.uk/aire/getting_started/start.html)
:::

---

## Serial and parallel programs

- Serial programs run on a single CPU core, solving one problem at a time.
- Parallel programs run across multiple CPU cores, splitting the workload between them and solving the problem faster.
- In order to use parallelism on an HPC system, you must organise your code/program to use multiple cores.

[**View Interactive Slides: Serial vs Parallel Programs**](what-is-hpc-parallel-slides.qmd){.btn .btn-primary target="_blank"}

---

## Why use HPC?

HPC systems enable researchers to:

- **Scale up computations**: Run calculations that would be impossible on a single computer
- **Speed up results**: Reduce time from months/years to days/weeks
- **Handle big data**: Process datasets too large for regular computers
- **Enable new discoveries**: Solve previously intractable problems

However, remember:

- **It's not magic** - there's no fairy dust that can just make code run on hundreds of processors
- **Effort required** - A lot of work has to be put into using these systems effectively
- **Potential transformation** - It could transform your workflow if you put the effort in

[**View Interactive Slides: Parallelisation and Amdahl's Law**](what-is-hpc-amdahl-slides.qmd){.btn .btn-primary target="_blank"}

---

# Exercises

Test your understanding of the key concepts from this session:

::: {.callout-tip collapse="true"}
## Q1: What is the main difference between a cluster and a single computer?

**Answer:** A cluster is made up of many separate computers (nodes) connected by a high-speed network, allowing them to work together on problems.

A single computer has limited processing power and memory, while a cluster can scale up to handle much larger computational tasks.
:::

::: {.callout-tip collapse="true"}
## Q2: Why can't you just add more cores to make any program run faster?

**Answer:** Not all programs can be parallelised effectively!

Some parts of a program must run serially or sequentially (one after another), and there's overhead in coordinating between cores.

According to Amdahl's Law, the maximum speedup is limited by the portion of the program that cannot be parallelised.
:::

::: {.callout-tip collapse="true"}
## Q3: What's the difference between task parallelism and data parallelism?

**Answer:**

- **Task parallelism:** Different processors work on different types of tasks or calculations
- **Data parallelism:** Multiple processors perform the same operation on different subsets of data

:::

::: {.callout-tip collapse="true"}
## Q4: A program is 80% parallelizable. What's the maximum theoretical speedup you could achieve?

**Answer:** According to Amdahl's Law, if 80% of a program can be parallelized, the maximum speedup is 1/(1-0.8) = 5 times faster, regardless of how many cores you use.
:::

::: {.callout-tip collapse="true"}
## Q5: When might HPC not be the right solution for your research problem?

**Answer:** HPC might not be suitable when:

- Your program cannot be parallelized effectively
- The problem is small enough to run quickly on a regular computer
- Your workflow requires frequent interaction or visualization

:::

---

# Summary

::: {.callout-note}
## Key Takeaways

- **HPC enables research** that would be impossible on regular computers
- **Parallelization is key** - breaking problems into smaller parts that run simultaneously
- **Amdahl's Law** sets limits on speedup based on the serial portion of your code
- **Different types of parallelism** (task vs. data) suit different problems
- **HPC isn't always the answer** - consider if your problem truly needs it
- **Aire provides powerful resources** for University of Leeds researchers
:::

---

## Next Steps

Ready to start using HPC? Let's move on to [Session 2: Logging on and Linux Recap](logging-on-and-linux-recap.qmd) to learn how to access and navigate the Aire system.

## Additional Resources

- [Aire Documentation](https://arcdocs.leeds.ac.uk/aire/welcome.html)
- [Research Computing Training](https://arc.leeds.ac.uk/courses/)
- [Contact Research Computing](mailto:ask-rc@leeds.ac.uk)
- [Parallel Computing Concepts](https://en.wikipedia.org/wiki/Parallel_computing)
- [Amdahl's Law Explained](https://en.wikipedia.org/wiki/Amdahl%27s_law)