# An Intro To Hoffman2 With R

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## Overview

This guide will provide you with the tools to kickstart your High Performance Computing (HPC) journey. The purpose is not to make you an HPC expert, but to provide an inexperienced reader with enough information to start using the Hoffman2 computing cluster at UCLA, and to point you to helpful resources to build upon the knowledge provided here.

While Hoffman2 can be used to run a variety of analyses using different tools and computing languages, this guide is written specifically to help users who want to run R scripts. This is meant to be a “living document”, so if you would like to see a tutorial for something that isn’t already here, please shoot me an email at btonelli “at” ucla “dot” edu.

Throughout the document, there are a number of links to helpful resources that are also linked here:

## Brief Overview of High Performance Computing

### What is a Computing Cluster?

In simple terms, a computing cluster is an amalgamation of many separate computers that can be used to help accelerate certain tasks. While each individual computer that is a part of this cluster is necessarily much (if at all) faster than a desktop computer, the power comes in leveraging many of these computers to run a single analysis. The user (you) can connect to UCLA’s version of this resource, the Hoffman2 cluster, remotely using a secure shell (or SSH) through the command line. If you don’t know what a secure shell is, don’t worry, that will be covered! After connecting, you can then interact with the cluster to run your scripts.

### What ways can the cluster help me?

The cluster has two major advantages over running something on your local laptop or desktop: 1) Run scripts in a way that reduces the overall runtime. Analysis can be run in parallel using the many cores available to the user, or take advantages of gobs of memory (RAM) that are available on the cluster. 2) Free up your computer for other tasks. Scripts that have excessively long run times can be off-loaded to the cluster, even if they don’t necessary run any faster on the cluster. These types of tasks (like running a Bayesian model with lots of data) may not be able to be easily sped up, but will allow you to use your own personal computer while the cluster does all the work.

### Step 1: Get an account

Hoffman2 is open open to all UCLA-affiliated staff and students, but you do need to apply for a user account. To get a user account, you also need to be sponsored by your PI.

To apply for an account just click this link and follow the instructions: https://www.hoffman2.idre.ucla.edu/Accounts/Requesting-an-account.html

Once you have an approved Hoffman account, you will get a username and password.

### Step 2: Connect using SSH

SSH is a way for your computer to connect with resources remotely from the command line. This step may seem daunting if you haven’t used the command line before, but don’t be discouraged - it is pretty easy once you get the hang of it! How you implement SSH will depend on your operating system (e.g., Windows, MacOS), though the rest of the steps should be the same.

#### Step 2a:

For Windows: Download PuTTY For Mac: Open Terminal (can be found in the Applications/ folder)

#### Step 2b:

At the command line prompt, type in “ssh”, space, your Hoffman2 username, then the “address” of the cluster. It’ll look something like: ssh USERNAME@hoffman2.idre.ucla.edu

Press Enter, and then type in you password when prompted.

Once you’re in, you’ll get a message welcoming you to the Hoffman2 Cluster. You did it!

#### Step 3: Accessing your files

When you register for a Hoffman2 account, you will have a designated space on the cluster. You can think of this like hard drive space. This space will be your “home directory”. It is a folder with all of your personal thigns (scripts, data, results, etc.). If you are comfortable navigating through directors (i.e. folders) and files from the command line, you can do that. A basic tutorial of the command line can be found here.

If you are a normal person, using the command line to navigate this way will be a daunting prospect. That’s where Cyberduck comes in. Cyberduck is a third-party platform that provides a Graphical User Interface (GUI) that connects with your space on Hoffman2. Once Cyberduck (or another platform like it) is set up, you can drag and drop your files directly into, or out of, Hoffman2. Handy!

Open Cyberduck on your computer, click “Open Connection”, choose the SFTP protocol, and enter:

Once you are in, you should see your home directory. You can then start dragging and dropping like crazy.

#### Step 4: Using Hoffman2 to run scripts

Running scripts on Hoffman2 can be done in a variety of ways. Which approach you decide to use will depend on what type of task you are trying to perform (running thousands of simulations, Bayesian models, or processing a batch of images), and what works best for you!

Here, the examples are all R-based (because that’s what’s most applicable to the Tingley Lab), but there are certainly other ways to run jobs in all sorts of languages.

The Hoffman2 documentation page has a wide range of examples, but some of the basic ways are explored below:

##### Method 1) submit.sh -> example.R

One of the easier ways to run a script is by submitting a “batch job”. To do this, we will create and run a submission script. In this way, we create a .sh script that tells the cluster to execute our .R script (the code we actually wan to run). Using this method, we need to be sure to write all output from our .R script to a file (like a .csv or .rds) to make sure we save our results.

An example of a submission script is provided here. This submission script basically acts as the “Run all” in Rstudio, executing all the code in a .R script. But the submission file also includes important information about the computational resources you are requesting to use on the cluster.

Regardless of the method for running jobs on the cluster, you will need to request resources, which brings us to…

#### Computational Resources Interlude

Because you aren’t using your personal computer to run your script, you need to tell the cluster how much memory, how many cores, and how much time it should allocate to your task. Making these requests can be daunting for a number of reasons. First, it’s often hard to know how long or how much memory your job will run for! So how do you pick numbers???

Importantly, if your task exceeds the amount of resources you requested, it will be terminated by the cluster. You don’t want this to happen, especially when your script needs to run for an extended period of time. Once you allocate resources, it CANNOT be changed, which means that if you allocated 9 days for your task that will take 10, you will be very sad when your task terminates when it is 90% finished.

Same thing goes for memory. If your script tries to load a massive 10GB file into memory, and you only allocated 2GB of memory, Hoffman2 will boot you off. Sad =(.

Conversely, if you request TOO many resources, your script will (practically) never run. The cluster uses a scheduling algorithm to decide which order your tasks execute in. The more resources you request, the farther down your task is placed the to-do list of the cluster.

So you want to optimize by requests just enough resources to get your task to run without failing, and to get it running as quickly as possible. How you do that?

Testing your code first with a smaller dataset, running only a few simulations, or otherwise using less resources than you expect to use in your final task can give you an indication of what resources you should request. Yes, this is an extra step, but the last thing you want is to wait 3 days for your task to start running, only to have it fail within 10 seconds because you didn’t request enough memory!

##### Method 2) Run R directly using an interactive session

This option can be helpful to run scripts that may not take a very long time, scripts where you need to interact with the results (exploratory analyses), or do testing for a bigger job.

Interactive sessions allow you to “interact” with R just like you would if you were on your laptop. To open up an interactive session, you need to:

1. Connect to the cluster using SSH

2. Request resources using the line:

qrsh -l h_rt=3:00:00,h_data=4G

(where 4G is the total memory allocated to the session, and 3:00:00 is 3 hours)