What Is An HPC Cluster?
At the most basic level, a compute cluster is a group of uniformly configured compute nodes that work in concert, usually over high bandwidth network interconnects, to solve large computational problems which require more memory or more CPU than is available on a typical workstation or single server. When clustered together, these nodes can be thought of as a single system. This allows researchers to solve complex problems in much shorter amounts of time.
Compute clusters can be comprised of anywhere from several to thousands of compute nodes, a few "login" or "staging" nodes, and one or more head nodes, which coordinate the scheduling, distribution and data sharing of jobs.
What Is A Scheduler?
In order to fairly distribute a cluster's resources amongst a group of users, a software tool known as a scheduler is used. Cluster resources are divided into environments and resources. For example, users may choose between different available parallel environments (known as a PE) or special resources such as a co-processor, GPU, or extra memory. PEs add in extra functionality for parallel processing which manages inter-node process communication. Different types of parallel jobs may require a certain configuration of parallel environment, such as with the use of OpenMPI.
Schedulers generally work with a queueing system (Queues can be organized in various ways, but generally are organized by resource or by user access), which distributes jobs to the cluster as resource become available. Schedulers monitor user job submissions, then check available resources to determine which jobs can be run at a given time.
Other factors are also taken into consideration, depending on how the scheduler may be configured. For example, schedulers can prioritize submissions based on how frequently a particular user may be using the system, and will put a slightly higher priority on jobs from users who do not use the system as frequently. This ensures that the system will not be dominated by only a few very active users.
Helium & Neon Cluster Systems
The University of Iowa has two shared clusters available for campus researchers to use. The systems are run primarily by ITS-Research Services. Our clusters are capable of running both High Performance jobs and High Throughput jobs. Collectively, the two systems comprise over 550 compute nodes with more than 6500 total processor cores. More detailed information on the clusters is available here: /wiki/spaces/hpcdocs/pages/76514722.
What are the differences between High Throughput Computing (Shared Memory) and High Performance Computing (Distributed Memory)?
High Performance computing enables a user to solve a single, large problem by harnessing a large number of processors and memory across multiple compute nodes. These types of problems are typically broken down into pieces and processed in parallel, with different compute nodes working on a different part of the problem. Each node communicates with the other nodes working on the problem via a high-speed interconnect -- in our case, we use Infiniband. Parallel processing typically requires code modification in order to utilize a library such as MPI which in turn facilitates parallel communication between the nodes working to solve the problem. Examples of problems that use High Performance Computing are Computational Fluid Dynamics and Molecular Dynamics. More information on using MPI on our HPC systems is available here: MPI Implementation.
High Throughput computing allows a user to use multiple compute nodes in a coordinated fashion to solve a high number of individual problems. The jobs that make up this sort of computation typically do not communicate with each other. This provides the ability to analyze many data sets simultaneously, and also allows the user to efficiently perform a parameter sweep, which refers to running the same program multiple times, but with varying inputs.
Examples of High Throughput problems are Genomics data processing and Image Analysis.
I just received my HPC account, what are my next steps?
Getting started with Linux
Our shared clusters run CentOS Linux 5 (Helium) and CentOS Linux 6 (Neon). In order to make use of the clusters, users will need a basic understanding of how to interact with a Linux system at the command line. At a minimum, you will need to know how to move around the system, copy and edit files. There are many resources on the Internet devoted to helping you learn your way around a Linux system. One of the best resources available is a book called The Linux Command Line, which is available for a free download here. For a quicker overview of basic Linux commands, there is a good Linux Cheat Sheet here.
Mapping your work to one of the clusters
If your problem is not tractable on a desktop or lab workstation, uses a large amount of memory, requires a rapid turnaround of results, would benefit from being scheduled, then an HPC cluster may be a good fit for you. The next steps are to determine if your job runs on Linux, can be run in batch mode (non-interactively) and whether it is a high performance (parallel) or high throughput (serial) job. Determining the answers to these questions will help decide how to go about requesting and utilizing HPC resources. Some additional questions to consider are:
- Will you need to recompile your code to run on our cluster?
If you are brining code over from another system, you may need to recompile it to work on our systems, especially if you are uisng MPI (of which we offer a few different varieties). We have some additional notes on compiling here: Compiling Software - What software will your job need, and is it available centrally, or could it be installed in your home directory?
Our list of installed software is here: Software List. If you don't see a package you need, please let us know, and if it is broadly applicable to a number of users, we my install it centrally, or we will help install it into your home directory. - Can you estimate how much memory your job will need?
Knowing approximately how many processes you will need or how much memory to request will help ensure you request enough resources to get your job to complete. One way to discover this is to run a small version of the job to see how much memory it uses and then calculate how much it would use if you were to double or triple it in size. We also offer a small sandbox queue on each of our HPC clusters that you may submit small jobs to to see how things go, and then tweak your resource requests accordingly.
Getting your data into the cluster
If your data is not large, the quickest way to get your data onto one of the clusters is to use scp, rsync or sftp from the command line or via an application such as Fetch (Mac based) or IPSwitch (windows based). If your account is on Helium, and you have larger data sets (larger meaning several Gigabytes or more), then you can utilize our Globus Online connection. For transfers to Neon, you must use one of the other methods listed above.
Basic job submission
Software
Once your data is uploaded to the cluster, you are ready to work on getting a job submitted to the cluster. If you are going to use one of our centrally installed software packages, you'll need to load the module for it into your environment. More in-depth information on this is on our Modules page. Basically, you'd use the command
$module avail
To list the modules available to choose from. Then you'd use
$module load <module-name>
To load that module into your environment. Note that some modules are not compatible, and will not load together. For example, openmpi_intel_1.4.1 will only work with intel_11.1.072, so attempting to load a newer intel module will fail. The moduling system is aware of most conflicts, however and will automatically load the correct dependent modules. You may also use the
$module show <module-name>
To see what other modules will be loaded along with it, and also what modifications it will make to your environment.
Sizing Your Job To Fit The Cluster
Another factor to consider is how many resources you need to request from the cluster for your job. In High Performance Computing, resources are parceled out in units called "slots". A slot is a combination of a cpu & ram allocated based on the memory available from the nodes where your job will be running. Each of our clusters has different types of machines inside of it which are defined by the number of cores and the amount of ram that each offers. For example, Neon has 3 different node types: standard (16 cores, 64G), medium memory (16 cores, 256G) and high memory (24 cores, 512G). Slots from each resource will be defined accordingly. A standard node slot will be 1CPU + 4G, while a medium memory node slot would be 1CPU + 16G RAM. More detailed information on slots is available in the /wiki/spaces/hpcdocs/pages/76514711. Once you have an idea of how many processors, or how much memory your computation will need, you can use this information to calculate how many slots you will need to request for your job.
For example, if your computational problem is to process data from thousands of large image files, you'd need to first figure out how much memory is required to process one file, and extrapolate accordingly. If processing each image requires 2G RAM, and a standard node offers 4G per slot, you could request one slot for each image on Neon. On Helium, slots are smaller, so you'd need to request 2 slots per image.
You may find that doing small prototyping jobs are necessary in order to come up with an accurate resource request. For this, both clusters offer a small "sandbox" queue where you may run small versions of your jobs, or run interactively in order to get an idea of how your job will run on the clusters.
Launching Your Job
Our clusters use the SGE scheduler to match job submissions with available resources. There is extensive documentation on using SGE and all the options available. We offer pages on Basic Job Submission and Advanced Job Submission for both clusters. Common ways of launching jobs are via qsub on the command line, or via a job submission script which contains special commands which are then passed to the scheduler for controlling your job. A qsub script can be very simple, as much as a few commands, or very complex, depending on your needs.
You may also forego the use of a script, and simply use the "qsub" command with the desired options on the command line. Note that if you use qsub with a script, then any additional options you pass to qsub on the command line when you launch the script will override those same settings inside the script. For example, if your script specifies the UI queue with #$ -q UI, but you would like to do a submission to the sandbox queue for prototyping, you can override the UI queue on the command line with qsub:
qsub -q sandbox <myscript.sh>
Monitoring Your Job
Once you have launched your job using qsub, you will want to be able to monitor its progress on the cluster. There are a couple of simple ways to do this. First, you need to find out what the "jobid" is of your job. A jobid is a unique number assigned by SGE to each job. To get this information, you can use the qstat command like so:
$ qstat -u naomi
which will produce the following output:
[naomi@neon-login-0-1 espresso]$ qstat -u naomi job-ID prior name user state submit/start at queue slots ja-task-ID ------------------------------------------------------------------------ 8853637 0.00000 QE-CO2-Tes naomi qw 01/05/2015 16:42:09 16
Note the "job-ID" is in the leftmost column. Other columns of note are the "state" column, which tells you what state your job is in. In this case the job is in "qw" or "queue wait" state, which means it is waiting to launch. The "queue" column in this case is blank as the job has not yet launched in any queue. Once the job launches, the queue will be listed. The "slots" column tells how many slots the job is requesting. Use the qstat -j <jobid> command to view additional details about your job (note the below is abbreviated output):
The above
We also offer consulting services, so if you have questions about anything on this page, about HPC in general or would simply like additional assistance getting started, please do not hesitate to contact our staff: hpc-sysadmins@iowa.uiowa.edu, and one of us will be happy to help you.