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Glossary of High Performance Computing Terminology

High Performance and related computing paradigms has its own set of terminology which might be confusing to those new to these fields. This page tries to define the terminology as used throughout this web site in simple language.

Terms are listed in alphabetical order, with the following links
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

(hardware) accelerator
While traditionally in computing the computation takes place on a general purpose CPU, many workloads can obtain great improvements in performance when run on specialized processors like GPUs, field-programmable gate arrays and other technologies. The general term for these specialized processors is accelerators or hardware accelerators.

These accelerators typically must be placed in a node containing a generalized CPU, and programs must be specifically coded to take advantage of these technologies. These accelerators can often be expensive, with nodes containing the accelerators generally costing several times the cost of a comparable node without the accelerators, but the performance gains for suitable workloads can be very significant. Because of this, typically only a small subset of the nodes have accelerators, and you will need to instruct the scheduler to use those nodes.

Account
The term "account" can have two disparate usages. The first usage, sometimes distinguished with the terms "user" or "login", refers to the user credentials that you use to log into the cluster, and other attributes, etc. connected with your user identity on the cluster. This sense of the word "account" is similar to the login/user accounts on other computer systems.

The second usage refers to allocation accounts, sometimes simply referred to as allocaitons.

Allocation
Allocations are the means by which HPC resources are made available to users. Every allocation consists of a set of resources to which HPC members have been granted access; this consists not only of the type of resource, but also the amount of that resource that can be used. E.g., allocations will typically include access to compute resources; this will include the specification of the cluster access is granted to, as well as the number of SUs over a time period (and what time period) the members of the allocation have access to. Allocations typically will give access to storage resources as well; this will include the specification of which storage resources (e.g. high-performance or SHELL storage, and the limits on the amount of data that can be stored there.

Every allocation is contained within a single project which represents the research group the allocation is for. Every member of an allocation must also belong to the containing project. So every user of the cluster must be a member of a project and a member of an allocation in that project which provides resources on that cluster.

Auristor file system
The Auristor file system is the distributed file system used to back the SHELL storage tier on the Zaratan cluster. It is an enhanced version of the AFS file system long used on the campus Glue/TerpConnect Unix clusters. It uses Kerberos based authentication, and provides a globally distributed file system.

See also:

B

Backfilling of jobs
Backfilling is a technique used by the scheduler to increase the utilization of the cluster. When enabled, it allows the scheduler to run smaller, shorter jobs from further down the queue when the job at the top of the queue is waiting for resources, provided that the smaller, shorter jobs are expected to complete before the additional resources needed by the larger job are expected to become available.

For example, consider a small cluster with ten nodes (node #1 to node #10), and five of the nodes (nodes #1 to #5) are currently in use by jobs which have another 12 hours of walltime remaining, and the remaining five nodes (#6 to #10) are idle. At the top of the queue is a job (job #1) that needs 8 nodes --- since only five nodes are idle, it is currently pending and the scheduler estimates that it will remain pending for another 12 hours (when some of the jobs currently running finish and at least three of nodes #1 to #5 become idle). Next in the queue are a bunch of small jobs (jobs #2-6) that need only node for 2 hours.

Without backfilling, the scheduler would hold nodes #6 through #10 for job #1, and wait until at least three of the nodes #1 through #5 become idle (which will happen when the jobs running on them finish), and then allocate nodes to and start job #1. Since it is expected take about 12 hours for the jobs on nodes #1 through #5 to finish, nodes #6 through #10 will remain idle for about 12 hours.

With backfill, the scheduler sees that jobs #2 through #6 are small, and that if it started them on the nodes #6 through #10, they could all run and finish within 2 hours (since the scheduler will kill them if they run past 2 hours), and so running those jobs should not delay the start of job #1. This allows for more efficient use of resources.

Of course, in reality things are not quite so black and white. The scheduler knows the walltimes requested by jobs, but a job can finish before its walltime is up, and indeed they usually will (since the scheduler will terminate jobs if they exceed their requested walltimes, users are encouraged to provide a bit of padding in their walltime estimates to ensure the job will finish before then). However, if the walltimes provided by the users are somewhat reasonable estimates of the actual runtime, even if padded by 20 or 30% for safety, this backfill process can help keep the queue moving without introducing much delay into the larger jobs.

BeeGFS file system
BeeGFS is a high-performance, parallel file system. It is used to provide the scratch tier storage resource on the Zaratan high-performance computing cluster.

See also:

Bit
A bit (from binary digit) is the smallest unit of information in a classical computer system. It can hold one of two values, typically denoted as 0 or 1 (although other labels like true/false or on/off are equally valid). As such, a bit can be represented by a simple switch which can be in either an on or off position, which in the most elementary sense forms the basis of digital electronics. Although a single bit can only hold either a 0 or 1, it is possible to string multiple bits together to hold arbitrarily large numbers (limited only by the number of bits); a common example is the byte. See also:
  • Glossary entry for byte
  • Glossary entry for qubit
  • Wikipedia entry for bit
Byte
A byte is a collection of 8 bits, and as such it can take on any of 2^8 distinct values, typically expressed as the integers between 0 and 255. Bytes are also commonly used to represent a single character in many language sets, or instructions in the machine level language for computer codes, etc. Bytes are typically the smallest addressable unit of information, and as such are often used as the unit by which digital information is quantified; i.e. the size of memory used by a program or job, or the size of files stored on disk, etc. --- since a byte is a rather small unit by today's standards, this typically is used with a metric multiplier prefix like k (1000x), M (1,000,000x), G(10^9x), T(10^12x), etc. See also:

C

Checkpointing
Checkpointing is the process by which a program or job periodically saves its state to disk so that if the code is terminated for some reason, the state information can be used to resume the calculation from when the checkpoint was made. This is commonly used to provide some resilience to jobs, especially long jobs, from hardware and other failures --- when the job is resubmitted, it can (with appropriate configuration) resume from the last checkpoint rather than from the very beginning. See also:
compute nodes
A compute node is a node on the cluster which is intended for heavy computation. These are the "work horses" of the cluster, where all the real computation gets done and most of the nodes fall into this category. These nodes can be further specialized into compute nodes with specific features, like nodes with large amounts of RAM (large memory nodes) or nodes which have GPUs or other specialized accelerators (e.g. GPU nodes).
Controlled Unclassified Information (CUI)
Controlled Unclassified Information (CUI) is an unmbrella term to describe data which is not classified by the US government but which still has some data sensitivity which requires it to safeguarded. This encompasses various categories of sensitive data which is not formally classified, including personally identifiable information (PII), HIPAA, and similar.
core
A computer core is the basic unit for processing on a computer processor or GPU. Each core is capable of performing an independent calculation at the same time, so in the high performance computing paradigm we typically want to parallelize code with each parallel thread of execution running on a separate core to gain maximum performance.

From a software perspective, each core looks much like a CPU , and the cores are sometimes referred to as CPUs, but for this website we will try to always use CPUs to refer to the chip/socket, and cores (or CPU cores) for the basic processing units within the CPU.

See also:

CPU
The acronym CPU stands for the "central processing unit" of the system, traditionally the "brain" behind the computer. It is where most of the processing traditionally occurs.

The CPU consists of one or more cores (modern CPUs almost always consist of multiple cores, the CPUs on the Deepthought2 cluster have 10 cores per CPU, some recent chips have upwards of 60 cores per CPU). The CPU is the physical "chip"; many nodes have multiple CPUs. The CPU is sometimes referred to as a "socket" to more clearly distinguish it from the cores --- from a software perspective the cores look like CPUs, and some places use the term CPU to refer to cores, so the socket terminology is useful. On this web site, we will try to always use the term CPU to refer to the chip/socket, and refer to cores as cores (or sometimes CPU cores). Note that the Slurm scheduler (e.g. sbatch and other commands), tends to use the term CPU to refer to a core, and uses the term socket when refering to the physical processor.

See also:

CUDA
CUDA is a platform for parallel computing, primarily used for doing general purpose computing on GPUs. CUDA can be accessed via specific libraries, use of compiler directives like OpenACC, and the specialized nvcc compiler.

See also:

CUDA compute capability
Different GPUs support different features and command sets. Generally this is a cumulative hierarchy, so later GPUs support all of the features of earlier GPUs, and have some additional capabilities not supported by the earlier ones. This hierarchy is encapsulated as the CUDA compute capability, which is a simple version number which specifies the features supported by a given GPU and CUDA driver library. E.g., double precision floating point operations were introduced in CUDA compute capability 1.3; GPUs supporting only 1.0, 1.1, or 1.2 do not support double precision, but GPUs supporting 1.3 or higher do.

The following list gives the CUDA compute capability for GPUs on the various UMD clusters:

  • K20 GPUs: 3.5 (Deepthought2 cluster)
  • K80 GPUs: 3.7 (Bluecrab cluster)
  • P100 GPUs: 6.0 (Bluecrab and Juggernaut clusters)
  • V100 GPUs: 7.0 (Juggernaut cluster)
  • A100 GPUs: 8.0 (successor to Deepthought2 ???)

See also:

D

data transfer nodes (DTNs)
These are nodes optimized for the transferring of the large amounts of data, both data sets needed for jobs running on the cluster, and moving results of finished jobs off the cluster. You should not run any computationally intensive processes on the login nodes.
Distributed Memory Parallelism
Distributed memory parallelism is a paradigm for parallel computing in which the parallel processes do not all share a global memory address space. Because of this, communication among the different processes cannot occur only over shared memory. This is in contrast with shared memory parallelism which can use the shared memory space for inter-process communication.

Distributed memory parallelism is typically implemented by using the Message Passing Interface (MPI) for communication between the tasks. Although programming using MPI is generally harder than using shared memory paradigms like OpenMP, it has the advantage that it is not restricted to a single node.

E

Embarrassingly parallel
A workload or problem is embarrassingly parallel (also referred to a perfectly, delightfully, or pleasingly parallel) is a workload such that there no or only minimal communication required between the parallel tasks. Because of this, such workloads can be parallelized with only mimimal effort.

Grid searches of a parameter space are often a common case of an embarrassingly parallel problem. Consider a case wherein you have an N dimensional grid of parameters, and you need to evaluate some (potentially complicated) function for the parameters at each point in the grid. Typically the evaluation at any point in the grid is independent of any other evaluation, and these evaluations can be perofrmed as a large number of sequential jobs. The sequential jobs can be sequentially or simultaneously or in some combination of the two.

See also:

F

FLOPS
FLOPS, short for FLoating-point OPerations per Second is a measurement of computer performance, and as the name suggests, basically measures the maximum number of calculations using floating point arithmetic that a computer can perform per second. This is the measure commonly used to compare the performance of HPC clusters, except that we usually talk in terms of teraFLOPS (1 TFLOPS = 10^12 FLOPS) or petaFLOPS (1 PFLOPS = 10^15 FLOPS) or even exaFLOPS (1 EFLOPS = 10^18 FLOPS).

See also:

G

Gibibyte (GiB)
This is the binary unit corresponding to the decimal unit gigabyte, with 1 GiB = 2^10 MiB = 1024 MiB = 2^30 B = 1024^3 B = 1,073,741,824 B = 1.073 GB. This is approximately the size of half an hour of video. See also:
Gigabyte (GB)
This is the decimal unit corresponding to one billion (10^9) bytes. This is approximately the size of half an hour of video. See also:
Graphics Processing Unit (GPU)
Graphics Processing Units (GPUs) are hardware accelerators which were initially designed to facilitate the creation of graphics for output to a display device. However, their highly parallel structure makes them more efficient then general purpose CPUs for certain types of algorithms, and so they are quite useful for processing certain "number crunching" workloads.

High-end GPUs can be expensive, and are rapidly evolving, so only a small subset of nodes typically have GPUs, and if you wish to use them you will need to instruct the scheduler to use those nodes. Although a GPU contains a large number of cores, they are not compatible with the standard Intel x86 architecture, and codes need to written (and compiled) especially for these devices, typically using the CUDA or OpenCL platforms. Some of the applications in the standard software library, but even in those cases you need to use the versions which specifically support CUDA.

See also:

H

high-performance storage system (HPFS) (aka scratch filesystem)
A high-performance storage system is a disk-like storage system designed to support sustained high level of input/output operations from many processes across many nodes. This is generally achieved by spreading data across many file servers. In particular, with appropriate settings, the data for a single file may be striped across multiple file servers, thereby potentially allowing the many different tasks of a large parallel job to access different parts of the same file without overwhelming a single file server. For this reason, HPFS systems are ofter referred to as parallel file systems.

The large number of file servers and high performance makes the HPFS tier of storage more expensive than less performant tiers. As such, HPFS tiers are often considered scratch storage, as they are only intended for the storage of data (either input to or output from) jobs that are currently running or that would be running in the near future. Typically, input data should be downloaded to the HPFS (or copied from medium term or longer tiered storage), the job submitted, the job runs to completion, and then the inputs are deleted (or returned to longer term storage tiers), temporary files from the job are deleted, and precious output is moved to longer term storage. It is the policy on UMD HPC systems that scratch space is only for data related to active jobs on the cluster, i.e. jobs that are running, in the queue, that completed recently, or that you plan to run in the near future. Please delete all unneeded files in a timely fashion, and move and data off the scratch file system when no longer needed for active jobs.

See also:

hybrid parallelism
Hybrid parallelism is a paradigm for parallel computing in which some of the parallelism is done using distributed memory parallelism for some of the parallelism, and shared-memory parallelism for the rest. This is commonly done by using MPI for the distributed-memory type, with each MPI task being multithreaded.

This technique is not common, but typically occurs when the problem can be decomposed for parallelization in two distinct manners, with the code using one paradigm for one decomposition and the other for the other.

Even for codes which support hybrid parallelization, then benefits and the performance gains might be highly dependent on the problem being solved. So it is advisable to benchmark to find the optimal configuration.

See also

High Performance Computing (HPC)
High Performance Computing is a general term for computing with a high level of performance. Generally high performance computing specifically refers to running jobs which are very parallel, often running on hundreds or even thousands of cores.

However, then term is often used more generally to encompass not only traditional high performance computing but also high-throughput computing and various machine learning paradigms. The Deepthought clusters are generally designed for High Performance Computing, and are referred to as such, but they also support High Throughput Computing as well.

I

Infiniband
Infiniband is a data interconnect used by many high=performance computing (HPC) systems because of its high bandwidth and low latency. These factors are important in HPC systems because the different tasks in an MPI code typically need to exchange significant amounts of data with each other. While more traditional interconnects like ethernet can sometimes be competitive on bandwidth, they typically have significantly more latency, which can degrade performance in many MPI based codes.

See also:

J

Job Priority
Every job submitted to the scheduler has an associated priority which determines the order in which the scheduler will try to run them. While broadly speaking jobs are run in a first-come, first-served order, there are exceptions. Jobs submitted to the debug partition get the highest priority (i.e. the scheduler tries to run those first) --- these jobs have highly restrictive time limits and a small dedicated pool of nodes, so this generally allows users to get these jobs started quickly, to promote rapid debugging of issues.

Most remaining jobs are run at standard priority --- these comprise the bulk of the jobs on the cluster. Basically, unless you specifically request otherwise (by either submitting to the debug partition above, or the scavenger partition below), the job will run at standard priority. Typically jobs submitted at this priority run in a first-come/first-serve fashion. However, if the next job X in line to run is waiting on resources, the scheduler may "backfill" some smaller jobs onto resources reserved for the job X f such jobs would finish before the remaining resources needed to run job X are expected to be ready.

Jobs submitted to the scavenger partition run at the lowest priority, and are subject to preemption, but to compensate for that such jobs do not incur charges to the allocation account.

See also:

K

Kerberos
Kerberos is an authentication system which makes use of tickets to grant access to computer resources on the network. Typcially, you will automatically get Kerberos tickets when you enter your password to log into a Kerberos enabled system. These tickets have expiration dates (typically on the order of a day), although they can be renewed. Kerberos tickets for the basis for obtaining tokens to access the Auristor file system which is used for the SHELL medium term storage tier.

See also:

kibibyte (kiB)
A kibibyte is 1024 bytes, as distinct from a kilobyte (kB) which is 1000 bytes. Because certain parts of computer hardware (e.g. memory) typically tend to favor numbers which are powers of 2, there is a need for binary units like kibibytes. In the early days of personal computers, usage was sloppy, and many used the term kilobyte to refer to 1024 bytes (even though the metric kilo- prefix means 1000x), but this became problematic as sizes increased, with each successive prefix adding about 2% to the difference between purely decimal and purely binary units. Eventually the standards committees settled on this system of binary units. This is about the size of the text of Jabberwocky or a small icon image. See also:
kilobyte (kB)
A kilobyte is 1000 bytes. Sometimes this term is used for 1024 (2^10) bytes, but that is more properly referred to as a kibibyte (kiB) as the metric prefix k is meant to be a multiplier of exactly 1000. A kilobyte is about the size of the text of Jabberwocky or a small icon image.
kSU
Please see service unit (SU).

L

Login node
These are the nodes you can actually log into to submit jobs, edit files, monitor your jobs, view the results of jobs, etc. They are *not* intended to do number crunching or real computation. If the cluster has distinct DTNs, then most file transfers (certainly all transfer of large amounts of data) should be done on the DTNs. You should not run any computationally intensive processes on the login nodes.
Lustre
Lustre is a high-performance, parallel file system. It was used to provide the scratch tier storage resource on the Deepthought2 high-performance computing cluster, as well as for the Juggernaut cluster.

See also:

M

Mebibyte (MiB)
This is the binary unit corresponding to the megabyte, with 1 MiB = 2^10 kiB = 1024 kiB = 2^20 B = 1024^2 B = 1,048,576 B. This is approximately the size of the content of a typical Harry Potter novel. See also:
Medium-term storage (SHELL storage tier)
The SHELL storage tier is a medium-term storage tier provided on the Zaratan cluster. Unlike the scratch tier, data on the medium-term storage cluster does not have to be needed by a job which is running or about to run. However, it is not long-term or archival storage --- the storage it not automatically backed up by DIT, nor are there guarantees that the storage will last beyond the lifetime of the Zaratan cluster (5 years or so). It is intended for the storage of large amounts of data related to active research on the cluster, even if the data is not related to jobs which are running or will be run in the near future.

Please note that the SHELL tier is not designed to the same level of performance as the HPFS/scratch tier. To prevent users from overloading the small number of fileservers forming this tier by having many tasks from large parallel jobs do large amounts of I/O to it, the SHELL tier is not accessible from the compute nodes. It is accessible from the login and data transfer nodes. Furthermore, since the underyling filesystem is the Auristor file system, it can be securely accessed with appropriate credentials from any system with the appropriate file client, even systems outside cluster.

Megabyte (MB)
This is the decimal unit corresponding to one million (10^6) bytes. This is approximately the size of the content of a typical Harry Potter novel. See also:
Memory
Just like people, when talking about computers memory is the means by which computers and computer programs keep track of things for future use. With computer systems, the term memory typically refers to RAM, which is a specific type/family of memory chips which provide relatively high speed access. Although both memory and disk storage are usually measured in bytes, disk storage is typically not considered memory (just like one normally does not refer to notes in a notebook as a person's memory).

Note that GPUs typically have memory which is distinct from the main system (CPU) memory, usually an order of magnitude or so smaller.

Message Passing Interface (MPI)
The Message Passing Interface (MPI) is a standardized and portable standard for communication between tasks in a distributed memory parallel job. It has become a de-facto standard for programs which can run across multiple nodes.

There are a number of implementations of this interface. UMD HPC clusters typically provide:

  • For GNU compilers, OpenMPI: an open-source MPI implementation
  • For Intel compilers, Intel MPI: Intel's implementation of MPI

Both implementations include wrappers for C, C++, and Fortran compilers, as well as bindings for many scripting languages including Python and R.

See also:

MPI Communicator
MPI jobs group the various processes which comprise the job into objects called communicators. The default communicator is MPI_COMM_WORLD which consists of all of the processes that were in the job when it first started. For many jobs, all of the processes are closely coupled, and that is the only communicator you need, but MPI provides other mechanisms which are needed in some more complicated cases.

Each communicator has a size, which is the number of processes in the communicator. Every process in the communicator has an unique rank, which is an integer between 0 and the size - 1 of the communicator. See also:

MSU
Please see service unit (SU).
Multi-instance GPU
Modern represent a significant amount of computational power, and sometimes jobs cannot fully utilize a full GPU. However, a single GPU cannot be split between jobs, so in this situation the GPU goes underutilized. Certain models of NVidia GPUs support something called multi-instance GPU which allows one physical GPU to be split into multiple, less powerful, virtual GPUs. These virtual GPUs can each be assigned to different jobs, thereby allowing multiple jobs to share one of these powerful GPUs, increasing utilitization of the device.

The A100 GPUs support this feature, and each physical can be split into various smaller virtual devices, up to seven smaller units. We have currently divided the A100s on some of the nodes into seven smaller virtual units:

> 7 SU
GPU name in Slurm DescriptionGPU Compute units GPU Memory Cost per hour
gpu:a100 Full physical A100 8 40 GB 48 SU
gpu:a100_1g.5gb One seventh of an A100 1 5 GB

N

node
In HPC terminology, we use the term node to refer to a complete computer system, comprising of at least one CPU, memory, power supply, various buses, networking devices and generally some local disk storage. This is the direct analog to your laptop or workstation.

On the clusters, we have differentiated the nodes into various specializations:

See also:

O

OpenACC
OpenACC is a programming standard designed for parallel computing heterogeneous CPU/GPU systems as well as other accelerators.

See also:

OpenCL
OpenCL is a programming framework for writing code to execute across heterogeneous platforms of CPUs, GPUs, and other hardware accelerators. It is an open standard.

See also:

OpenMP
OpenMP is an application programming interface for shared memory parallelism using threads. (Should not be confused with OpenMPI.)

OpenMP is implemented in the compiler, adding some pragmas to the language with which you can instruct the compiler which sections of the code are parallelizable and which must be run sequentially.

See also:

OpenMPI
OpenMPI is an open source implementation of the MPI library.

P

Parallel Computing
Parallel computing is a type of computation wherein many calculations are carried out simultaneously. This contrasts with sequential computing wherein only a single calculation occurs at any given time.

Obviously, a parallel computation should be faster than a sequential computation. The speedup of an algorithm when parallelized is the ratio of the runtime of the sequential code to that of the parallelized code. Although one might naively expect the speedup (compared to the sequential calculation) of a computation parallelized over N CPU cores would be N, this is rarely the case. Typically, there are parts of an algorithm which cannot take advantage of parallelization, causing the speedup to be less than N. Generally, the addition of CPU cores will start having diminishing returns at some point, depending on the algorithm.

See also:

Parallel File System
Please see High-Performance File System.
Partition
In HPC terminology, a partition is a set of compute nodes. Clusters typically divide their nodes into partitions to reflect different hardware availabilities, or to associate priority and quality of service settings to jobs submitted to the partition.

The term "queue" is sometimes used synonymously with "partition" (because in some sense the scheduler maintains a distinct queue for each partition).

See also:

Petabyte (PB)
This is the decimal unit corresponding to one quadrillion (10^15) bytes. This is about the size of 2000 years of MP3-encoded music. See also:
Preemption of Jobs
Most jobs on the cluster are submitted to the scheduler, wait in the queue until resources are available, and then once the job is started, it runs until it either completes, crashes, or is cancelled due to exceeding its requested walltime. Even if a higher priority job is submitted afterwards which "wants" the resources being used by the lower priority job which already started, the running job is allowed to run to its successful or unsuccessful completion, and the higher priority job is forced to wait until it finishes (or other resources become available).

An exception to this is jobs submitted to the scavenger partition. These jobs, in addition to be the lowest priority, are also preemptible. This means that even after the scavenger job starts, if another, higher priority job (and every non-scavenger job is higher priority) "wants" the resources being used by the scavenger job, the higher priority job can kick the scavenger job off of the node(s) it is using, and take those node(s) for its own use. This is called preemption. Typically, the preempted scavenger job is put back in the queue, and will eventually start again (it is advisable that such scavenger jobs do checkpointing so that they do not start over from the very beginning but can make some progress towards completion in these intermittent runs).

Again, normal jobs will not get preempted, only scavenger jobs. Scavenger jobs are subject to preemption, but your allocation account does not get charged for scavenger jobs.

See also:

Priority of Jobs
Please see job priority.
Project
A project is a collection of allocations for a research group, i.e. under a single principal investigator (PI). In theory, a single project can have allocations across multiple clusters. Usually a faculty member will have a single project; although exceptions exist. If the faculty member has requested use of the cluster for a class he/she is teaching, then that class will be in a separate project. Also, faculty members who manage college or departmental pools of HPC resources will see that as a distinct project.

Every user of the cluster is associated with one or more projects, and also with at least one of the allocation accounts for the cluster beneath the project.

Projects usually belong to research groups, although sometimes they belong to a department or a subset of a research group. They provide a means of organizing users and associating them with allocation accounts. Members of the same project also share membership in an Unix group, which can be used to facilitate sharing files (although by default your home and data directories are restricted to your login account only, they are generally group-owned by the project group, and you can use chmod to allow group members to read).

See also:

Q

Quarter (of a year)
This represents one fourth of a year. Some resources are meted out on a quarterly basis. This is generally to encourage the spreading out of the use of the resource over time. Quarters are labelled Q1, Q2, Q3, and Q4, starting on 1 Jan, 1 Apr, 1 Jul, and 1 Oct, respectively.
Qubit
A qubit (from quantum bit) is the smallest unit of information in a quantum computer system. When observed/read out at the end of a calculation, it returns a binary value (typically denoted as 0 or 1) just like a classical , but during processing it can be a coherent superposition of both of those states simultaneously, thereby containing much more information. See also:
Queue
In Britain, a queue is a line of people, etc. waiting their turn for something. This definition is borrowed in HPC to refer to the list of jobs that are waiting to be allocated resources and started by the scheduler.

When you submit a job to the cluster and then check its status, unless you are lucky enough to have submitted it when the cluster is very idle, you will see that the job is in a "pending" state --- i.e. the job is waiting in the queue.

The amount of time a job spends in the queue depends on how busy the cluster is, how many other jobs are in the queue, how many resources your job is requesting and for how long, the priority of the job, and many other factors. A sizable production job requesting a day or more of runtime can easily spend a couple of hours in the queue. Very short jobs (under 15 minutes) should look into the debug partition which generally has much lesser wait times.

In some sense, the different partitions each have their own queue, and so sometimes the term queue is also used to refer to a partition. Some schedulers tend to use the term queue in that fashion more than others.

See also:

R

RAM
RAM is an acronym for Random-access memory. This is particular form of computer memory which can be both read and written, and for which the access times are basically irrespective of the location of the data within the memory. Most typically, this takes the form of dynamic random access memory (DRAM), which is "volatile", meaning that the stored information is lost if power is removed from the chip.

RAM is typically where the machine code for the program typically resides when it is being run, along with all of the data that the program is actiing on, although in both cases this can be supplemented by disk storage as well. For static data like the code, the system just overwrites the memory storing the code when needed, and when the code is needed reads it back from the program file on disk. For general data, the program can write out chunks of memory to a swap area on the disk, overwrite the memory thus freed for a while, and read the memory back from disk when needed. In either case (but especially the latter case), this can significantly degrade performance, as disks are much slower than RAM, and so high performance codes typically wish to avoid swapping to disk.

See also:

S

Scavenger partition
The scavenger partition is a special partition that allows users to submit very low priority partition is a special partition that allows users to submit very low priority job that is subject to preemption, but in return does not incur any charges against the allocation account.

Because scavenger jobs do not incur charges against the allocation account, these are an useful mechanism to run jobs in excess of your SU allotment. But because of the preemption, you are strongly encouraged to make use of checkpointing in order that your job can make progress during its piecemeal run time.

Scheduler
The scheduler is a critical component of an HPC environment. It is a complex piece of code responsible for allocating resources to the various jobs in the queue in a timely fashion.

The scheduler is in some ways analagous to the host or hostess at a restaurant, with the restaurant customers being like jobs, the tables being like the compute resources, and the waiting list being like the queue. If the restaurant is not busy, a new customer can just come in and the host/hostess will seat them almost immediately, just like when the cluster is idle the scheduler will allocate resources to a new job almost immediately. When things are busy, the host/hostess will place customers on a waiting list, and customers will be seated in roughly the order they came in. But there are exceptions --- some parties are too big for some tables or might have special requests (e.g. indoor vs outdoor seating, etc), so sometimes smaller or less particular parties can jump ahead in the queue.

In the same way, the scheduler tries to allocate resources to jobs roughly in the order they came in, but modified because the jobs have different sizes (from single cores to thousands of cores) and can requestaGPUs or large amounts of memory. Also, unlike most restaurants, the scheduler will allow in some cases for multiple jobs to be placed on the same table. As an added twist, jobs are required to specify the maximum amount of time for which they will run, and the scheduler can use this to backfill jobs.

There are many different schedulers available for HPC clusters. At UMD, we have been using the Slurm scheduler since 2014. Slurm is an open source product used at many clusters in the world, including most of the TOP500 systems.

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Secure copy protocol (scp)
The secure copy protocol and command (scp) are a protocol and command for transfering files from one computer system to another over the network. The secure in the name comes from the fact that the authentication and the transferred file data are both encrypted over the wire, thereby protecting the data from wire tapping attacks.
Scratch file system
Please see the High-Performance File system
Sequential Job or Process
A sequential job or process is a job or process which does not do any form of parallelism, i.e. the code processes one instruction, then the next, and so on. These are also referred to as single core jobs or processes, as they can efficiently run on a single CPU core.
Service node
Service node is a catch-all phrase for any of the nodes in the cluster which provide various services needed to keep the cluster running. Generally you will never directly run processes on these nodes, but they provide essential services. E.g. the scheduler, file servers, web servers, etc.
SHELL file system
Please see Medium-term storage
Solid-state drive
Solid-state drives(SSDs) are a form of mass storage which use integrated circuits for the actual storage instead of spinning magnetic media. These devices are of interest in HPC applications primarily because they have much higher input and output speeds and much lower latency than traditional spinning media. Solid state drives can either use traditional hard disk drive (HDD) interfaces like SAS or SATA, but they can also use newer interfaces like NVMe which can better support the parallelism available in modern SSDs and therefore even further improve performance. The downside is that currently SSDs are more expensive per storage unit than traditional HDDs.
Secure Shell Protocol (SSH)
The Secure Shell Protocol is a cryptographic network protocol for communicating between systems. It is also the name of the most common remote login client application ssh using this protocol. Typically one uses the ssh command to open a command line terminal session on a remote system. This client application is usually installed by default (as ssh) on circuits for the actual storage instead of spinning magnetic media. These devices are of interest in HPC applications primarily because they have much higher input and output speeds and much lower latency than traditional spinning media. Solid state drives can either use traditional hard disk drive (HDD) interfaces like SAS or SATA, but they can also use newer interfaces like NVMe which can better support the parallelism available in modern SSDs and therefore even further improve performance. The downside is that currently SSDs are more expensive per storage unit than traditional HDDs.
SSH public key authentication
Solid-state drivesa(SSDs) are a form of mass storage which use integrated circuits for the actual storage instead of spinning magnetic media. These devices are of interest in HPC applications primarily because they have much higher input and output speeds and much lower latency than traditional spinning media. Solid state drives can either use traditional hard disk drive (HDD) interfaces like SAS or SATA, but they can also use newer interfaces like NVMe which can better support the parallelism available in modern SSDs and therefore even further improve performance. The downside is that currently SSDs are more expensive per storage unit than traditional HDDs.
Striping (of a file)
High-performance file systems (like BeeGFS or Lustre) typically achieve their high performance by striping the data in a file across multiple file servers, or at least disk arrays on a file server. I.e., different portions or chunks of a file will get written to different disk arrays, and usually to disk arrays on different file servers. Generally, there is a chunk size and a number of stripes set for a file (either by default or specified by the user), and the system will assign the file to a number of disk arrays and/or file servers equal to the number of stripes, and write the first chunk size amount of data to the first such disk array, and when that is done write the second chunk size amount of data to the second, and so forth. After the chunk is written to the last of the assigned disk arrays, things cycle back around to the first again. This spreads out the I/O load among multiple disks, and usually across multiple servers, so in theory the maximum I/O rate is the sum of the I/O rates for the individual disk arrays. Of course, in practice the data rates do not quite reach the theoretical maximums, but process does allow for substantial increases in I/O throughput.

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Service Unit (SU)
A service unit (commonly abbreviated SU) is the basic unit used in HPC clusters to measure an amount of computation. It is basically defined as 1 SU = 1 walltime hour of use of a single CPU core, although this definition can be tweaked somewhat. On clusters with many different CPU models, an additional factor maybe included to account for the different computational power of the different CPU models; e.g. often the above formula only holds for the slowest CPU model on the cluster, and more powerful CPU cores will have an additional factor (>1) applied to normalize. E.g., if a job takes one hour on the slower core (so consumes 1 SU), and only takes 30 minutes on the faster core, the faster core might have a factor of 2 added in the above formula (so the SU cost on that core would be twice the number of CPU hours, or 2 * 1 core * 0.5 hour = 1 SU, so the job costs the same on either core).

Similarly, the above formula will be tweaked to account for other resources. E.g., memory is a restricted resource, and so a job which is using twice the average memory per CPU core might be charged as if twice as many cores were in use. GPUs are a valuable resource, typically with each GPU having thousands of cores. (E.g. an Nvidia A100 GPU has almost 7000 CUDA cores and over 400 Tensor cores.) Although each GPU core is less powerful than a single CPU core, the sheer number of such cores makes the GPUs very powerful for some alorithms. We do not charge 1 SU for each GPU core used for 1 hour, but we do charge significantly more for the use of an entire GPU for 1 hour than we do for a single CPU core. The exact factor depends on the GPU being used.

Generally, your job will be charged an hourly SU rate given by the maximum of: job (in hours) times the

For example, a job requesting 6 CPU cores, no GPUs, and 8 GB/CPU-core would be charged an hourly rate of 12 SU/hour (in this case, the total memory allocated would be 6 CPU * 8 GB/CPU = 48 GB, and the maximal value in the list above would be 48 GB * 0.25 SU/hr/GB = 12 SU/hour). Similarly, a job requesting two A100 GPUs along with 64 CPU cores and 4 GB/CPU core would be dominated by the GPU costs, and result in 48 SU/hour/GPU * 2 GPU = 96 SU/hour.

This rate is multiplied by the actual walltime the job used (in hours) to calculate the SU cost of the job. Note that the actual walltime is used; if your job requested 24 hours of walltime but completed in 20 hours, your job is charged for 20 hours, not 24 hours. Note also that the hourly SU rate is based on the amount of resources allocated to the job, not the resources actually used. E.g., if you submit a single core job requesting 8 GB or RAM and no GPUs, you will be charged for 8 GB of RAM whether your job uses all 8 GB or not --- these resources are reserved for your job (and therefore unavailable to other users) for the duration of the job and so you will be charged for them. In particular, submitting a job in exclusive mode (which disallows any other jobs from sharing the node with your job) will result in your being charged for all the resource on the node whether you are using them or not.

Compared to the amount of computation done on the cluster, an SU is a fairly small unit, so one often talks in terms of kSU or even MSU, where 1 kSU = 1000 SU and 1 MSU = 1,000,000 SU. See also:

Shared memory parallelism
Shared memory parallelism is a paradigm for parallel computing in which all of the parallel processes share a global, shared memory address space which they use to communicate among each other. It contrasts with distributed memory parallelism techniques where no memory space is shared among all of the processes.

Shared memory parallelism is typically implemented by threads using OpenMP or Threading Building Blocks.

Note: in order to use this paradigm, all of the processes using shared memory parallelism must be running on the same node, otherwise they would not be able to share memory.

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SHELL filesystem
Please see medium term file system (SHELL).
Speedup
The speedup of a code when parallelized is a measurement of how much performance improvement one obtains when parallelizing a job. It is given by the ratio of the runtime for a sequential version of the algorithm and the parallelized computation.

I.e., if the sequential code takes T_seq seconds, and the parallel code takes T_par seconds, the speedup is given by S = T_seq/T_par.

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SU
Please see service unit

T

task
MPI jobs consist of a number of tasks, which is the smallest unit of execution within MPI. All processes, etc. for a given task run on the same node (other tasks can also be on the same or different nodes, but the processes for a given task will not be split across multiple nodes), and each task uses the MPI for communication with the other tasks, so therefore the tasks can all be running on different nodes (although they can also run on the same node).

In a pure MPI job, the tasks are sequential, and so the job will use a number of cores equal to the number of tasks.

Hybrid jobs can also exist, in which each MPI task uses a shared memory parallelism technique (OpenMP or other threading paradigms) for additional parallelism. In this case, each task needs multiple CPU cores on the same node. Although all cores for a task will always be assigned on the same node, the default is one CPU core per task, so you will need to specify the number of CPU cores per task.

Tebibyte (TiB)
This is the binary unit corresponding to one terabyte (TB) or roughly one trillion bytes (B). It is equivalent to 1 TiB = 2^40 B = 1024^4 B = 1024^3 kiB = 1099511627776 B ~ 1.1 TB. Note that on this scale, the difference between the binary (TiB) and decimal (TB) units is almost 10%. See also:
Terabyte (TB)
This is the decimal unit corresponding to one trillion (10^12) bytes. The largest consumer hard drives available in 2007 were only 1 TB, and even in 2022 most laptops only have drives in the 1/4 to 1 TB range. See also:
thread
A thread, often called a "lightweight process", is a thread of execution, which is the smallest sequence of instructions which the operating system can manage.

From an HPC perspective, a multi-threaded process can have multiple threads of execution running simultaneously, assuming that the node the job is running on has available cores for each thread. Technically, a multi-threaded process can run on fewer cores than threads, but in that case you will not get full advantage of the parallelism. Generally for HPC purposes, you want a separate CPU core for each thread, to get the maximum speedup.

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Threading Building Blocks
Threading Building Blocks (aka TBB or oneTBB) is a C++ template library developed by Intel for shared memory parallelism.

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Walltime
The term walltime refers to the amount of time that the job actually run. The "wall" in the term means that this is normal, everyday time you are familiar with, as for example might be measured by a clock on the wall. (This is to distinguish from other, more specialized "times" sometimes used in refering to processes running on a computer system.) I.e., a job which starts to run at 1:05 PM and finishes at 3:27 PM on the same day will have a walltime of 3:27 - 1:05 = 2:22 or 2.37 hours.

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