Application Development Environment

The application development environment on Cirrus is primarily controlled through the modules environment. By loading and switching modules you control the compilers, libraries and software available.

This means that for compiling on Cirrus you typically set the compiler you wish to use using the appropriate modules, then load all the required library modules (e.g. numerical libraries, IO format libraries).

Additionally, if you are compiling parallel applications using MPI (or SHMEM, etc.) then you will need to load one of the MPI environments and use the appropriate compiler wrapper scripts.

By default, all users on Cirrus start with no modules loaded.

Basic usage of the module command on Cirrus is covered below. For full documentation please see:

Linux manual page on modules

Using the modules environment

Information on the available modules

Finding out which modules (and hence which compilers, libraries and software) are available on the system is performed using the module avail command:

[user@cirrus-login0 ~]$ module avail
...

This will list all the names and versions of the modules available on the service. Not all of them may work in your account though due to, for example, licencing restrictions. You will notice that for many modules we have more than one version, each of which is identified by a version number. One of these versions is the default. As the service develops the default version will change.

You can list all the modules of a particular type by providing an argument to the module avail command. For example, to list all available versions of the Intel Compiler type:

[user@cirrus-login0 ~]$ module avail intel-*/compilers

--------------------------------- /work/y07/shared/cirrus-modulefiles --------------------------------
intel-19.5/compilers  intel-20.4/compilers

If you want more info on any of the modules, you can use the module help command:

[user@cirrus-login0 ~]$ module help mpt

-------------------------------------------------------------------
Module Specific Help for /usr/share/Modules/modulefiles/mpt/2.25:

The HPE Message Passing Toolkit (MPT) is an optimized MPI
implementation for HPE systems and clusters.  See the
MPI(1) man page and the MPT User's Guide for more
information.
-------------------------------------------------------------------

The simple module list command will give the names of the modules and their versions you have presently loaded in your environment, e.g.:

[user@cirrus-login0 ~]$ module list
Currently Loaded Modulefiles:
1) git/2.35.1(default)                                  
2) epcc/utils
2) /mnt/lustre/e1000/home/y07/shared/cirrus-modulefiles/epcc/setup-env

Loading, unloading and swapping modules

To load a module to use module add or module load. For example, to load the intel 20.4 compilers into the development environment:

module load intel-20.4/compilers

This will load the default version of the intel compilers.

If a module loading file cannot be accessed within 10 seconds, a warning message will appear: Warning: Module system not loaded.

If you want to clean up, module remove will remove a loaded module:

module remove intel-20.4/compilers

You could also run module rm intel-20.4/compilers or module unload intel-20.4/compilers. There are many situations in which you might want to change the presently loaded version to a different one, such as trying the latest version which is not yet the default or using a legacy version to keep compatibility with old data. This can be achieved most easily by using "module swap oldmodule newmodule".

Suppose you have loaded version 19 of the Intel compilers; the following command will change to version 20:

module swap intel-19.5/compilers intel-20.4/compilers

Available Compiler Suites

Note

As Cirrus uses dynamic linking by default you will generally also need to load any modules you used to compile your code in your job submission script when you run your code.

Intel Compiler Suite

The Intel compiler suite is accessed by loading the intel-*/compilers module, where * references the version. For example, to load the v20 release, you would run:

module load intel-20.4/compilers

Once you have loaded the module, the compilers are available as:

ifort - Fortran
icc - C
icpc - C++

See the extended section below for further details of available Intel compiler versions and tools.

GCC Compiler Suite

The GCC compiler suite is accessed by loading the gcc/* modules, where * again is the version. For example, to load version 10.2.0 you would run:

module load gcc/10.2.0

Once you have loaded the module, the compilers are available as:

gfortran - Fortran
gcc - C
g++ - C++

Compiling MPI codes

MPI on Cirrus is currently provided by the HPE MPT library.

You should also consult the chapter on running jobs through the batch system for examples of how to run jobs compiled against MPI.

Note

By default, all compilers produce dynamic executables on Cirrus. This means that you must load the same modules at runtime (usually in your job submission script) as you have loaded at compile time.

Using HPE MPT

To compile MPI code with HPE MPT, using any compiler, you must first load the "mpt" module.

module load mpt

This makes the compiler wrapper scripts mpicc, mpicxx and mpif90 available to you.

What you do next depends on which compiler (Intel or GCC) you wish to use to compile your code.

Note

We recommend that you use the Intel compiler wherever possible to compile MPI applications as this is the method officially supported and tested by HPE.

Note

You can always check which compiler the MPI compiler wrapper scripts are using with, for example, mpicc -v or mpif90 -v.

Using Intel Compilers and HPE MPT

Once you have loaded the MPT module you should next load the Intel compilers module you intend to use (e.g. intel-20.4/compilers):

module load intel-20.4/compilers

The compiler wrappers are then available as

mpif90 - Fortran with MPI
mpicc - C with MPI
mpicxx - C++ with MPI

Note

The MPT compiler wrappers use GCC by default rather than the Intel compilers:

When compiling C applications you must also specify that mpicc should use the icc compiler with, for example, mpicc -cc=icc. Similarly, when compiling C++ applications you must also specify that mpicxx should use the icpc compiler with, for example, mpicxx -cxx=icpc. (This is not required for Fortran as the mpif90 compiler automatically uses ifort.) If in doubt use mpicc -cc=icc -v or mpicxx -cxx=icpc -v to see which compiler is actually being called.

Alternatively, you can set the environment variables MPICC_CC=icc and/or MPICXX=icpc to ensure the correct base compiler is used:

export MPICC_CC=icc
export MPICXX_CXX=icpc

Using GCC Compilers and HPE MPT

Once you have loaded the MPT module you should next load the gcc module:

module load gcc

Compilers are then available as

mpif90 - Fortran with MPI
mpicc - C with MPI
mpicxx - C++ with MPI

Note

HPE MPT does not support the syntax use mpi in Fortran applications with the GCC compiler gfortran. You should use the older include "mpif.h" syntax when using GCC compilers with mpif90. If you cannot change this, then use the Intel compilers with MPT.

Using Intel MPI

Although HPE MPT remains the default MPI library and we recommend that first attempts at building code follow that route, you may also choose to use Intel MPI if you wish. To use these, load the appropriate MPI module, for example intel-20.4/mpi:

module load intel-20.4/mpi

Please note that the name of the wrappers to use when compiling with Intel MPI depends on whether you are using the Intel compilers or GCC. You should make sure that you or any tools use the correct ones when building software.

Note

Although Intel MPI is available on Cirrus, HPE MPT remains the recommended and default MPI library to use when building applications.

Using Intel Compilers and Intel MPI

After first loading Intel MPI, you should next load the appropriate Intel compilers module (e.g. intel-20.4/compilers):

module load intel-20.4/compilers

You may then use the following MPI compiler wrappers:

mpiifort - Fortran with MPI
mpiicc - C with MPI
mpiicpc - C++ with MPI

Using GCC Compilers and Intel MPI

After loading Intel MPI, you should next load the gcc module you wish to use:

module load gcc

You may then use these MPI compiler wrappers:

mpif90 - Fortran with MPI
mpicc - C with MPI
mpicxx - C++ with MPI

Using OpenMPI

There are a number of OpenMPI modules available on Cirrus; these can be listed by running module avail openmpi. You'll notice that the majority of these modules are intended for use on the GPU nodes.

The fact that OpenMPI is open source means that we have full control over how the OpenMPI libraries are built. Indeed the OpenMPI configure script supports a wealth of options that allow us to build OpenMPI for a specific CUDA version, one that is fully compatible with the underlying NVIDIA GPU device driver. See the link below for an example how an OpenMPI build is configured.

Build instructions for OpenMPI 4.1.6 on Cirrus

All this means we build can OpenMPI such that it supports direct GPU-to-GPU communications using the NVLink intra-node GPU comm links (and inter-node GPU comms are direct to Infiniband intead of passing through the host processor).

Hence, the OpenMPI GPU modules allow the user to run GPU-aware MPI code as efficiently as possible, see Compiling and using GPU-aware MPI.

OpenMPI modules for use on the CPU nodes are also available, but these are not expected to provide any performance advantage over HPE MPT or Intel MPI.

Compiler Information and Options

The manual pages for the different compiler suites are available:

GCC
Fortran man gfortran , C/C++ man gcc

Intel
Fortran man ifort , C/C++ man icc

Useful compiler options

Whilst difference codes will benefit from compiler optimisations in different ways, for reasonable performance on Cirrus, at least initially, we suggest the following compiler options:

Intel
-O2

GNU
-O2 -ftree-vectorize -funroll-loops -ffast-math

When you have a application that you are happy is working correctly and has reasonable performance you may wish to investigate some more aggressive compiler optimisations. Below is a list of some further optimisations that you can try on your application (Note: these optimisations may result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions):

Intel
-fast

GNU
-Ofast -funroll-loops

Vectorisation, which is one of the important compiler optimisations for Cirrus, is enabled by default as follows:

Intel
At -O2 and above

GNU
At -O3 and above or when using -ftree-vectorize

To promote integer and real variables from four to eight byte precision for Fortran codes the following compiler flags can be used:

Intel
-real-size 64 -integer-size 64 -xAVX (Sometimes the Intel compiler incorrectly generates AVX2 instructions if the -real-size 64 or -r8 options are set. Using the -xAVX option prevents this.)

GNU
-freal-4-real-8 -finteger-4-integer-8

Using static linking/libraries

By default, executables on Cirrus are built using shared/dynamic libraries (that is, libraries which are loaded at run-time as and when needed by the application) when using the wrapper scripts.

An application compiled this way to use shared/dynamic libraries will use the default version of the library installed on the system (just like any other Linux executable), even if the system modules were set differently at compile time. This means that the application may potentially be using slightly different object code each time the application runs as the defaults may change. This is usually the desired behaviour for many applications as any fixes or improvements to the default linked libraries are used without having to recompile the application, however some users may feel this is not the desired behaviour for their applications.

Alternatively, applications can be compiled to use static libraries (i.e. all of the object code of referenced libraries are contained in the executable file). This has the advantage that once an executable is created, whenever it is run in the future, it will always use the same object code (within the limit of changing runtime environment). However, executables compiled with static libraries have the potential disadvantage that when multiple instances are running simultaneously multiple copies of the libraries used are held in memory. This can lead to large amounts of memory being used to hold the executable and not application data.

To create an application that uses static libraries you must pass an extra flag during compilation, -Bstatic.

Use the UNIX command ldd exe_file to check whether you are using an executable that depends on shared libraries. This utility will also report the shared libraries this executable will use if it has been dynamically linked.

Intel modules and tools

There are a number of different Intel compiler versions available, and there is also a slight difference in the way different versions appear.

A full list is available via module avail intel.

The different available compiler versions are:

intel-19.5/* Intel 2019 Update 5
intel-20.4/* Intel 2020 Update 4

We recommend the most up-to-date version in the first instance, unless you have particular reasons for preferring an older version.

For a note on Intel compiler version numbers, see this Intel page

The different module names (or parts thereof) indicate:

cc C/C++ compilers only
cmkl MKL libraries (see Software Libraries section)
compilers Both C/C++ and Fortran compilers
fc Fortran compiler only
itac Intel Trace Analyze and Collector
mpi Intel MPI
pxse Intel Parallel Studio (all Intel modules)
tbb Thread Building Blocks
vtune VTune profiler - note that in older versions (intel-*/18.0.5.274, intel-*/19.0.0.117 VTune is launched as amplxe-gui for GUI or amplxe-cl for CLI use)