Unifying HPC and Cloud Systems; A Containerized Approach for the Integrated Forecast System (IFS)

The IFS (Integrated Forecast System) is a global numerical weather prediction system maintained by the European Centre for Medium-Range Weather Forecasts (ECMWF). Traditionally, ECMWFs high-performance computing facility (HPCF) is responsible for operationally supporting the IFS cycles. However, with the emergence of new cloud technologies, initiatives such as Destination Earth (DestinE), and growth of OpenIFS users within Europe and around the globe, the need to run IFS outside of ECMWF's computing facilities becomes more evident. Concerning such use cases, IFSTestsuite allows for the complete IFS system and its dependencies (e.g. ecCodes) to be built and tested outside of ECMWF's HPCF and designed to be self-contained, eliminating the need for external tools like MARS or ecCodes. Despite the need for users to perform multiple steps and the dependency of the software availability and versions on the host operating system, this indicates that there might be a potential for more generic and broader approach. Containerization might provide the much-needed portability and disposable environments to trigger new cycles with the desired compiler versions, or even with different compilers. In addition, pre-built container images can be executed on any platform, provided there is a compatible container runtime installed on the target system that adheres to Open Container Initiative (OCI) standards like Singularity or Docker. Another benefit of using container images is container image layers which can significantly reduce the image build time. Lastly, despite their differences, both Singularity and Docker adhere to the OCI standards, and converting one container image to another is straightforward. However, despite the clear advantages, there are several crucial design choices to keep in mind. Notably, the available hardware and software stacks varies greatly across different HPC systems. When performance is important, this heterogeneous landscape limits the portability of containers. The libraries and drivers inside the container must be specially selected with regard to the hardware and software stack of a specific host system to maximize performance on that system. If this is done correctly, theperformance of containerized HPC applications can match native applications. We demonstrate this process with the use of a hybrid containerization strategy where compatible MPI stacks and drivers are built inside the containers. The binding of host libraries into containers is also used on systems where proprietary software cannot be rebuilt inside the container. In this study we present a containerized solution which balances portability and efficient performance,with examples of containerizing the IFS on a variety of systems including cloud systems with generic x86-64 architecture, such as European Weather Cloud (EWC) and Microsoft Azure, on EuroHPC systems such as Leonardo and LUMI and provided container image recipes for OpenIFS.