SPH-EXA2: Smoothed Particle Hydrodynamics at Exascale
PI: Florina Ciorba (University of Basel)
Co-PIs: Lucio Mayer, Ruben Cabezon
July 1, 2021 – June 30, 2024
The goal of the SPH-EXA2 project is to scale the Smoothed Particle Hydrodynamics (SPH) method implemented in SPH-EXA1 to enable Tier-0 and Exascale simulations. To reach this goal we deﬁne four concrete and interrelated objectives: physics, performance, correctness, and portability & reproducibility.
We aim at coupling relevant physics modules with our SPH framework enabling the possibility of addressing both long-standing and cutting-edge problems via beyond state state-of-the-art simulations at extreme scales in the ﬁelds of Cosmology and Astrophysics. Such simulations include the formation, growth, and mergers of super-massive black holes in the early universe which would greatly impact the scientiﬁc community (for instance, the 2020 Nobel Prize in Physics has been awarded for pioneering research on super-massive black holes). Moreover, the ability to simulate planet formation with high resolution models will play an important role in consolidating Switzerland’s position as leader in experimental physics and observational astronomy. Additional targets will be related to explosive scenarios such as core-collapse and Type Ia supernovas, in which Switzerland has also maintained an long record of international renown. These simulations would be possible with a Tier-0-ready SPH code and would have a large impact on projects such as the current NCCR PlanetS funded by the SNF.
The long-term and ambitious vision of the SPH-EXA consortium is to study ﬂuid and solid mechanics in a wide range of research ﬁelds, that nowadays are unfeasible (with the current models, codes, and architectures). To this end, in SPH-EXA2 we build on SPH-EXA1 and develop a scalable bare-bones SPH simulation framework, and refer to it as SPH-EXA. In Switzerland, within the framework of the PASC SPH-EXA (2017-2021) project, we developed the SPH-EXA miniapp as a scalable SPH code that employs state-of-the-art parallel programming models and software engineering techniques to exploit the current HPC architectures, including accelerators. The current SPH-EXA mini-app performs pure hydrodynamical simulations with up to 1 trillion SPH particles using only CPUs on 4,096 nodes on Piz Daint at CSCS. With relatively limited memory per GPU, the miniapp can still scale up to 250 billion SPH particles.
In terms of performance, the use of accelerators is necessary to meet the above SPH-EXA2 goal and objectives. Oﬄoading computationally-intensive steps to hardware accelerators, such as self-gravity evaluation and ancillary physics, will enable SPH-EXA to simulate increasingly complex cosmological & astrophysical scenarios. We envision that various types of hardware accelerators will be deployed on the supercomputers that we will use in this project, such as NVIDIA GPUs (in Piz Daint) or AMD GPUs (in LUMI). Portability across GPUs will be ensured by using OpenACC and OpenMP target oﬄoading, which is supported by diﬀerent GPU vendors.
Scheduling & load balancing and fault tolerance are major challenges on the way to Exascale . We will address these challenges in SPH-EXA2 by employing locality-aware data decomposition, dynamic & adaptive scheduling and load balancing, and advanced fault tolerance techniques. Speciﬁcally, we will schedule & load balance the computational load across heterogeneous CPUs, various NUMA domains (e.g., multiple sockets or memory controllers, multi-channel DRAM, and NV-RAM), and between CPUs and GPUs.
To achieve correctness, we will examine and verify the eﬀectiveness of the new MPI 4.0 and beyond standard support for fault tolerance, in addition to selective particle replication  (SPR) and optimal checkpointing (to NV-RAM or SSD).
To ensure performance portability & reproducibility, we will benchmark SPH-EXA1’s performance on a wide variety of platforms, as well as build oﬀ-the-shelf SPH-EXA containers that can easily be deployed with no additional setup required. This will also enlarge the SPH-EXA code user base.
The primary advantage of the SPH-EXA2 project is its scientiﬁc interdisciplinarity. The project involves computer scientists, computer engineers, astrophysicists, and cosmologists. This is complemented by a holistic co-design, which involves applications (cosmology, astrophysics, CFD), algorithms (SPH, domain decomposition, load balancing, scheduling, fault tolerance, etc.), and architectures (CPUs, GPUs, etc.) as opposed to the traditional binary software-hardware co-design.
The methodology employed to achieve the goal and objectives of SPH-EXA2 is a unique combination of:
(a) State-of-the-art SPH implementation. The SPH-EXA frameworkill integrate the most recent advances in the SPH technique (namely, pairing resistant interpolation kernels, accurate evaluation of gradients via an integral formalism, adaptive generalized volume elements, and artiﬁcial viscosity switches), which are at the core of the current SPH-EXA mini-app.
(b) State-of-the-art computer science methods. Domain decomposition for hybrid architectures (CPUs and GPUs), preserving data locality (by locality-aware domain traversal using space-ﬁlling curves), adaptive (not only dy-namic) load balancing, and failure (e.g., permanent) and error (e.g., silent) detection and correction will be part of SPH-EXA2.
(c) Trillion particles simulations. The target for SPH-EXA2 is to be the ﬁrst SPH code to simulate 1012 particles on hybrid Tier-0 computing architectures.
(d) Ease of use and adoption. A key design goal for SPH-EXA2 is to develop it as a heavily-templated header-only code and to minimize, as much as possible, dependencies to third-party software libraries.
The impact of the SPH-EXA2 project is not limited to Cosmology and Astrophysics. The implementation of the SPH technique will target future exascale infrastructures and result in an exascale-ready hydrodynamics code to support the long term vision of the SPH-EXA consortium. Therefore, its inﬂuence on all ﬁelds that employ SPH (namely, geodynamics, hydraulic engineering, marine engineering, ballistics, physiology, chemistry, aeronautics, robotics, and others) is expected to be signiﬁcant.
SPH-EXA2 will also have a signiﬁcant impact on the computer science community. The complex compu-tational requirements of Cosmology and Astrophysics simulations demand highly scalable algorithms that rely on (mul-tilevel) parallel programming models that are both compute-centric and data-aware. The challenges found when scaling the current approaches for memory management, domain decomposition, global and local synchronization, scheduling & load balancing, and fault tolerance will be addressed in the SPH-EXA2 project and will yield signiﬁcant contributions in the High Performance Computing (HPC) ﬁeld. Moreover, SPH-EXA2 will uncover untapped performance potential as well as limitations in the current compute-centric and data-unaware parallel programming models.
The success of SPH-EXA2 will be in producing the ﬁrst (to the best of our knowledge) PASC-funded Tier-0 SPH code. The SPH-EXA2 will produce a code that is easily usable by other scientists. This accessibility is strengthened by the adoption of the newest standards for parallel programming which include: C++20, CUDA 11.x, OpenMP 5.1, MPI 4.x, and OpenACC 2.x. This transition guarantees a long lifetime of the SPH-EXA framework and a more eﬃcient implementation in particular cases.