32 research outputs found
Improving initialization and evolution accuracy of cosmological neutrino simulations
Neutrino mass constraints are a primary focus of current and future
large-scale structure (LSS) surveys. Non-linear LSS models rely heavily on
cosmological simulations -- the impact of massive neutrinos should therefore be
included in these simulations in a realistic, computationally tractable, and
controlled manner. A recent proposal to reduce the related computational cost
employs a symmetric neutrino momentum sampling strategy in the initial
conditions. We implement a modified version of this strategy into the
Hardware/Hybrid Accelerated Cosmology Code (HACC) and perform convergence tests
on its internal parameters. We illustrate that this method can impart
numerical artifacts on the total matter field on small
scales, similar to previous findings, and present a method to remove these
artifacts using Fourier-space filtering of the neutrino density field.
Moreover, we show that the converged neutrino power spectrum does not follow
linear theory predictions on relatively large scales at early times at the
level, prompting a more careful study of systematics in particle-based
neutrino simulations. We also present an improved method for backscaling linear
transfer functions for initial conditions in massive neutrino cosmologies that
is based on achieving the same relative neutrino growth as computed with
Boltzmann solvers. Our self-consistent backscaling method yields sub-percent
accuracy in the total matter growth function. Comparisons for the non-linear
power spectrum with the Mira-Titan emulator at a neutrino mass of
are in very good agreement with the expected level
of errors in the emulator and in the direct N-body simulation.Comment: 33 pages, 8 figures, 1 table. To be submitted to JCA
Numerical Discreteness Errors in Multi-Species Cosmological N-body Simulations
We present a detailed analysis of numerical discreteness errors in
two-species, gravity-only, cosmological simulations using the density power
spectrum as a diagnostic probe. In a simple setup where both species are
initialized with the same total matter transfer function, biased growth of
power forms on small scales when the solver force resolution is finer than the
mean interparticle separation. The artificial bias is more severe when
individual density and velocity transfer functions are applied. In particular,
significant large-scale offsets in power are measured between simulations with
conventional offset grid initial conditions when compared against converged
high-resolution results where the force resolution scale is matched to the
interparticle separation. These offsets persist even when the cosmology is
chosen so that the two particle species have the same mass, indicating that the
error is sourced from discreteness in the total matter field as opposed to
unequal particle mass. We further investigate two mitigation strategies to
address discreteness errors: the frozen potential method and softened
interspecies short-range forces. The former evolves particles under the
approximately "frozen" total matter potential in linear theory at early times,
while the latter filters cross-species gravitational interactions on small
scales in low density regions. By modeling closer to the continuum limit, both
mitigation strategies demonstrate considerable reductions in large-scale power
spectrum offsets.Comment: Accepted for publication in MNRA
A Performance-Portable SYCL Implementation of CRK-HACC for Exascale
The first generation of exascale systems will include a variety of machine
architectures, featuring GPUs from multiple vendors. As a result, many
developers are interested in adopting portable programming models to avoid
maintaining multiple versions of their code. It is necessary to document
experiences with such programming models to assist developers in understanding
the advantages and disadvantages of different approaches.
To this end, this paper evaluates the performance portability of a SYCL
implementation of a large-scale cosmology application (CRK-HACC) running on
GPUs from three different vendors: AMD, Intel, and NVIDIA. We detail the
process of migrating the original code from CUDA to SYCL and show that
specializing kernels for specific targets can greatly improve performance
portability without significantly impacting programmer productivity. The SYCL
version of CRK-HACC achieves a performance portability of 0.96 with a code
divergence of almost 0, demonstrating that SYCL is a viable programming model
for performance-portable applications.Comment: 12 pages, 13 figures, 2023 International Workshop on Performance,
Portability & Productivity in HP