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LeeoBianchi authored May 18, 2024
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# Summary

Spherical Harmonic Transforms (SHTs) can be seen as Fourier Transforms' spherical, two-dimensional counterparts, casting real-space data to the spectral domain and vice versa.
As in Fourier analysis a function is decomposed into a set of amplitude coefficients, an SHT allows to decompose any spherically-symmetric field, defined in real space, into a set of complex harmonic coefficients $a_{\ell, m}$, commonly referred to as alms, each quantifying the contribution of the corresponding spherical harmonic function.
As in Fourier analysis where a function is decomposed into a set of amplitude coefficients, an SHT allows any spherically-symmetric field, defined in real space, to be decompose into a set of complex harmonic coefficients $a_{\ell, m}$, commonly referred to as alms, where each quantifies the contribution of the corresponding spherical harmonic function.

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@danielskatz

danielskatz May 19, 2024

"to be decompose" should be "to be decomposed" - sorry if this came from a typo on my part


SHTs are important for a wide variety of theoretical and practical scientific applications, including particle physics, astrophysics, and cosmology.
However, SHTs are generally computationally expensive operations and thus often constitute the *bottleneck* of the scientific software they are part of.
For this reason, much effort has been spent over the last couple of decades to obtain fast and efficient SHTs implementations.
For this reason, much effort has been spent over the last couple of decades to obtain fast and efficient SHT implementations.
In such a setting, parallel computing naturally comes into play, especially for time-consuming software to be run on large High-Performance Computing (HPC) clusters.

The Julia package `HealpixMPI.jl` constitutes an extension package of `Healpix.jl` [@Healpix_jl], efficiently parallelizing its SHT-related functionalities.
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In particular, a strong-scaling scenario is analyzed: given a problem of fixed size, the wall time improvement is measured as the number of cores exploited in the computation is increased.

To obtain a reliable measurement of massively parallel spherical harmonics wall time is certainly nontrivial, especially for tests employing a high number of cores; intermittent operating system activity (aka, jitter) can significantly distort the measurement of short time scales.
For this reason, the benchmark tests were carried out by timing a batch of 20 `alm2map` + `adjoint_alm2map` SHTs pairs.
For this reason, the benchmark tests were carried out by timing a batch of 20 `alm2map` + `adjoint_alm2map` SHT pairs.
For reference, the scaling shown here is relative to unpolarized spherical harmonics with $\mathrm{N}_\mathrm{side} = 4096$ and $\ell_{\mathrm{max}} = 12287$ and were carried out on the [Hyades cluster](https://www.mn.uio.no/astro/english/services/it/help/basic-services/compute-resources.html) of the University of Oslo.
The benchmark results are quantified as the wall time multiplied by the total number of cores, shown in a 3D plot (\autoref{fig:bench}) as a function of the number of local threads and MPI tasks (always one per node).

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