# Dark-era: Dataflow Algorithm aRchitecture co-design of SKA pipeline for Exascale Radio Astronomy

Nicolas Gac, Ophélie Renaud

► To cite this version:

Nicolas Gac, Ophélie Renaud. Dark-era: Dataflow Algorithm aRchitecture co-design of SKA pipeline for Exascale Radio Astronomy. SaLF 2025 - 11th Annual conference on Science At Low Frequencies, Dec 2025, Orléans, France. hal-05430702

HAL Id: hal-05430702

https://hal.science/hal-05430702v1

Submitted on 23 Dec 2025

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Dataflow for SDP Prototyping

Two projects inside

<figure><img src="image_2.png" /></figure>

computing lab

<figure><img src="image_3.png" /></figure>

**Design Space Exploration through SimSDP**

Nicolas Gac, Univ. Paris Saclay, SATIE

Ophélie Renaud, IRISA/SATIE

<figure><img src="image_4.png" /></figure>

**Sustainable & Energy Aware Methods for SKA**

Mickael Dardaillon, INSA Rennes, IETR

December 12 2025, SALF, Orléans
16 french research teams federated on astronomy computing in a laboratory
*outside the walls*

<figure><img src="image_5.png" /></figure>
¹IETR (INSA Rennes), ²IRISA (ENS Rennes), ³L2S (CentraleSupélec, Univ. Paris-Saclay), ⁴Lagrange (Univ. Côte d'Azur), ⁵ORN (Obs. Paris)

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

ANR-20-CE46-0001-01

Daniel Charlet**5 (IJCLab), Léo Cosseron², Karol Desnos¹, Mickael Dardaillon¹, André Ferrari⁴, Chiara Ferrari⁴, Nicolas Gac³ (now SATIE), Adrien Gougeon², David Guibert* (Eviden), Shan Mignot⁴, Nicolas Monnier³ (now IAS), Jean-François Nezan¹, François Orieux³, Simon Prunet⁴, Martin Quinson², Erwan Raffin* (Eviden), Ophélie Renaud¹ then 2, Cyril Tasse**5 (LUX), Cédric Viou⁵, Sunrise Wang³ then 4

December 12 2025, SALF - Orléans

https://dark-era.pages.centralesupelec.fr
<figure><img src="image_8.png" /></figure>

credit: C. Ferrari, The SKAO: a new observatory to explore the radio sky, SKAO France.
SDP supercomputer

* Huge computing requirements to process a real-time streaming data

* Limited energy budget, *nr. 1 MW for each SDP*

* Huge algorithm model gap between programming languages used by astronomers and HPC developers, *Python vs MPI/CUDA/*...
## SDP supercomputer

* Huge computing requirements to process a realtime streaming data

* Limited energy budget, nr. 1 MW for each SDP

* Huge algorithm model gap between programming languages used by astronomers and HPC developers, Python vs MPI/CUDA/...

## A software/hardware co-design challenge

* New complex scientific dataflow algorithms under development

* Not-yet-existing large scale heterogeneous computing platform
## SDP supercomputer

* Huge computing requirements to process a **realtime streaming data**

* Limited energy budget, *nr. 1 MW for each SDP*

* **Huge algorithm model gap** between programming languages used by astronomers and HPC developers, *Python vs MPI/CUDA/*...

## A software/hardware co-design challenge

* New complex scientific dataflow algorithms under development

* Not-yet-existing **large scale heterogeneous computing** platform

⇒ Call for rapid prototyping tools providing **early time**, memory and energy performance assessments
## SimSDP

A rapid prototyping tool providing exascale simulations from dataflow algorithm descriptions.

⇒ Build as a co-simulation tool

**Input** : Dataflow algorithm description + architecture description

**Output** : Computing time + memory footprint + energy consumption

<figure><img src="image_9.png" /></figure>
# Design Space Exploration (DSE)

## Exploration of # major loop algorithms

* DFT : Direct Fourier Transform

* FFT : Gridding/Degidding with Fast Fourier Transform

* G2G : Proposed algorithm with grid/degrid designed a **single operator**

## Exploration of # distribution strategies

* Frequency distribution

* Facet distribution : DDFacet

* **Baseline distribution** [Wang2025]

## Exploration of # accelerators

### GPU or Low-power accelerators like FPGA or Kalray MPPA

S. Wang et al.. A Decentralized Framework for Radio-interferometric Image Reconstruction. The Astronomical Journal, 2025
DARK ERA

anr®

ANR-20-CT46-0001-01

<figure><img src="image_10.png" /></figure>

LAGRANGE

<figure><img src="image_11.png" /><figcaption>C. Tasté
(Obs-Paris)</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>S. Prunet
(Lagrange)</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>A. Ferrari
(Lagrange)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>F.Orieux (L2S)</figcaption></figure>

L2S

Laboratoire
Signaux &
Systèmes

SATIE

<figure><img src="image_15.png" /><figcaption>N.Gac (L2S then SATIE)</figcaption></figure>

SKA
FRANCE
SQUARE KILOMETRE ARRAY

Astronomy

<figure><img src="image_16.png" /></figure>

Station de
Radioastronomie
de Nançay

<figure><img src="image_17.png" /><figcaption>C. Viou (Obs Nançay) D. Charlet (IJCLab)</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>C.Ferrari (Lagrange)</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>M. Monnier
(L2S/Obs Paris,
now IA5)</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>O. Renaud
(IETR then IRISA/SATIE)</figcaption></figure>

<caption>S. Wang (L2S then
Lagrange)</caption>

Architecture design for embedded system

<figure><img src="image_21.png" /><figcaption>M. Dardailon
(IETR)</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>JF. Nezan
(IETR)</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>K. Desnos
(IETR)</figcaption></figure>

<figure><img src="image_24.png" /></figure>

EVIDEN

<figure><img src="image_25.png" /><figcaption>E. Raffin (Eviden)
&
D. Guibert (Eviden)</figcaption></figure>

<caption>D. Guibert (Eviden)</caption>

HPC

<figure><img src="image_26.png" /><figcaption>A. Gougeon
(IRISA)</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>M. Quinson
(IRISA)</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>L. Cosseron
(IRISA)</figcaption></figure>

<figure><img src="image_29.png" /></figure>
Inverse problem resolution

<figure><img src="image_30.png" /></figure>

v Measured visibilities

S Degidding

S† Gridding

F Fast Fourier Transform

F† Inverse Fast Fourier Transform

Ψ Deconvolution

x(k) Sky estimate
Inverse problem resolution

<figure><img src="image_31.png" /></figure>

$$ \nabla J_{STD}(x) = F^{\dagger} S^{\dagger} (v - SFx) $$
Major loop with the hessian $F^\dagger S^\dagger SF$

<figure><img src="image_32.png" /></figure>

$$ 
\begin{aligned}
\nabla J_{STD}(x) &= F^\dagger S^\dagger (v - SFx) \\
&= F^\dagger S^\dagger v - F^\dagger S^\dagger SFx \\
&= d - F^\dagger S^\dagger SFx
\end{aligned}
$$

d dirty image
Major loop with $S_{G2G} = S^\dagger S$ operator

<figure><img src="image_33.png" /></figure>

$$ 
\begin{align*}
\nabla J_{STD}(x) &= F^{\dagger} S^{\dagger}(v - SFx) \\
&= F^{\dagger} S^{\dagger} v - F^{\dagger} S^{\dagger} SFx \\
&= d - F^{\dagger} S_{G2G} Fx
\end{align*}
$$

d dirty image
$S_{G2G}$ Grid to Grid operator
Degidding ($S^\dagger$) and Gridding ($S$) decomposition

<figure><img src="image_34.png" /></figure>

Visibilités

<figure><img src="image_35.png" /></figure>

Décomposition du *degridding*

<figure><img src="image_36.png" /></figure>

Décomposition du *gridding*

[Monnier2023] Nicolas Monnier, François Orieux et al.. Fast Grid to Grid Interpolation for Radio Interferometric Imaging. Astronomy and Computing, 2023
$S_{G2G} = S^{\dagger}S$ : Gridding and degridding as a **single operator**

<figure><img src="image_37.png" /></figure>
G2G : GPU performance

<figure><img src="image_38.png" /></figure>

[Monnier2023] Nicolas Monnier, François Orieux et al.. Fast Grid to Grid Interpolation for Radio Interferometric Imaging. Astronomy and Computing, 2023