This week’s Spotlight is on Pierre Wahl, a PhD student at Vrije Universiteit Brussel.

As a member of the Brussels Photonics Team (B-PHOT), he designs energy-efficient optical interconnects and works closely with the NVIDIA Application Lab at the Forschungszentrum Jülich.

Pierre used CUDA to develop B-CALM, a GPU-accelerated Finite Difference Time Domain (FDTD) simulator.

Below is an excerpt from our interview (you can read the complete Spotlight here):

**NVIDIA: Pierre, what is B-CALM?**

** Pierre:** B-CALM stands for Belgium-California Light Machine and is an FDTD simulator to numerically solve electromagnetic problems using the fundamental Maxwell’s equations.

FDTD is particularly useful for problems where the electromagnetic waves interact with objects that are the same order of magnitude in size as the wavelength. Those problems can be very computationally intensive, especially when simulating the interaction of electromagnetic waves with metals, which is at the core of my research.

**NVIDIA: How have GPUs helped you in your research?
**

**Pierre:**I research avenues to make on-chip optical interconnects very energy efficient, because safely extracting the heat generated by regular metallic interconnects from chips gets increasingly difficult with the ever-increasing bandwidth requirements.

However, for optical interconnects to be competitive, optoelectronic components (modulators/photodetectors) have to have a very low electrical capacitance and must therefore be made very small. By using metals to guide and confine light (also referred to as plasmonics) optoelectronic components can have a size that is only a fraction of the wavelength and hence a very small electrical capacitance.

To be able to design coupling structures to sub-wavelength plasmonic optoelectronic components, very detailed and computationally intensive electromagnetic simulations have to be performed and a very fine grid has to be utilized.

Using our CPU FDTD code, we quickly ran into computational limitations. B-CALM was born by porting our FDTD code to GPUs using CUDA and we obtained an 80X speed-up.

Simulations that used to run overnight now take only ten minutes. This speed-up allowed us to optimize plasmonic coupling structures iteratively, which we were not able to do before porting our code to GPUs.

**NVIDIA: What ****types of parallel algorithms are being implemented?
**

**Pierre:**Fundamentally B-CALM is a Finite Difference algorithm with a halo of 1 that is parallelized using very fine-grained domain decomposition. Each CUDA thread computes only a very small part of the simulation domain. A white paper by Paulius Micikevicius of NVIDIA on the general implementation of Finite Difference algorithms that can be applied to many problems was very helpful.

**NVIDIA: What approaches did you find the most useful for CUDA development?
**

**Pierre:**I had little experience in parallel programming before I started to learn CUDA. Personally, I found the documented examples (such as matrix multiplication) in the SDK very useful to learn the fundamentals. Also, the CUDA Toolkit is very well documented so you can learn about the intricacies only when you need them while you are already working on your project.

**NVIDIA: What advice would you offer others in your field looking at CUDA?**

**Pierre:** CUDA C is really a great language to program on GPUs and understanding the fundamentals is enough to get started. Before you get started, take time to consider whether your problem is well-suited for GPUs. As a rule of thumb, I would say that well-suited problems need to access most of their data in a regular, and preferably non-data dependent, way.

**NVIDIA: Describe your partnership with the NVIDIA Application Lab at Jülich.**

** Pierre:** The NVIDIA Application Lab at Jülich helped us take B-CALM to the next level by assisting us in implementing a CUDA-aware MPI layer so that B-CALM could run on GPU clusters and by helping us to optimize the most important kernels. Their insights in terms of domain decomposition proved to be invaluable as well. In its current form, B-CALM scales up to 32 GPUs, but our models predict it could scale up to hundreds of GPUs.

Read the full interview. Read more CUDA Spotlights.