# High Performance Computing Strategies for Boundary Value Problems¶

Ever wonder if a GPU would make your code faster? Fast enough to justify the expense to your manager, adviser, or funding agency? This project can help answer your questions!

The example codes in this repository implement the same basic algorithm using whichever of the mainstream accelerator programming methods apply. Running the code on different parallel hardware configurations — CPU threading, GPU offloading, and CPU coprocessing — provides a benchmark of these tools using common computational materials science workloads. Comparing performance against the serial baseline will help you make informed decisions about which development pathways are appropriate for your scientific computing projects. Note that the examples do not depend on a particular simulation framework: dependencies are kept minimal, and the C functions are kept as simple as possible to enhance readability for study and reusability in other codes. The goal here is to learn how to use accelerators for materials science simulations, not to enhance or promote any particular software package.

## Accelerator Languages¶

There are six mainstream approaches to shared-memory parallel programming, with varying coding complexity and hardware dependencies:

MIMD-capable threading for multi-core CPU architectures. Challenging to properly implement, but with ample opportunity to tune performance. Provided by all compilers and compatible with any hardware configuration.

OpenMP

Loop-level parallelism for multi-core CPU architectures. Simple to implement for SIMD programs, but with little opportunity for performance tuning. Implementation simply requires prefixing target loops with #pragma directives. Provided by all compilers and compatible with any hardware configuration.

Loop-level parallelism for multi-core CPU architectures using C++. Similar to OpenMP, but requires a wrapper around parallel regions that is more complicated than a one-line #pragma. This provides more direct opportunities for performance tuning. Available as an open-source library.

OpenACC

Loop-level massive parallelism for GPU architectures. Like OpenMP, implementation requires prefixing target code with #pragma directives, with little opportunity for performance tuning. Provided in compilers from Cray, PGI, and GNU; depends on a compatible graphics card, drivers, and CUDA or OpenCL library installation.

CUDA

General-purpose massive parallelism for GPU architectures. Like POSIX threading but for GPUs, provides low-level capabilities and ample opportunity for performance tuning. Requires a purpose-built compiler (nvcc, gpucc), libraries, and a compatible graphics card or accelerator.

Generically speaking, OpenMP and OpenACC provide low barriers for entry into acceleration; CUDA and OpenCL require high investments for hardware and compilers, but offer the greatest capabilities for performance and optimization of a specific application. CUDA hardware can be emulated on the CPU using the MCUDA framework. Proof-of-concept trials on GPU and KNL hardware can be run on Amazon’s EC2, Rescale’s ScaleX, and equivalent HPC cloud computing platforms. Most of the current generation of research supercomputers contain GPU or KNL accelerator hardware, including Argonne National Labs’ Bebop, NERSC Cori, TACC Stampede2, and XSEDE.

## Basic Algorithm¶

Diffusion and phase-field problems depend extensively on the divergence of gradients, e.g.

$\frac{\partial c}{\partial t} = \nabla\cdot D\nabla c$

When D is constant, this simplifies to

$\frac{\partial c}{\partial t} = D\nabla^2 c$

This equation can be discretized, e.g. in 1D:

$\frac{\Delta c}{\Delta t} \approx D\left[\frac{c_{+} - 2c_{\circ} + c_{-}}{\left(\Delta x\right)^2}\right]$

This discretization is a special case of convolution, wherein a constant kernel of weighting coefficients is applied to an input dataset to produce a transformed output.

1D Laplacian

1

-2

1

2D Laplacian

5-point stencil

0

1

0

1

-4

1

0

1

0

2D Laplacian

9-point stencil*

1

4

1

4

-20

4

1

4

1

* This canonical 9-point (3×3) stencil uses first- and second-nearest neighbors. There is a 9-point (4×4) form that uses first- and third-nearest neighbors, which is also implemented in the source code; it is less efficient than the canonical form.

In addition, computing values for the next timestep given values from the previous timestep and the Laplacian values is a vector-add operation. Accelerators and coprocessors are well-suited to this type of computation. Therefore, to demonstrate the use of this hardware in materials science applications, these examples flow according to the following pseudocode:

Start
Allocate arrays using CPU
Apply initial conditions to grid marked "old" using CPU
While elapsed time is less than final time
Do
Apply boundary conditions using CPU
Compute Laplacian using "old" values using accelerator
Solve for "new" values using "old" and Laplacian values using accelerator
Increment elapsed time by one timestep
If elapsed time is an even increment of a specified interval
Then
Write an image file to disk
Endif
Done
Write final values to disk in comma-separated value format
Free arrays
Finish


### Source Code Documentation¶

You are encouraged to browse the source for this project to see how it works. This project is documented using Doxygen, which can help guide you through the source code layout and intent. This guide is included as hiperc_guide.pdf. To build the documentation yourself, with Doxygen, LaTeX, and Make installed, cd into doc and run make. Then browse the source code to your heart’s content.

## Running the Demonstration Programs¶

This repository has a flat structure. Code common to each problem type are lumped together, e.g. in common-diffusion. The remaining implementation folders have three-part names: architecture-threading-model. To compile code for your setup of interest, cd into its directory and run make (note that this will not work in the common folders). If the executable builds, i.e. make returns without errors, you can make run to execute the program and gather timing data. If you wish to attempt building or running all the example codes, execute make or make run from this top-level directory: it will recursively call the corresponding make command in every sub-directory.

### What to Expect¶

As the solver marches along, an indicator will display the start time, progress, and runtime in your terminal, e.g.

Fri Aug 18 21:05:47 2017 [• • • • • • • • • • • • • • • • • • • •] 0h: 7m:15s

If the progress bar is not moving, or to check that the machine is working hard, use a hardware monitoring tool. Here is a brief, definitely not comprehensive list of options:

• CPU: any system monitor provided by your operating system will work. Look for CPU utilization greater than 100%, but moderate memory consumption. On GNU/Linux systems, htop provides a rich interface to running processes in the terminal, which is helpful if you’re running remotely.

• GPU: use a GPU monitor designed for your hardware. Some options include nvidia-smi, radeontop, and intel_gpu_top.

• KNL: the same monitor used for the CPU should also report load on the Knights Landing processor.

As it runs, the code will write a series of PNG image files (diffusion.00?0000.png) in the same directory as the running executable resides; at the end, it will write the final values to diffusion.0100000.csv. It will also write a summary file, runlog.csv, containing the following columns:

• iter: number of completed iterations

• sim_time: elapsed simulation time (with $$\Delta t=1$$, the first two columns are equal)

• wrss: weighted sum-of-squares residual between the numerical values and analytical solution

• conv_time: cumulative real time spent computing the Laplacian (convolution)

• step_time: cumulative real time spent updating the composition (time-stepping)

• IO_time: cumulative real time spent writing PNG files

• soln_time: cumulative real time spent computing the analytical solution

• run_time: elapsed real time

At timestep 10,000 the expected wrss=0.002895 (0.2%) using the 5-point stencil; the rendered initial and final images should look like these (grayscale, 0 is black and 1 is white):

$$t=0\cdot\Delta t$$

$$t=10000\cdot\Delta t$$

The boundary conditions are fixed values of 1 along the lower-left half and upper-right half walls, no flux everywhere else, with an initial value of 0 everywhere. These conditions represent a carburizing process, with partial exposure (rather than the entire left and right walls) to produce an inhomogeneous workload and highlight numerical errors at the boundaries.

If your compiler returns warnings or errors, your simulation results do not look like this, or if wrss at $$t=10000\cdot\Delta t$$ is greater than 0.5% or so, something may be wrong with the installation, hardware, or implementation. Please file an issue and share what happened. You probably found a bug!

Note that a flat field of zeros at $$t=10000\cdot\Delta t$$, about as wrong an answer as possible, gives wrss=0.07493 (7.5%) relative to the analytical solution. Small differences in wrss indicate large errors.

## Reusing the Demonstration Code¶

The flat file structure is intended to make it easy for you to extract code for modification and reuse in your research code. To do so, copy the three-part folder corresponding to your setup of interest, e.g. gpu-cuda-diffusion, to another location (outside this repository). Then copy the contents of the common folder it depends upon, e.g. common-diffusion, into the new folder location. Finally, edit the Makefile within the new folder to remove references to the old common folder. This should centralize everything you need to remix and get started in the new folder.

## Completed Examples¶

### Diffusion Equation¶

• CPU - analytical - serial - OpenMP - TBB

• GPU - CUDA - OpenAcc - OpenCL

• CPU - OpenMP

• GPU - CUDA

## Contributions and Contact¶

Forks of this git repository are encouraged, and pull requests providing patches or implementations are more than welcome. Please review the Contributing Guidelines. Questions, concerns, and feedback regarding this source code should be addressed to the community on Gitter, or directly to the developer (Trevor Keller).

## Disclaimer¶

Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose.