This page was last updated on April 27, 2022.
The most recent draft can be found here; readers are encouraged to leave comments and suggestions, or to contact members of the P3HPC committee via e-mail.
Motivation and Goals
The intersection of performance, portability and productivity (P3) is of broad interest to many in the high performance computing (HPC+AI) community. We invite submissions from a diverse group of authors hailing from a wide range of fields – compiler, language and runtime experts; performance engineers; and domain scientists – and it is important to reflect this diversity of experience in the P3HPC program committee.
Such diversity brings with it some challenges; experts in different fields may disagree on which aspects of P3 are most challenging or most important, and there have been instances where it has been difficult to ensure consistency across submission quality and reviewer feedback.
The goals of this document are twofold:
To establish the key shared principles and expectations shared by all members of the P3HPC community, to provide clearer guidance to authors and reviewers.
To capture the remaining sources of disagreement within the community, to help inform future research directions and provide helpful clarity to drive the community even closer to a shared vision.
Adherence to Scientific Principles
P3 practitioners should avoid vague claims like “good performance” or “higher productivity”. To facilitate discussion and ensure forward progress, papers should support claims with data, and justify why the selected data supports the claim(s).
Comparisons that are both apples to apples, that use benchmarks or workloads that are meaningfully representative, and that are made on current hardware and software generations make a much more compelling basis for P3 claims. This does not exclude cross-generational studies showing the value of techniques in creating code that is portable over time, but those studies are stronger when using a control sample (e.g. code written for previous generation hardware run on current generation hardware to demonstrate cross-generational portability).
P3 practitioners should state their goals clearly, to assist readers and reviewers in evaluating it.
What is the performance target (and why)?
Is platform-specific code acceptable to the authors and their intended audience, or not? What level of expertise is considered acceptable?
Which aspect of programmer productivity is the focus?
While views may vary in these goals and expectations, assumptions about target audiences should be clearly stated and supportable.
Raw performance numbers and speed-ups provide little insight into an application’s ability to execute across different architectures. P3 practitioners should demonstrate how their achieved performance compares to the effective peak capabilities of each architecture, and/or to the state-of-the-art.
Effective peak capabilities include both hardware capabilities and software tuning. For example, McCalpin Stream Triad is a reasonable measure of attainable peak bandwidth that may be more meaningful than hardware peak specifications, and effective use of available (including platform-specific) programming models is a more meaningful measure of effective performance than ninja coding in assembly unless it occurs in vendor libraries.
It may also be important to consider how achieved performance compares to the state-of-the-art. Demonstrating that an application achieves a high fraction of effective peak is not equivalent to proving that the application is using appropriate algorithms on each architecture. Attention to this distinction is particularly important for high-level abstractions.
Sources of Disagreement
A summary of ongoing discussion from the living draft is presented here. For more information, readers should consult the draft.
Definitions and Metrics
There remains some disagreement about what each of the three Ps means, and how to measure them. Should the community strive for shared definitions and metrics, or is it sufficient that authors provide explanations and justifications?
Another common disagreement is the importance of performance consistency (i.e. spread) across architectures vs architecture utilization. How desirable is an application that achieves the same (low) fraction of achievable peak performance across multiple architectures, or which runs at the same speed everywhere but does not run “fast enough” to be useful?
Discussions of performance typically focus on quantifying performance measurements (e.g. percent of peak), and ignore the inherent efficiency of an algorithm. Is the code doing the minimum number of arithmetic operations, or the minimum number of loads/stores? Is it efficiently parallelized? How much extra work/memory do we incur by threading? How much extra do we pay for synchronization?
An often overlooked component of performance portability is a code’s ability to make use of available parallelism. Different architectures can have wildly different amounts of parallelism that need to be utilized, leading to very different efficient workloads on different platforms. Smaller, lower dimensional workloads may run at peak efficiency on CPU architectures, but are completely incapable of running efficiently on GPU architectures. Does that mean the code isn’t portable? Since available parallelism is tied to problem size, how do strong- and weak-scaling fit into the notion of a code’s performance and portability?
“Productivity” is arguably the hardest P to define and measure. It could mean fewer lines of code, less time to develop a new algorithm, less time to port existing code to another architecture, less time to maintain, and/or less time to onboard a new developer. Are these definitions compatible, and what are the trade-offs? If a code is highly portable and compact (in lines of code) but is extremely complex, is that better than a code that is less portable but easier to debug and maintain?