master-thesis/thesis/chapters/evaluation.tex

\chapter{Evaluation}
\label{cha:evaluation}

This thesis aims to determine whether one of the two GPU evaluators is faster than the current CPU evaluator. This chapter describes the performance evaluation process. First, the environment in which the performance benchmarks are conducted is explained. Next the individual results for the GPU interpreter and transpiler are presented individually. This section also includes the performance tuning steps taken to achieve these results. Finally, the results of the GPU evaluators are compared to those of the CPU evaluator to answer the research questions of this thesis.

\section{Benchmark Environment}
In this section, the benchmark environment used to evaluate the performance is outlined. To ensure the validity and reliability of the results, it is necessary to specify the details of the environment. This includes a description of the hardware and software configuration as well as the performance evaluation process. With this, the variance between the results is minimised, which allows for better reproducibility and comparability between the implementations.

\subsection{Hardware Configuration}
The hardware configuration is the most important aspect of the benchmark environment. The capabilities of both the CPU and GPU can have a significant impact on the resulting performance. The following sections outline the importance of the individual components as well as the hardware used for the benchmarks.

\subsubsection{GPU}
Especially the GPU is important, as different microarchitectures typically require different optimisations. While the evaluators can generally run on any Nvidia GPU with a compute capability of at least 6.1, they are tuned for the Ampere microarchitecture with a compute capability of 8.6. Despite the evaluators being tuned for this microarchitecture, more modern ones can be used as well. However, additional tuning is required to ensure the evaluators can utilise the hardware to its fullest potential.

Tuning must also be done on a per-problem basis. Especially the number of variable sets can impact how well the hardware is utilised. Therefore, it is important to see which configuration performs the best. In Section \ref{sec:results} a strategy for tuning the configuration to a new problem is described.

\subsubsection{CPU}
Although the GPU plays a crucial role, work is also carried out on the CPU. The interpreter mainly uses the CPU for data transfer and the pre-processing step and is therefore more GPU-bound. However, the transpiler additionally needs the CPU to perform the transpilation step. This step produces a kernel for each expression and also involves sending these kernels to the driver for compilation, a process which is also performed by the CPU. By contrast, the interpreter only has one kernel that needs to be converted into PTX and compiled by the driver only once. Consequently, the transpiler is much more CPU-bound and variations in the used CPU have a much greater impact. Therefore, using a more powerful CPU benefits the transpiler more than the interpreter.

\subsubsection{System Memory}
In addition to the hardware configuration of the GPU and CPU, system memory (RAM) also plays a crucial role. While RAM does not directly contribute to the overall performance, it can have a noticeable indirect impact due to its role in caching. Insufficient RAM forces the operating system to use the page file, which is stored on a much slower SSD. This results in slower cache access, thereby reducing the overall performance of the application.

As seen in the list below, only 16 GB of RAM were available during the benchmarking process. This amount is insufficient to utilise caching to the extent outlined in Chapter \ref{cha:implementation}. More RAM was not available, which means some caching had to be disabled, which will be further explained in Section \ref{sec:results}.

\subsubsection{Hardware}
With the requirements explained above in mind, the following hardware is used to perform the benchmarks for the CPU-based evaluator, which was used as the baseline, as well as for the GPU-based evaluators:
\begin{itemize}
	\item Intel i5 12500
	\item Nvidia RTX 3060 Ti
	\item 16 GB 4400 MT/s DDR5 RAM
\end{itemize}


\subsection{Software Configuration}
Apart from the hardware, the performance of the evaluators can also be significantly affected by the software. Primarily these three software components or libraries are involved in the performance:
\begin{itemize}
	\item GPU Driver
	\item Julia
	\item CUDA.jl
\end{itemize}

Typically, newer versions of these components include performance improvements, among other things. This is why it is important to specify the version which is used for benchmarking. The GPU driver has version \emph{561.17}, Julia has version \emph{1.11.5}, and CUDA.jl has version \emph{5.8.1}. As with the hardware configuration, this ensures that the results are reproducible and comparable to each other.


\subsection{Performance Evaluation Process}
With the hardware and software configuration being set, the process of benchmarking the implementations can be described. The process is designed to simulate the load and scenario these evaluators will be used in. The Nikuradse dataset \parencite{nikuradse_laws_1950} has been chosen as the source of the data. The dataset itself models the laws of flow in rough pipes and provides $362$ variable sets, with each set containing two variables. This dataset has first been used by \textcite{guimera_bayesian_2020} to benchmark a symbolic regression algorithm.

Because only the evaluators are benchmarked, the expressions to be evaluated, need to already exist. Generating the expressions is done, using the exhaustive symbolic regression algorithm proposed by \textcite{bartlett_exhaustive_2024} and the Nikuradse dataset. This ensures that the expressions are exemplary of what needs to be evaluated in a real use-case.

With roughly $250\,000$ expressions, the second-largest set has been used as the first benchmark. This means that all $250\,000$ expressions are evaluated in a single run, which is much more than what would be evaluated in a typical run. This benchmark is designed to show how the evaluators can handle large amounts of data. However, evaluating such high amount of expressions also has some drawbacks as will be explained in Section \ref{sec:results}.

A second benchmark with slight adaptations to the first one is also performed. Because GPUs are very good at executing work in parallel, the number of variable sets is increased in this benchmark. Therefore, the second benchmark consists of the same $250\,000$ expressions, but the number of variable sets has been increased by a factor of four to a total of $1\,4448$.

Lastly a third benchmark will be performed. This benchmark should mimic a realistic load. Therefore, the number of expressions has been reduced to roughly $10\,000$ and the number of variable sets is again $362$. The reason for this benchmark is to demonstrate how the evaluators will most likely perform in a typical run.

All three benchmarks will also simulate a parameter optimisation step, as this is the scenario, these evaluators will be used in. For parameter optimisation, $100$ steps have been used. This means, that all expressions will be evaluated $100$ times. During the benchmark, this process is simulated by re-transmitting the parameters instead of generating new ones. Generating new parameters is not part of the evaluators and is therefore not implemented. However, because the parameters are re-transmitted every time, the overhead of sending the data is taken into account. This is part of the evaluators and additional overhead the CPU implementation does not have and is therefore important to be measured.

\subsubsection{Measuring Performance}
The performance measurements are taken, using the BenchmarkTools.jl\footnote{\url{https://juliaci.github.io/BenchmarkTools.jl/stable/}} package. It is the standard for benchmarking applications in Julia, which makes it an obvious choice for measuring the performance of the evaluators.

It offers extensive support for measuring and comparing results of different implementations and versions of the same implementation. Benchmark groups allow to categorise the different implementations, take performance measurements and compare them. When taking performance measurements, it also supports setting a timeout and most importantly, set the number of samples to be taken. This is especially important, as it ensures to produce stable results by combating run-to-run variance. For this thesis, a sample size of $50$ has been used. This means that each of the previously-mentioned benchmarks, gets executed $50$ times. 

\section{Results}
\label{sec:results}
talk about what we will see now (results only for interpreter, then transpiler and then compared with each other and the CPU interpreter)

BECAUSE OF RAM CONSTRAINTS, CACHING IS NOT USED TO THE FULL EXTEND AS IN CONTRAST TO HOW IT IS EXPLAINED IN THE IMPLEMENTATION CHAPTER. I hope I can cache the frontend. If only the finished kernels can not be cached, move this explanation to the transpiler section below and update the reference in subsubsection "System Memory"

% TODO: Do one run with 
% 	- 250k expressions
% 	- increase variables to be 4 times as large (nr. of varsets should be 362 * 4)
% 	- compare CPU with interpreter (probably also transpiler, but only to see if it takes even longer, or roughly the same considering that resources are still available on the GPU)
% 	- This should demonstrate that bigger varsets lead to better performance (although I kinda doubt considering that the hardware is already fully utilised)

% TODO: Do another run with
% 	- 10 000 expressions choose the file that is closest to these 10k
% 	- nr. var sets stays the same
% 	- compare CPU, interpreter and transpiler
% 	- do a second run with kernel compilation being performed before parameter optimisation step (as 10 000 expressions shouldn't fill up the memory as much)
% 	- depending on how much time I have, also do a run with 4 times as much var sets (if this is done, adapt the above subsection "Performance Evaluation Process")

\subsection{Interpreter}
Results only for Interpreter (also contains final kernel configuration and probably quick overview/recap of the implementation used and described in Implementation section)
\subsection{Performance Tuning}
Document the process of performance tuning

Initial: no cache; 256 blocksize; exprs pre-processed and sent to GPU on every call; vars sent on every call; frontend + dispatch are multithreaded

1.) Done before parameter optimisation loop: Frontend, transmitting Exprs and Variables (improved runtime)
2.) tuned blocksize to have as little wasted threads as possible (new blocksize 121 -> 3-blocks -> 363 threads but 362 threads needed per expression) (128 should lead to the same results. Talk here a bit what to look out for, so block-size should be a multiple of 32 and should divide the nr. of varsets as best as possible to a whole number without going over)
3.) Minor optimisations. Reduced stacksize; reduced memory allocations on the CPU; reduced GC pressure

CPU and GPU are almost all the time at 100\% utilisation (GPU every now and then drops to 70\%), meaning it is quite balanced.
Uncached but multithreaded frontend only makes up a small percentage of the total runtime (optimisations there are not really needed, which is good because enabling caching took up too much RAM)
Most of the time is spent doing the parameter optimisation step

\subsection{Transpiler}
Results only for Transpiler (also contains final kernel configuration and probably quick overview/recap of the implementation used and described in Implementation section



\subsection{Performance Tuning}
Document the process of performance tuning

Initial: no cache; 256 blocksize; exprs pre-processed and transpiled on every call; vars sent on every call; frontend + transpilation + dispatch are multithreaded

1.) Done before parameter optimisation loop: Frontend, transmitting Exprs and Variables (improved runtime)
2.) All expressions to execute are transpiled first (before they were transpiled for every execution, even in parameter optimisation scenarios). Compilation is still done every time, because too little RAM was available (compilation takes the most time, so this is only a minor boost). Also tried blocksize of 121. However, kernel itself is very fast anyway, so this didn't make a difference (further proof that the CPU is the bottleneck here)

CPU at 100\% GPU at around 30\%. Heavily CPU bottlenecked. Mainly due to PTX compilation taking by far the longest (while kernels are finished more or less instantly)

\subsection{Comparison}
Comparison of Interpreter and Transpiler as well as Comparing the two with CPU interpreter

talk about that compute portion is just too little. Only more complex expressions with higher var set count benefit well (make one or two performance evaluations, with 10 larger expressions and at least 1k var sets and present that here as point for that statement)