phd-defense/slides/20_foundations/main.tex

\section{Foundations}

\begin{frame}{Bird's eye view of a CPU}
    \centering
    \includegraphics[height=0.94\textheight]{cpu_big_picture.svg}
\end{frame}

\begin{frame}{Possible bottlenecks}
    \begin{columns}
        \begin{column}{0.37\textwidth}
            \begin{center}
                \includegraphics[width=\textwidth]{cpu_big_picture_truncate.svg}
            \end{center}
        \end{column}
        \hfill
        \begin{column}{0.62\textwidth}
            \begin{tightitemize}{0pt}
                \begin{itemize}
                    \item \alert{Frontend:} \uops{} not issued fast enough
                        \bigskip

                    \item \alert{Backend:} saturated execution units
                        \bigskip

                    \item \alert{Dependencies:} computation is stalled waiting
                        for previous results
                \end{itemize}
            \end{tightitemize}
        \end{column}
    \end{columns}
\end{frame}

%\begin{frame}{Dependencies and the ROB}
%    \begin{columns}
%        \begin{column}{0.35\textwidth}
%            \begin{center}
%                \includegraphics[width=\textwidth]{cpu_frontend.svg}
%            \end{center}
%        \end{column}
%        \hfill
%        \begin{column}{0.64\textwidth}
%            \begin{tightitemize}{0pt}
%                \begin{itemize}
%                    \item Dependencies can stall execution
%                    \item Maybe instructions further down can be executed right now?
%                \end{itemize}
%                \begin{center}
%                    \textbf{\alert{$\to$ Out-of-Order CPUs}}
%                \end{center}
%                \begin{itemize}
%                    \item ROB: circular buffer of \uops{}
%                    \item First possible instruction is issued
%                \end{itemize}
%            \end{tightitemize}
%        \end{column}
%    \end{columns}
%\end{frame}

%\begin{frame}{How do we get insights from this complex system?}
%    \textbf{Hardware counters}
%    \begin{itemize}
%        \item Built-in hardware, counters gathered at runtime
%        \item Very accurate
%        \item Available data varies from model to model
%        \item May not even be available at all
%    \end{itemize}
%
%    \textbf{Simulation?}
%    \begin{itemize}
%        \item A modern CPU is \alert{$\sim$\,100e9 transistors}: very complex
%            models!
%        \item Very expensive, even for manufacturers for design validation
%        \item CPU design is industrial secret $\leadsto$ not available anyway
%        \item \ldots{}\ie{} not feasible.
%    \end{itemize}
%\end{frame}

\begin{frame}{Code analyzers}
    \begin{itemize}
        \item That predict performance of a piece of assembly
        \item Features microarchitectural models
        \item Most often static analyzers
        \item Predict at least the \emph{reverse-throughput} $\cyc{\kerK}$ of a
            kernel $\kerK$ (cycles per iteration)
        \item May derive further useful metrics, \eg{} bottlenecks, by
            inspecting their model at will
    \end{itemize}
\end{frame}

\begin{frame}{What can be analyzed?}
    Pieces of code referred as \alert{``microkernels''}:

    \begin{itemize}
        \item body of an (assumed) infinite loop;
        \item in steady-state;
        \item straight-line code (branches assumed not taken);
        \item L1-resident (memory model is out of scope).
    \end{itemize}

    Reasonable hypotheses for the category of codes worth optimizing this way!
\end{frame}

\begin{frame}{Existing code analyzers}
    \begin{itemize}
        \item Intel \alert{\iaca{}}: proprietary, Intel CPUs only. First
            ``good'' code analyzer, now deprecated. Was (is?) widely used.
        \item \alert{\llvmmca{}}: FOSS, production-grade, many
            microarchitectures. Based on data from the \texttt{llvm} compiler.
        \item \alert{\uica{}} and \alert{\uopsinfo{}}: research, good accuracy.
            Intel CPUs.
        \item \alert{\ithemal{}}: machine-learning based.
    \end{itemize}
    \bigskip

    Except Ithemal, \alert{all} are (to some extent) based on manually-made
    models!\\
    \pause{}
    \bigskip{}
    \begin{center}
        \textbf{\alert{Ambition:}} \alert{automated} model generation.
    \end{center}
\end{frame}

\begin{frame}{When I started my PhD\ldots}
    \centering
    \includegraphics[height=0.9\textheight]{patate_placeholder.jpg}
\end{frame}