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Libunwind, formalisation, perf

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Théophile Bastian 2018-09-05 00:43:15 +02:00
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slides/slides.tex
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@ -241,24 +241,6 @@ $1 = 84
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Compilation Strategy}
\begin{frame}{Interface: libunwind}
\begin{itemize}
\item \alert{libunwind}: \textit{de facto} standard library for
unwinding
\item Uses DWARF in background
\item \texttt{libunwind-eh\_elf}: alternative implementation using
\ehelfs{}
\item{} Result: \alert{alternative implementation} of libunwind, nearly
plug-and-play for existing projects!
\begin{itemize}
\item[$\leadsto$] It is \alert{easy} to use \ehelfs{}: just
link against the right library!
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}{Compilation overview}
\begin{itemize}
\item Compiled to \alert{C code}
@ -296,6 +278,24 @@ $1 = 84
\end{itemize}
\end{frame}
\begin{frame}{Interface: libunwind}
\begin{itemize}
\item \alert{libunwind}: \textit{de facto} standard library for
unwinding
\item Uses DWARF in background
\item \texttt{libunwind-eh\_elf}: alternative implementation using
\ehelfs{}
\item{} Result: \alert{alternative implementation} of libunwind, nearly
plug-and-play for existing projects!
\begin{itemize}
\item[$\leadsto$] It is \alert{easy} to use \ehelfs{}: just
link against the right library!
\end{itemize}
\end{itemize}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Outlining}
@ -335,6 +335,19 @@ $1 = 84
\lstinputlisting[language=C]{src/fib7/fib7.eh_elf_outline.c}
\end{frame}
\subsection{A word on formalization}
\begin{frame}{A word on formalization}
\begin{itemize}
\item First task: \alert{writing semantics} for DWARF, written as
mapping to C code.
\item DWARF5 specification: \alert{plain English}, no proper semantics
\item Compiled code is in substance equivalent to semantics
\item What remains to prove is mostly \alert{simple or classic
optimisations}
\end{itemize}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Benchmarking}
@ -358,34 +371,20 @@ $1 = 84
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Unwinding using perf}
\begin{frame}{Presentation of perf}
\textbf{A profiler is used to\ldots}
\begin{itemize}
\item get readings of the \alert{time spent in each function}
\item detect ``hot paths'': functions you ought to optimize
\item \ldots{}and \alert{benchmark \ehelfs{}}!
\end{itemize}
\vspace{1em}\pause{}
\textbf{How does it work?}
\begin{itemize}
\item{} \alert{Polling profiler}: stops at regular intervals to perform
analyses
\item{} Upon polling, \alert{dumps the stack} to a file
\item{} In the analysis phase (after the program terminated),
\alert{unwinds all the stacks gathered} to get call paths
\end{itemize}
\end{frame}
\begin{frame}{perf instrumentation}
\textbf{\alert{perf} is a polling profiler.}
\begin{itemize}
\item{} used to get readings of the time spent in each function
\item{} works by regularly stopping the program, unwinding its stack,
then aggregating the gathered data
\end{itemize}
\pause{}\bigskip{}
\textbf{Instrumenting perf matches all the requirements!}
\vspace{1em}\pause{}
\begin{itemize}
\item{} \alert{Plug \ehelfs{} into perf}: use \ehelfs{} instead of
DWARF to analyze stack dumps
DWARF to unwind the stack
\item{} Implement \alert{unwinding performance counters} inside perf
\bigskip{}