Add Ryzen execution times example.

manual/tracy.tex

@@ -520,6 +520,22 @@ Be very careful when using AVX2 or AVX512.
 
 More information can be found at \url{https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html}, \url{https://en.wikichip.org/wiki/intel/frequency_behavior}.
 
+\paragraph{Summing it up}
+\label{ryzen}
+
+Power management schemes employed in various CPUs make it hard to reason about true performance of the code. For example, figure~\ref{ryzenimage} contains a histogram of function execution times (as described in chapter~\ref{findzone}), as measured on an AMD Ryzen CPU. The results ranged from 13.05~\si{\micro\second} to 61.25~\si{\micro\second} (extreme outliers were not included on the graph, limiting the longest displayed time to 36.04~\si{\micro\second}).
+
+\begin{figure}[h]
+\centering
+\includegraphics[width=0.5\textwidth]{images/ryzen.png}
+\caption{Example function execution times on a Ryzen CPU}
+\label{ryzenimage}
+\end{figure}
+
+We can immediately see that there are two distinct peaks, at 13.4~\si{\micro\second} and 15.3~\si{\micro\second}. A reasonable assumption would be that there are two paths in the code, one that can omit some work, and the second one which must do some additional job. But here's a catch -- the measured code is actually branchless and is always executed the same way. The two peaks represent two turbo frequencies between which the CPU was aggressively switching.
+
+We can also see that the graph gradually falls off to the right (representing longer times), with a small bump near the end. This can be attributed to running in power saving mode, with differing reaction times to the required operating frequency boost to full power.
+
 \subsection{Building the server}
 
 The easiest way to get going is to build the data analyzer, available in the \texttt{profiler} directory. With it you can connect to localhost or remote clients and view the collected data right away.
@@ -2371,6 +2387,14 @@ logo=\bclampe
 You may press \keys{\ctrl + F} to open or focus the find zone window and set the keyboard input on the search box.
 \end{bclogo}
 
+\begin{bclogo}[
+noborder=true,
+couleur=black!5,
+logo=\bcattention
+]{Caveats}
+When using the execution times histogram you must be aware about the hardware peculiarities. Read section~\ref{checkenvironmentcpu} for more detail.
+\end{bclogo}
+
 \subsubsection{Timeline interaction}
 
 When the zone statistics are displayed in the find zone menu, matching zones will be highlighted on the timeline display. Highlight colors match the histogram display. Bright blue highlight is used to indicate that a zone is in the optional selection range, while the yellow highlight is used for the rest of zones.