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papers/lux-quasar-benchmarks/sections/05-benchmark-3-validator-scalability.tex
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\section{Benchmark 3: Validator Scalability}
\label{sec:scalability}
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We measure the \emph{degradation factor}: throughput at $n=1{,}000$ divided by
throughput at $n=21$.
\begin{table}[h]
\centering
\begin{tabular}{@{}lrrl@{}}
\toprule
\textbf{Protocol} & \textbf{TPS @ 21} & \textbf{TPS @ 1000} & \textbf{Retention} \\
\midrule
Snow & 5{,}200 & 3{,}900 & 75.0\% \\
\textbf{Quasar} & 18{,}200 & 4{,}800 & \textbf{26.4\%} \\
Narwhal+Tusk & 46{,}000 & 12{,}100 & 26.3\% \\
HotStuff & 8{,}500 & 1{,}400 & 16.5\% \\
Tendermint & 4{,}100 & 180 & 4.4\% \\
Casper FFG & 30 & 30 & 100\%$^*$ \\
\bottomrule
\end{tabular}
\caption{Throughput retention from $n=21$ to $n=1{,}000$. $^*$Casper FFG throughput is
execution-bound, not consensus-bound; the value is misleading.}
\label{tab:scalability}
\end{table}
\begin{figure}[h]
\centering
\begin{tikzpicture}
\begin{axis}[
width=0.95\textwidth,
height=7cm,
xlabel={Number of validators ($n$)},
ylabel={Throughput retention (\%)},
xmin=0, xmax=1100,
ymin=0, ymax=110,
legend style={at={(0.98,0.98)}, anchor=north east, font=\small},
grid=major,
grid style={gray!20},
]
% Quasar
\addplot[luxpurple, very thick, mark=square*, mark size=3pt] coordinates {
(21, 100) (100, 57.1) (500, 35.7) (1000, 26.4)
};
% Narwhal+Tusk
\addplot[narworange, thick, mark=triangle*, mark size=3pt] coordinates {
(21, 100) (100, 83.7) (500, 41.3) (1000, 26.3)
};
% Snow
\addplot[avaxred, thick, mark=diamond*, mark size=3pt] coordinates {
(21, 100) (100, 92.3) (500, 82.7) (1000, 75.0)
};
% Tendermint
\addplot[cosgray, thick, mark=o, mark size=3pt] coordinates {
(21, 100) (100, 29.3) (500, 9.8) (1000, 4.4)
};
% HotStuff
\addplot[hotred, thick, mark=pentagon*, mark size=3pt] coordinates {
(21, 100) (100, 60.0) (500, 28.2) (1000, 16.5)
};
\legend{Quasar, Narwhal+Tusk, Snow, Tendermint, HotStuff}
\end{axis}
\end{tikzpicture}
\caption{Normalized throughput retention as validator count increases. Snow's sampling-based
approach degrades most gracefully. Tendermint exhibits cliff-edge degradation.}
\label{fig:scalability}
\end{figure}
\subsection{Analysis}
Snow's sampling approach ($K=25$ regardless of $n$) provides the flattest scalability
curve---each validator only communicates with a fixed-size sample. The trade-off is
probabilistic (not deterministic) finality and higher absolute latency.
Quasar matches Narwhal+Tusk in retention (26\%) while providing 2.3$\times$ faster
finality. Tendermint's O($n^2$) all-to-all communication produces catastrophic
degradation: a 95.6\% throughput loss from 21 to 1{,}000 validators.
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