\begin{small}
\begin{center}
\begin{tabular}{||c|c|c|c|c||} \hline \hline
{\bf Mean} & {\bf Mean } & \multicolumn{2}{c|}{\bf Time} & 
{\bf Space} \\ 
{\bf comm.} & {\bf file access} & \multicolumn{2}{c|}{\bf overhead} & 
{\bf (MB/hour)} \\ \cline{3-4}
{\bf period} & {\bf period} & {\bf In-memory} & {\bf Asynch. file}  & \\
{\bf (ms)} & {\bf (ms)} & {\bf tracing} & {\bf tracing} & \\ \hline \hline
30 & 183 & 0.21\% & 0.55\% & 0.72 \\ \hline
3  & 159 & 0.15\% & 0.69\% & 2.51 \\ \hline
87 & 67  & 0.03\% & 0.05\% & 1.50 \\ \hline \hline
\end{tabular}
\end{center}
\end{small}
