I have tried to make a several pictures in one big figure. However, they are placed very close to each other. I need to add more space between them (between pictures in horizontal and in vertical).
Code:
\documentclass{article}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}
\begin{figure}[h]
\centering
\begin{subfigure}[b]{0.4\textwidth}
\centering
\includegraphics[scale=0.6]{figs/solutions/transaction_solution.png}
\caption{Transaction level isolation}
\label{fig:subim1}
\end{subfigure}
\begin{subfigure}[b]{0.4\textwidth}
\centering
\includegraphics[scale=0.6]{figs/solutions/locking_solution.png}
\caption{Record locking}
\label{fig:subim2}
\end{subfigure}
\begin{subfigure}[b]{0.4\textwidth}
\centering
\includegraphics[scale=0.6]{figs/solutions/cache_solution.png}
\caption{Shared cache synchronization}
\label{fig:subim3}
\end{subfigure}
\begin{subfigure}[b]{0.4\textwidth}
\centering
\includegraphics[scale=0.6]{figs/solutions/message_broker_solution.png}
\caption{Message broker sequentializing}
\label{fig:subim4}
\end{subfigure}
\captionsetup{justification=centering,margin=1cm}
\caption{\label{fig:knng} Existing synchronization approaches for multiple server applications}
\end{figure}
\end{document}