\documentclass[12pt,a4paper]{article}
\usepackage{amsmath}    % need for subequations
\usepackage{listings}
\usepackage{tikz}
\usetikzlibrary{positioning,shapes.geometric}
\begin{document}
\title{[CE538] Parallel Computer Architecture\\Homework 2}
\author{Vasiliadis Vasilios\\ \small{vasiliad@inf.uth.gr}}
\maketitle
\clearpage

\section{Exercise 1}
\begin{tikzpicture}[%
    ->,
    shorten >=2pt,
    >=stealth,
    node distance=1cm,
    noname/.style={%
      ellipse,
      minimum width=5em,
      minimum height=3em,
      draw
    }
  ]
    \node[noname] (1)              {L1};
    \node[noname] (2) [right=of 1] {L2};
		\node[noname] (3) [below=of 2] {M1};
		\node[noname] (4) [right=of 3] {L3};
		\node[noname] (5) [below=of 3] {A1};
		\node[noname] (6) [below=of 5] {S1};
		\node[noname] (7) [right=of 6] {A2};
		\node[noname] (8) [below=of 7] {S2};

    \path (1) edge node {} (3)
          (2) edge node {} (3)
					(4) edge node {} (5)
					(5) edge node {} (6)
					(3) edge node {} (5)
		;
  \end{tikzpicture}
\section{Exercise 2}

\section{Exercise 3}

\section{Exercise 4}
\end{document}