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\author{Andreas van Cranenburgh (0440949) \\
		Job Jonkergouw (5613949)}

\date{October 2008}

\title{Process Scheduling}

\begin{document}
\maketitle

\tableofcontents

\abstract{Scheduling is an important part in the design of operating systems.
At any time many applications are running on a computer with many of them
needing access to system recourses such as memory or I/O services. Because a
computer's resources are limited not all applications can have access to their
resources at the same time. A system to assign the available resources to the
applications is needed. This is called scheduling. We will use a simulated
operating system to test different types of scheduling. This paper describes 
an implementation of different methods of scheduling and measures their
performance.}

\section{Simulation}
Instead of testing the scheduling directly on a real operating system we used a
simulator. This has the advantage that we have to worry less about the
technical side of the operating system but instead we can focus on the
scheduling alone. This simulator was provided as the object-file
\texttt{simul.o}, which is the simulator itself, the random-number generator
\texttt{amt19937ar.o} and \texttt{mem\_alloc.o} which is used to simulate the
memory management. Those provided files were closed sources to prevent us from
cheating the measurements of the scheduling; we could have adjusted the
simulator to create bugged processes which could be handled very easy by some
schedulers. Moreover, to give be able to edit only a few files makes it easier
to focus our attention in the right way. However, there was a downside of the
closed sources: we didn't get access to the main() function so direction of
input and output was difficult to handle. Instead, we needed write additional
scripts to manage the in- and output.

The provided simulator gives detailed output of the performance. To simplify
matters we have focussed on only two variables of performance: turn around time
and the number of finished processes. Turn around time is the total time from
submission of a process until time of completion.

\section{The Operating System}
The scheduler itself was written in the files \texttt{schedule.c} and
\texttt{schedule.h}, which were provided. The simulated operating system acts
according to the following queues: \texttt{new\_proc},
\texttt{ready\_proc}, \texttt{io\_proc} and \texttt{defunct\_proc}. These
queues are pointers to the their first element with the queue itself being a
linked list. Processes move from queue to queue according to this diagram:


\begin{figure}[h!]
	\includegraphics[width=0.75\textwidth]{queues.png}
	\centering
	\caption{State transitions of processes}
\end{figure}


The functions for enqueueing and dequeueing are used to handle the switching of
a process to one queue to another and are defined in in \texttt{schedule.c}.

\section{Scheduling algorithms}
Our scheduler has different modes, which can be chosen at run-time. This makes
it possible to combine different parameters as needed.

\subsection{First Fit First Serve}
The first algorithm allocates memory to processes and skips over processes
which do not fit in memory. The whole queue is processed in this fashion, after
which they will be executed.

The obvious downside to this is that processes requesting large amounts of
memory will be skipped, even though it should be their turn to run. This can
lead to starvation when a lots of memory are requested.

\subsection{First Come First Serve}
This option is similar to the previous one but instead of skipping over
processes that do not fit in memory it will defer further allocation and
execute those processes which have already been admitted.

\subsection{Shortest Job First}
Another method is to make the bias for small processes explicit. This algorithm
actively searches for the process requesting the least amount of memory in
order to allocate as many processes as possible.

A small modification was made, however, when we discovered that visiting each
process in the queue only once yielded much better results than restarting
at the front of the queue after dequeueing the smallest process (so even though
the second smallest process might be before the smallest procces, with this
modification only processes after the smallest proces will be evaluated).

\subsection{Mixed mode}
As an experiment an option was added to use the preceeding three algorithms
repeatedly, in turn (using increment modulus 3).

\subsection{Monte Carlo}
Monte Carlo scheduling (also known as lottery scheduling) simply picks a random
process from the queue of waiting processes until a process is encounterd which
does not fit in memory. The downside to this is that it is obviously unfair:
processes which have been waiting for longer might not get a turn before others
which have just arrived in the queue. On the other hand it solves the problem
of starvation, every process will have an equal oppurtunity.

\section{Optional features}
The following three features can be enabled or disabled with any of the
preceeding scheduling algorithms.

\subsection{Queueing strategy}
Processes which have been granted memory can be queued either at the back or at
the front of the ready queue. Normally they should be queued at the back,
because the other processes in the queue have been waiting longer, but queueing
at the front can be used as a trick to achieve very low turn-around times.
This will have detrimental results if the queue is large (unfairness will rise
exponentially).

\subsection{Time slices}
When enabled processes will get a specific time slice, based on the number of
processes (this is to ensure that scheduling has a constant frequency,
irrespective of number of processes). The formula for this is \texttt{100 /
number\_of\_processes}, in ``units of simulation'' (the actual units were not
documented in the simulator, so this number was arrived at by educated
guessing). The advantage of this is that processes will not hog the CPU for
long amounts of time, and this benefits the responsiveness of the system. The
downside is that many more context switches will be generated, especially with
a large number of concurrent processes.

\subsection{Priorities}
When this feature is enabled processes will be assigned a priority depending on
their I/O behavior. In order to determine the priority of a process two timers
are maintained in an extra struct in the process control block, the first for
cumulative CPU time, the other for cumulative I/O time. When a process starts
to do I/O, its CPU timer is stopped and its I/O time starts; and vice versa
when its I/O is finished.

Processes that spend a relatively long time doing I/O will be granted a higher
priority than other processes (the actual condition is \texttt{cputime > 1000 *
iotime}). The rationale for this is that processes waiting a lot for I/O should
be compensated by not having to wait for the CPU, as well as the heuristic that
I/O bound processes will generally not need a lot of CPU power.

\section{Results}

\begin{table}[h]
\begin{tabular}{ | l r r r r r r | } \hline
 & FCFS & FFFS & SJF & FFFS+slices & SJF+prio & SJF+frontq+prio \\
processes & 893   & 1003 & 934  & 989 & 929 & 969 \\
turn-around & 4770 & 1584 & 1498 & 889 & 823 & 788 \\
\hline
\end{tabular}
\caption{Some measurements. Processes is the number of finished processes (higher is better). Turn-around is the total turn around time (lower is better)}
\end{table}

See the graphs on the following pages.


\section{Discussion}
We have compared the different scheduling methods by their performance as
function of the CPU load. We considered the normal methods such as FCFS, FFFS,
SJF, Mixed and Monte Carlo but also two configurations we think give good
results. We see that by far FCFS is almost always the slowerst with Monte Carlo
coming close, but that is not something we can trust giving its random nature.

When comparing the use of round robin with FFFS we see that time slices only
gain the upper hand under a high CPU load, which is as expected because
round robin is only useful with CPU heavy processes or for time sharing
systems. A notable winner is SJF at the higher CPU load with priority checking
giving it an even better result. This may indicate a correlation between memory
size and CPU load of a process.

From the graphs it is clear that as the number of processes increases, only Shortest Job First, Mixed mode and Monte Carlo degrade gracefully. The other scheduling algorithms show faster growth of turn around time.

If the CPU load is increased instead only Shortest Job First shows promising
performance, the other algorithms break down at high loads. The clear winner is Shortest Job First combined with priorities.

Enabling priorities and time slices yields the best performance, as should be
expected.

%We have also compared the effect of the Priority and Time-slice settings on
%FCFS in particular. We can see that Time slices are notable slower in lower
%regions. Which is as expected because Round-Robin only has a benefit at high
%CPU loads otherwise the context switching time will work as a big disadvantage.
%Moreover, it seems that the priorities do not work well at the highest CPU
%load but works very well at lower loads.

We have also compared the effect of the Priority and Time-slice settings on SJF
in particular (since that algorithm seemed to perform the best). We can see
that Time slices are notably slower under lower loads. Which is as expected
because Round-Robin only has a benefit at high CPU loads, otherwise the context
switching time will work as a big disadvantage. Moreover, it seems that the
enabling priorities gives best performance even at the highest CPU load.


\begin{figure}[p!]
	\includegraphics[width=0.75\textwidth]{FCFSback.png}
	\centering
	\caption{Turn around time of FCFS under different CPU, memory and IO loads (lower is better).}
\end{figure}

\begin{figure}[p!]
	\includegraphics[width=0.75\textwidth]{5typesBACKprocnum.png}
	\centering
	\caption{Turn around time for each scheduling algorithm as a function of the number of processes (lower is better)}
\end{figure}

\begin{figure}[p!]
	\includegraphics[width=0.75\textwidth]{totalCPUload.png}
	\centering
	\caption{Turn around time for each scheduling algorithm and some combinations of options as a function of CPU load (lower is better)}
\end{figure}

\begin{figure}[p!]
	\includegraphics[scale=0.5]{SJFprioRR.png}
	\centering
	\caption{Turn around time for SJF with some combinations of options as a function of CPU load (lower is better)}
\end{figure}

\end{document}
