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Parameterised handlers section.

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Daniel Hillerström
2021-02-04 18:18:27 +00:00
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thesis.tex
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@@ -7730,14 +7730,19 @@ split on the process queue. There are three cases to consider.
\begin{enumerate}
\item The queue is empty. Then the function returns the list $done$,
which is the list of process return values.
\item The next process is blocked. Then the process is moved to the
end of the queue, and the function is applied recursively to the
scheduler state with the updated queue.
\item The next process is blocked. Then the process is appended on
to the end of the queue, and $\runNext$ is applied recursively to
the scheduler state $st'$ with the updated queue.
\item The next process is ready. Then the $q$ and $pid$ fields
within the scheduler state are updated accordingly. The updated
state is supplied as argument to the reified process.
within the scheduler state are updated accordingly. The reified
process $resume$ is applied to the updated scheduler state $st'$.
\end{enumerate}
%
Evidently, this function may enter an infinite loop if every process
is in blocked state. This may happen if we deadlock any two processes
by having them wait on one another. Using this function we can define
a handler that implements a process scheduler.
%
\[
\bl
\scheduler : \Record{\alpha \eff \{\Co;\varepsilon\};\Sstate~\alpha~\varepsilon} \Harrow^\param \List~\alpha \eff \varepsilon\\
@@ -7780,6 +7785,48 @@ split on the process queue. There are three cases to consider.
\el
\]
%
The handler definition $\scheduler$ takes as input a computation that
computes a value of type $\alpha$ whilst making use of the concurrency
operations from the $\Co$ signature. In addition it takes the initial
scheduler state as input. Ultimately, the handler returns a
computation that computes a list of $\alpha$s, where all the
$\Co$-operations have been handled.
%
In the definition the scheduler state is bound by the name $st$.
The $\return$ case is invoked when a process completes. The return
value $x$ is consed onto the list $done$. Subsequently, the function
$\runNext$ is invoked in order to the next ready process.
The $\UFork$ case implements the semantics for process forking. First
the child process is constructed by abstracting the parameterised
resumption $resume$ such that it becomes an unary state-accepting
function, which can be ascribed type $\Proc~\alpha~\varepsilon$. The
parameterised resumption applied to the process identifier $0$, which
lets the receiver know that it assumes the role of child in the
parent-child relationship amongst the processes. The next line
retrieves the unique process identifier for the child. Afterwards, the
child process is pushed on to the queue in ready state. The next line
updates the scheduler state with the new queue and a new unique
identifier for the next process. Finally, the parameterised resumption
is applied to the child process identifier and the updated scheduler
state.
The $\Wait$ case implements the synchronisation operation. The
parameter $pid$ is the identifier of the process that the invoking
process wants to wait on. First we construct an unary state-accepting
function. Then we check whether there exists a process with identifier
$pid$ in the queue. If there is one, then we enqueue the current
process in blocked state. If no such process exists (e.g. it may
already have finished), then we enqueue the current process in ready
state. Finally, we invoke $\runNext$ with the scheduler state updated
with the new process queue in order to run the next ready process.
The $\Interrupt$ case suspends the current process by enqueuing it in
ready state, and dequeuing the next ready process.
Using this handler we can implement version 2 of the time-sharing
system.
%
\[
\bl
@@ -7792,6 +7839,18 @@ split on the process queue. There are three cases to consider.
\el
\]
%
The computation $m$, which may perform any of the concurrency
operations, is handled by the parameterised handler $\scheduler$. The
parameterised handler definition is applied to the initial scheduler
state, which has an empty process queue, an empty done list, and it
assigns the first process the identifier $1$, and sets up the
identifier for the next process to be $2$.
With $\UFork$ and $\Wait$ we can implement the \emph{init} process,
which is the initial startup process in
\UNIX{}~\cite{RitchieT74}. This process remains alive until the
operating system is shutdown. It is the ancestor of every process
created by the operating system.
%
\[
\bl
@@ -7806,6 +7865,14 @@ split on the process queue. There are three cases to consider.
\el
\]
%
We implement $\init$ as a higher-order function. It takes a main
routine that will be applied when the system has been started. The
function first performs $\UFork$ to duplicate itself. The child branch
executes the $main$ routine, whilst the parent branch waits on the
child. We somewhat arbitrarily choose to exit the parent branch with
code $1$ such that we can identify the process in the completion list.
Now we can plug everything together.
%
\[
\ba{@{~}l@{~}l}
@@ -7820,10 +7887,10 @@ split on the process queue. There are three cases to consider.
\Let\;pid \revto \Do\;\UFork\,\Unit\;\In\\
\If\;pid = 0\\
\Then\;\bl
\su~\Alice; \Do\;\Wait~pid;
\quoteRitchie\,\Unit; \exit~2
\su~\Alice;
\quoteRitchie\,\Unit; \exit~3
\el\\
\Else\; \su~\Bob; \quoteHamlet\,\Unit;\exit~3))})))}
\Else\; \su~\Bob; \Do\;\Wait~pid; \quoteHamlet\,\Unit;\exit~2))})))}
\ea
\el \smallskip\\
\reducesto^+&
@@ -7839,10 +7906,10 @@ split on the process queue. There are three cases to consider.
\ba[t]{@{}l}
\Record{0;
\ba[t]{@{}l@{}l}
\texttt{"}&\texttt{To be, or not to be,\nl{}that is the question:\nl{}}\\
&\texttt{Whether 'tis nobler in the mind to suffer\nl{}}\\
&\texttt{UNIX is basically a simple operating system, but }\\
&\texttt{you have to be a genius to understand the simplicity.\nl"}}];
\texttt{"}&\texttt{UNIX is basically a simple operating system, but }\\
&\texttt{you have to be a genius to understand the simplicity.\nl}\\
&\texttt{To be, or not to be,\nl{}that is the question:\nl{}}\\
&\texttt{Whether 'tis nobler in the mind to suffer\nl{}"}}]
\ea\\
lnext=1; inext=1}}\\
\ea
@@ -7853,7 +7920,19 @@ split on the process queue. There are three cases to consider.
\ea
\]
%
The function provided to $\init$ forks itself. The child branch
switches user to $\Alice$ and invokes the $\quoteRitchie$ process
which writes to standard out. The process exits with status code
$3$. The parent branch switches user to $\Bob$ and waits for the child
process to complete before it invokes the $\quoteHamlet$ process which
also writes to standard out, and finally exiting with status code $2$.
%
It is evident from looking at the file system state that the writes to
standard out has not been interleaved as the contents of
$\strlit{stdout}$ appear in order. We can also see from the process
status list that Alice's process is the first to complete, and the
second to complete is Bob's process, whilst the last process to
complete is the $\init$ process.
\section{Related work}
\label{sec:unix-related-work}