This chapter shows the implementation of the \gls{MTASK} system by:
\begin{itemize}
\item showing the compilation and execution toolchain;
- \item elaborating on the implementation and architecture of the \gls{RTS} of \gls{MTASK};
\item showing the implementation of the byte code compiler for the \gls{MTASK} language;
+ \item elaborating on the implementation and architecture of the \gls{RTS} of \gls{MTASK};
\item and explaining the machinery used to automatically serialise and deserialise data to-and-fro the device.
\end{itemize}
\end{chapterabstract}
The microcontrollers usually have flash-based program memory which wears out fairly quickly.
For example, the flash memory of the popular atmega328p powering the \gls{ARDUINO} UNO is rated for \num{10000} write cycles.
While this sounds like a lot, if new tasks are sent to the device every minute or so, a lifetime of only seven days is guaranteed.
-Hence, for dynamic applications, storing the program in the \gls{RAM} of the device and interpreting this code is necessary in order to save precious write cycles of the program memory.
+Hence, for dynamic applications, storing the program in the \gls{RAM} of the device and thus interpreting this code is necessary in order to save precious write cycles of the program memory.
In the \gls{MTASK} system, the \gls{MTASK} \gls{RTS}, a domain-specific \gls{OS}, is responsible for interpreting the programs.
Programs in \gls{MTASK} are \gls{DSL} terms constructed at run time in an \gls{ITASK} system.
\Cref{fig:toolchain} shows the compilation and execution toolchain of such programs.
First, the source code is compiled to a byte code specification, this specification contains the compiled main expression, the functions, and the \gls{SDS} and peripheral configuration.
How an \gls{MTASK} task is compiled to this specification is shown in \cref{sec:compiler_imp}.
-This package is then sent to the \gls{RTS} of the device for execution, shown in \cref{sec:compiler_rts}.
+This package is then sent to the \gls{RTS} of the device for execution.
+In order to execute a task, first the main expression is evaluated in the interpreter, resulting in a task tree.
+Using small-step reduction, this task tree is continuously rewritten by the rewrite engine of the \gls{RTS}.
+At times, the reduction requires the evaluation of expressions, using the interpreter.
+During every rewrite step, a task value is produced.
On the device, the \gls{RTS} may have multiple tasks at the same time active.
+By interleavig the rewrite steps, parallel operation is achieved.
+The design, architecture and implementation of the \gls{RTS} is shown in \cref{sec:compiler_rts}.
\begin{figure}
\centering
\label{fig:toolchain}
\end{figure}
-\section{Run-time system}\label{sec:compiler_rts}
-The \gls{RTS} is a customisable domain-specific \gls{OS} that takes care of the execution of tasks.
-Furthermore, it also takes care of low-level mechanisms such as the communication, multitasking, and memory management.
-Once a device is programmed with the \gls{MTASK} \gls{RTS}, it can continuously receive new tasks without the need for reprogramming.
-The \gls{OS} is written in portable \ccpp{} and only contains a small device-specific portion.
-In order to keep the abstraction level high and the hardware requirements low, much of the high-level functionality of the \gls{MTASK} language is implemented not in terms of lower-level constructs from \gls{MTASK} language but in terms of \ccpp{} code.
-
-Most microcontrollers software consists of a cyclic executive instead of an \gls{OS}, this one loop function is continuously executed and all work is performed there.
-In the \gls{RTS} of the \gls{MTASK} system, there is also such an event loop function.
-It is a function with a relatively short execution time that gets called repeatedly.
-The event loop consists of three distinct phases.
-After doing the three phases, the devices goes to sleep for as long as possible (see \cref{chp:green_computing_mtask} for more details on task scheduling).
-
-\subsection{Communication phase}
-In the first phase, the communication channels are processed.
-The exact communication method is a customisable device-specific option baked into the \gls{RTS}.
-The interface is kept deliberately simple and consists of two layers: a link interface and a communication interface.
-Besides opening, closing and cleaning up, the link interface has three functions that are shown in \cref{lst:link_interface}.
-Consequently, implementing this link interface is very simple but it is still possible to implement more advanced link features such as buffering.
-There are implementations for this interface for serial or \gls{WIFI} connections using \gls{ARDUINO}, and \gls{TCP} connections for Linux.
-
-\begin{lstArduino}[caption={Link interface of the \gls{MTASK} \gls{RTS}.},label={lst:link_interface}]
-bool link_input_available(void);
-uint8_t link_read_byte(void);
-void link_write_byte(uint8_t b);
-\end{lstArduino}
-
-The communication interface abstracts away from this link interface and is typed instead.
-It contains only two functions as seen in \cref{lst:comm_interface}.
-There are implementations for direct communication, or communication using an \gls{MQTT} broker.
-Both use the automatic serialisation and deserialisation shown in \cref{sec:ccodegen}.
-
-\begin{lstArduino}[caption={Communication interface of the \gls{MTASK} \gls{RTS}.},label={lst:comm_interface}]
-struct MTMessageTo receive_message(void);
-void send_message(struct MTMessageFro msg);
-\end{lstArduino}
-
-Processing the received messages from the communication channels happens synchronously and the channels are exhausted completely before moving on to the next phase.
-There are several possible messages that can be received from the server:
-
-\begin{description}
- \item[SpecRequest]
- is a message instructing the device to send its specification and it is received immediately after connecting.
- The \gls{RTS} responds with a \texttt{Spec} answer containing the specification.
- \item[TaskPrep]
- tells the device a task is on its way.
- Especially on faster connections, it may be the case that the communication buffers overflow because a big message is sent while the \gls{RTS} is busy executing tasks.
- This message allows the \gls{RTS} to postpone execution for a while, until the larger task has been received.
- The server sends the task only after the device acknowledged the preparation by by sending a \texttt{TaskPrepAck} message.
- \item[Task]
- contains a new task, its peripheral configuration, the \glspl{SDS}, and the byte code.
- The new task is immediately copied to the task storage but is only initialised during the next phase.
- The device acknowledges the task by sending a \texttt{TaskAck} message.
- \item[SdsUpdate]
- notifies the device of the new value for a lowered \gls{SDS}.
- The old value of the lowered \gls{SDS} is immediately replaced with the new one.
- There is no acknowledgement required.
- \item[TaskDel]
- instructs the device to delete a running task.
- Tasks are automatically deleted when they become stable.
- However, a task may also be deleted when the surrounding task on the server is deleted, for example when the task is on the left-hand side of a step combinator and the condition to step holds.
- The device acknowledges the deletion by sending a \texttt{TaskDelAck}.
- \item[Shutdown]
- tells the device to reset.
-\end{description}
-
-\subsection{Execution phase}
-The second phase performs one execution step for all tasks that wish for it.
-Tasks are ordered in a priority queue ordered by the time a task needs to execute, the \gls{RTS} selects all tasks that can be scheduled, see \cref{sec:scheduling} for more details.
-Execution of a task is always an interplay between the interpreter and the rewriter.
-
-When a new task is received, the main expression is evaluated to produce a task tree.
-A task tree is a tree structure in which each node represents a task combinator and the leaves are basic tasks.
-If a task is not initialized yet, i.e.\ the pointer to the current task tree is still null, the byte code of the main function is interpreted.
-The main expression always produces a task tree.
-Execution of a task consists of continuously rewriting the task until its value is stable.
-
-Rewriting is a destructive process, i.e.\ the rewriting is done in place.
-The rewriting engine uses the interpreter when needed, e.g.\ to calculate the step continuations.
-The rewriter and the interpreter use the same stack to store intermediate values.
-Rewriting steps are small so that interleaving results in seemingly parallel execution.
-In this phase new task tree nodes may be allocated.
-Both rewriting and initialization are atomic operations in the sense that no processing on \glspl{SDS} is done other than \gls{SDS} operations from the task itself.
-The host is notified if a task value is changed after a rewrite step by sending a \texttt{TaskReturn} message.
-
-Take for example a blink task for which the code is shown in \cref{lst:blink_code}.
-
-\begin{lstClean}[caption={Code for a blink program.},label={lst:blink_code}]
-fun \blink=(\st->delay (lit 500) >>|. writeD d3 st >>=. blink o Not)
-In {main = blink true}
-\end{lstClean}
-
-On receiving this task, the task tree is still null and the initial expression \cleaninline{blink true} is evaluated by the interpreter.
-This results in the task tree shown in \cref{fig:blink_tree}.
-Rewriting always starts at the top of the tree and traverses to the leaves, the basic tasks that do the actual work.
-The first basic task encountered is the \cleaninline{delay} task, that yields no value until the time, \qty{500}{\ms} in this case, has passed.
-When the \cleaninline{delay} task yielded a stable value after a number of rewrites, the task continues with the right-hand side of the \cleaninline{>>\|.} combinator.
-This combinator has a \cleaninline{writeD} task at the left-hand side that becomes stable after one rewrite step in which it writes the value to the given pin.
-When \cleaninline{writeD} becomes stable, the written value is the task value that is observed by the right-hand side of the \cleaninline{>>=.} combinator.
-This will call the interpreter to evaluate the expression, now that the argument of the function is known.
-The result of the function is again a task tree, but now with different arguments to the tasks, e.g.\ the state in \cleaninline{writeD} is inversed.
-
-\begin{figure}
- \centering
- \includestandalone{blinktree}
- \caption{The task tree for a blink task in \cref{lst:blink_code} in \gls{MTASK}.}%
- \label{fig:blink_tree}
-\end{figure}
-
-\subsection{Memory management}
-The third and final phase is memory management.
-The \gls{MTASK} \gls{RTS} is designed to run on systems with as little as \qty{2}{\kibi\byte} of \gls{RAM}.
-Aggressive memory management is therefore vital.
-Not all firmwares for microprocessors support heaps and---when they do---allocation often leaves holes when not used in a \emph{last in first out} strategy.
-The \gls{RTS} uses a chunk of memory in the global data segment with its own memory manager tailored to the needs of \gls{MTASK}.
-The size of this block can be changed in the configuration of the \gls{RTS} if necessary.
-On an \gls{ARDUINO} UNO---equipped with \qty{2}{\kibi\byte} of \gls{RAM}---the maximum viable size is about \qty{1500}{\byte}.
-The self-managed memory uses a similar layout as the memory layout for \gls{C} programs only the heap and the stack are switched (see \cref{fig:memory_layout}).
-
-\begin{figure}
- \centering
- \includestandalone{memorylayout}
- \caption{Memory layout in the \gls{MTASK} \gls{RTS}.}\label{fig:memory_layout}
-\end{figure}
-
-A task is stored below the stack and its complete state is a \gls{CLEAN} record contain most importantly the task id, a pointer to the task tree in the heap (null if not initialised yet), the current task value, the configuration of \glspl{SDS}, the configuration of peripherals, the byte code and some scheduling information.
-
-In memory, task data grows from the bottom up and the interpreter stack is located directly on top of it growing in the same direction.
-As a consequence, the stack moves when a new task is received.
-This never happens within execution because communication is always processed before execution.
-Values in the interpreter are always stored on the stack.
-Compound data types are stored unboxed and flattened.
-Task trees grow from the top down as in a heap.
-This approach allows for flexible ratios, i.e.\ many tasks and small trees or few tasks and big trees.
-
-Stable tasks, and unreachable task tree nodes are removed.
-If a task is to be removed, tasks with higher memory addresses are moved down.
-For task trees---stored in the heap---the \gls{RTS} already marks tasks and task trees as trash during rewriting so the heap can be compacted in a single pass.
-This is possible because there is no sharing or cycles in task trees and nodes contain pointers pointers to their parent.
-
\section{Compiler}\label{sec:compiler_imp}
\subsection{Compiler infrastructure}
-The bytecode compiler interpretation for the \gls{MTASK} language is implemented as a monad stack containing a writer monad and a state monad.
+The byte code compiler interpretation for the \gls{MTASK} language is implemented as a monad stack containing a writer monad and a state monad.
The writer monad is used to generate code snippets locally without having to store them in the monadic values.
The state monad accumulates the code, and stores the state the compiler requires.
\Cref{lst:compiler_state} shows the data type for the state, storing:
}\end{lstClean}
% VimTeX: SynIgnore off
+\section{Run-time system}\label{sec:compiler_rts}
+The \gls{RTS} is a customisable domain-specific \gls{OS} that takes care of the execution of tasks.
+Furthermore, it also takes care of low-level mechanisms such as the communication, multitasking, and memory management.
+Once a device is programmed with the \gls{MTASK} \gls{RTS}, it can continuously receive new tasks without the need for reprogramming.
+The \gls{OS} is written in portable \ccpp{} and only contains a small device-specific portion.
+In order to keep the abstraction level high and the hardware requirements low, much of the high-level functionality of the \gls{MTASK} language is implemented not in terms of lower-level constructs from \gls{MTASK} language but in terms of \ccpp{} code.
+
+Most microcontrollers software consists of a cyclic executive instead of an \gls{OS}, this one loop function is continuously executed and all work is performed there.
+In the \gls{RTS} of the \gls{MTASK} system, there is also such an event loop function.
+It is a function with a relatively short execution time that gets called repeatedly.
+The event loop consists of three distinct phases.
+After doing the three phases, the devices goes to sleep for as long as possible (see \cref{chp:green_computing_mtask} for more details on task scheduling).
+
+\subsection{Communication phase}
+In the first phase, the communication channels are processed.
+The exact communication method is a customisable device-specific option baked into the \gls{RTS}.
+The interface is kept deliberately simple and consists of two layers: a link interface and a communication interface.
+Besides opening, closing and cleaning up, the link interface has three functions that are shown in \cref{lst:link_interface}.
+Consequently, implementing this link interface is very simple but it is still possible to implement more advanced link features such as buffering.
+There are implementations for this interface for serial or \gls{WIFI} connections using \gls{ARDUINO}, and \gls{TCP} connections for Linux.
+
+\begin{lstArduino}[caption={Link interface of the \gls{MTASK} \gls{RTS}.},label={lst:link_interface}]
+bool link_input_available(void);
+uint8_t link_read_byte(void);
+void link_write_byte(uint8_t b);
+\end{lstArduino}
+
+The communication interface abstracts away from this link interface and is typed instead.
+It contains only two functions as seen in \cref{lst:comm_interface}.
+There are implementations for direct communication, or communication using an \gls{MQTT} broker.
+Both use the automatic serialisation and deserialisation shown in \cref{sec:ccodegen}.
+
+\begin{lstArduino}[caption={Communication interface of the \gls{MTASK} \gls{RTS}.},label={lst:comm_interface}]
+struct MTMessageTo receive_message(void);
+void send_message(struct MTMessageFro msg);
+\end{lstArduino}
+
+Processing the received messages from the communication channels happens synchronously and the channels are exhausted completely before moving on to the next phase.
+There are several possible messages that can be received from the server:
+
+\begin{description}
+ \item[SpecRequest]
+ is a message instructing the device to send its specification and it is received immediately after connecting.
+ The \gls{RTS} responds with a \texttt{Spec} answer containing the specification.
+ \item[TaskPrep]
+ tells the device a task is on its way.
+ Especially on faster connections, it may be the case that the communication buffers overflow because a big message is sent while the \gls{RTS} is busy executing tasks.
+ This message allows the \gls{RTS} to postpone execution for a while, until the larger task has been received.
+ The server sends the task only after the device acknowledged the preparation by by sending a \texttt{TaskPrepAck} message.
+ \item[Task]
+ contains a new task, its peripheral configuration, the \glspl{SDS}, and the byte code.
+ The new task is immediately copied to the task storage but is only initialised during the next phase.
+ The device acknowledges the task by sending a \texttt{TaskAck} message.
+ \item[SdsUpdate]
+ notifies the device of the new value for a lowered \gls{SDS}.
+ The old value of the lowered \gls{SDS} is immediately replaced with the new one.
+ There is no acknowledgement required.
+ \item[TaskDel]
+ instructs the device to delete a running task.
+ Tasks are automatically deleted when they become stable.
+ However, a task may also be deleted when the surrounding task on the server is deleted, for example when the task is on the left-hand side of a step combinator and the condition to step holds.
+ The device acknowledges the deletion by sending a \texttt{TaskDelAck}.
+ \item[Shutdown]
+ tells the device to reset.
+\end{description}
+
+\subsection{Execution phase}
+The second phase performs one execution step for all tasks that wish for it.
+Tasks are ordered in a priority queue ordered by the time a task needs to execute, the \gls{RTS} selects all tasks that can be scheduled, see \cref{sec:scheduling} for more details.
+Execution of a task is always an interplay between the interpreter and the rewriter.
+
+When a new task is received, the main expression is evaluated to produce a task tree.
+A task tree is a tree structure in which each node represents a task combinator and the leaves are basic tasks.
+If a task is not initialized yet, i.e.\ the pointer to the current task tree is still null, the byte code of the main function is interpreted.
+The main expression always produces a task tree.
+Execution of a task consists of continuously rewriting the task until its value is stable.
+
+Rewriting is a destructive process, i.e.\ the rewriting is done in place.
+The rewriting engine uses the interpreter when needed, e.g.\ to calculate the step continuations.
+The rewriter and the interpreter use the same stack to store intermediate values.
+Rewriting steps are small so that interleaving results in seemingly parallel execution.
+In this phase new task tree nodes may be allocated.
+Both rewriting and initialization are atomic operations in the sense that no processing on \glspl{SDS} is done other than \gls{SDS} operations from the task itself.
+The host is notified if a task value is changed after a rewrite step by sending a \texttt{TaskReturn} message.
+
+Take for example a blink task for which the code is shown in \cref{lst:blink_code}.
+
+\begin{lstClean}[caption={Code for a blink program.},label={lst:blink_code}]
+fun \blink=(\st->delay (lit 500) >>|. writeD d3 st >>=. blink o Not)
+In {main = blink true}
+\end{lstClean}
+
+On receiving this task, the task tree is still null and the initial expression \cleaninline{blink true} is evaluated by the interpreter.
+This results in the task tree shown in \cref{fig:blink_tree}.
+Rewriting always starts at the top of the tree and traverses to the leaves, the basic tasks that do the actual work.
+The first basic task encountered is the \cleaninline{delay} task, that yields no value until the time, \qty{500}{\ms} in this case, has passed.
+When the \cleaninline{delay} task yielded a stable value after a number of rewrites, the task continues with the right-hand side of the \cleaninline{>>\|.} combinator.
+This combinator has a \cleaninline{writeD} task at the left-hand side that becomes stable after one rewrite step in which it writes the value to the given pin.
+When \cleaninline{writeD} becomes stable, the written value is the task value that is observed by the right-hand side of the \cleaninline{>>=.} combinator.
+This will call the interpreter to evaluate the expression, now that the argument of the function is known.
+The result of the function is again a task tree, but now with different arguments to the tasks, e.g.\ the state in \cleaninline{writeD} is inversed.
+
+\begin{figure}
+ \centering
+ \includestandalone{blinktree}
+ \caption{The task tree for a blink task in \cref{lst:blink_code} in \gls{MTASK}.}%
+ \label{fig:blink_tree}
+\end{figure}
+
+\subsection{Memory management}
+The third and final phase is memory management.
+The \gls{MTASK} \gls{RTS} is designed to run on systems with as little as \qty{2}{\kibi\byte} of \gls{RAM}.
+Aggressive memory management is therefore vital.
+Not all firmwares for microprocessors support heaps and---when they do---allocation often leaves holes when not used in a \emph{last in first out} strategy.
+The \gls{RTS} uses a chunk of memory in the global data segment with its own memory manager tailored to the needs of \gls{MTASK}.
+The size of this block can be changed in the configuration of the \gls{RTS} if necessary.
+On an \gls{ARDUINO} UNO---equipped with \qty{2}{\kibi\byte} of \gls{RAM}---the maximum viable size is about \qty{1500}{\byte}.
+The self-managed memory uses a similar layout as the memory layout for \gls{C} programs only the heap and the stack are switched (see \cref{fig:memory_layout}).
+
+\begin{figure}
+ \centering
+ \includestandalone{memorylayout}
+ \caption{Memory layout in the \gls{MTASK} \gls{RTS}.}\label{fig:memory_layout}
+\end{figure}
+
+A task is stored below the stack and its complete state is a \gls{CLEAN} record contain most importantly the task id, a pointer to the task tree in the heap (null if not initialised yet), the current task value, the configuration of \glspl{SDS}, the configuration of peripherals, the byte code and some scheduling information.
+
+In memory, task data grows from the bottom up and the interpreter stack is located directly on top of it growing in the same direction.
+As a consequence, the stack moves when a new task is received.
+This never happens within execution because communication is always processed before execution.
+Values in the interpreter are always stored on the stack.
+Compound data types are stored unboxed and flattened.
+Task trees grow from the top down as in a heap.
+This approach allows for flexible ratios, i.e.\ many tasks and small trees or few tasks and big trees.
+
+Stable tasks, and unreachable task tree nodes are removed.
+If a task is to be removed, tasks with higher memory addresses are moved down.
+For task trees---stored in the heap---the \gls{RTS} already marks tasks and task trees as trash during rewriting so the heap can be compacted in a single pass.
+This is possible because there is no sharing or cycles in task trees and nodes contain pointers pointers to their parent.
+
+
\section{C code generation}\label{sec:ccodegen}
All communication between the \gls{ITASK} server and the \gls{MTASK} server is type parametrised.
From the structural representation of the type, a \gls{CLEAN} parser and printer is constructed using generic programming.
\section{Conclusion}
It is not straightforward to execute \gls{MTASK} tasks on resources-constrained \gls{IOT} edge devices.
-To achieve this, the terms in the \gls{DSL} are compiled to domain-specific byte code.
+To achieve this, the terms in the \gls{DSL} are compiled to compact domain-specific byte code.
This byte code is sent for interpretation to the light-weight \gls{RTS} of the edge device.
-The \gls{RTS} first evaluates the main expression.
+The \gls{RTS} first evaluates the main expression in the interpreter.
The result of this evaluation, a run time representation of the task, is a task tree.
-This task tree is rewritten according to rewrite rules until a stable value is observed.
+This task tree is rewritten according to small-step reduction rules until a stable value is observed.
Rewriting multiple tasks at the same time is achieved by interleaving the rewrite steps, resulting in seamingly parallel execution of the tasks.
-Furthermore, the \gls{RTS} automates communication and coordinates multi tasking.
+All communication, including the serialisation and deserialisation, between the server and the \gls{RTS} is automated.
+From the structural representation of the types, printers and parsers are generated for the server and the client.
\input{subfilepostamble}
\end{document}
repositories / phd-thesis.git / blobdiff