source: doc/proposals/concurrency/text/together.tex@ 6ba16fa

% ======================================================================
% ======================================================================
\chapter{Putting It All Together}
% ======================================================================
% ======================================================================


\section{Threads As Monitors}
As it was subtly alluded in section \ref{threads}, \code{thread}s in \CFA are in fact monitors, which means that all monitor features are available when using threads. For example, here is a very simple two thread pipeline that could be used for a simulator of a game engine :
\begin{figure}[H]
\begin{cfacode}[caption={Toy simulator using \code{thread}s and \code{monitor}s.},label={lst:engine-v1}]
// Visualization declaration
thread Renderer {} renderer;
Frame * simulate( Simulator & this );

// Simulation declaration
thread Simulator{} simulator;
void render( Renderer & this );

// Blocking call used as communication
void draw( Renderer & mutex this, Frame * frame );

// Simulation loop
void main( Simulator & this ) {
        while( true ) {
                Frame * frame = simulate( this );
                draw( renderer, frame );
        }
}

// Rendering loop
void main( Renderer & this ) {
        while( true ) {
                waitfor( draw, this );
                render( this );
        }
}
\end{cfacode}
\end{figure}
One of the obvious complaints of the previous code snippet (other than its toy-like simplicity) is that it does not handle exit conditions and just goes on forever. Luckily, the monitor semantics can also be used to clearly enforce a shutdown order in a concise manner :
\begin{figure}[H]
\begin{cfacode}[caption={Same toy simulator with proper termination condition.},label={lst:engine-v2}]
// Visualization declaration
thread Renderer {} renderer;
Frame * simulate( Simulator & this );

// Simulation declaration
thread Simulator{} simulator;
void render( Renderer & this );

// Blocking call used as communication
void draw( Renderer & mutex this, Frame * frame );

// Simulation loop
void main( Simulator & this ) {
        while( true ) {
                Frame * frame = simulate( this );
                draw( renderer, frame );

                // Exit main loop after the last frame
                if( frame->is_last ) break;
        }
}

// Rendering loop
void main( Renderer & this ) {
        while( true ) {
                   waitfor( draw, this );
                or waitfor( ^?{}, this ) {
                        // Add an exit condition
                        break;
                }

                render( this );
        }
}

// Call destructor for simulator once simulator finishes
// Call destructor for renderer to signify shutdown
\end{cfacode}
\end{figure}

\section{Fibers \& Threads}
As mentioned in section \ref{preemption}, \CFA uses preemptive threads by default but can use fibers on demand. Currently, using fibers is done by adding the following line of code to the program~:
\begin{cfacode}
unsigned int default_preemption() {
        return 0;
}
\end{cfacode}
This function is called by the kernel to fetch the default preemption rate, where 0 signifies an infinite time-slice, i.e., no preemption. However, once clusters are fully implemented, it will be possible to create fibers and \glspl{uthread} in the same system, as in listing \ref{lst:fiber-uthread}
\begin{figure}
\begin{cfacode}[caption={Using fibers and \glspl{uthread} side-by-side in \CFA},label={lst:fiber-uthread}]
//Cluster forward declaration
struct cluster;

//Processor forward declaration
struct processor;

//Construct clusters with a preemption rate
void ?{}(cluster& this, unsigned int rate);
//Construct processor and add it to cluster
void ?{}(processor& this, cluster& cluster);
//Construct thread and schedule it on cluster
void ?{}(thread& this, cluster& cluster);

//Declare two clusters
cluster thread_cluster = { 10`ms };                     //Preempt every 10 ms
cluster fibers_cluster = { 0 };                         //Never preempt

//Construct 4 processors
processor processors[4] = {
        //2 for the thread cluster
        thread_cluster;
        thread_cluster;
        //2 for the fibers cluster
        fibers_cluster;
        fibers_cluster;
};

//Declares thread
thread UThread {};
void ?{}(UThread& this) {
        //Construct underlying thread to automatically
        //be scheduled on the thread cluster
        (this){ thread_cluster }
}

void main(UThread & this);

//Declares fibers
thread Fiber {};
void ?{}(Fiber& this) {
        //Construct underlying thread to automatically
        //be scheduled on the fiber cluster
        (this.__thread){ fibers_cluster }
}

void main(Fiber & this);
\end{cfacode}
\end{figure}