ThreadedDiffuser.java

/*
  Copyright 2006 by Sean Luke and George Mason University
  Licensed under the Academic Free License version 3.0
  See the file "LICENSE" for more information
*/

package sim.app.heatbugs;
import sim.engine.*;
import sim.field.grid.*;

/** A dual-threaded version of Diffuser for use on machines with two processors.
    Holds a ParallelSequence which in turn holds two dummy Steppables which each
    call diffuse(...) with different start and end values.  That way basically we
    split the array diffusion up among two processors, one taking the first half
    of the array and one taking up the second half.  Because Diffuser takes up
    nearly all our time, this results in a dramatic increase in speed on a
    dual-processor machine. */

public /*strictfp*/ class ThreadedDiffuser implements Steppable
    {
    private static final long serialVersionUID = 1;

    public ParallelSequence diffusers;
    
    public ThreadedDiffuser(final int numThreads)
        {
        Steppable[] threads = new Steppable[numThreads];
        
        // load all but the last thread
        for(int i = 0; i < numThreads - 1; i++)
            {
            final int _i = i;
            threads[i] = new Steppable ()
                { 
                public void step(SimState state) 
                    {
                    HeatBugs heatbugs = (HeatBugs)state;
                    int _gridWidth = heatbugs.valgrid.getWidth();  // read-only, so threadsafe
                    diffuse(heatbugs, _gridWidth / numThreads * _i, _gridWidth / numThreads * (_i + 1));
                    }
                };
            }
        
        // load the last thread
        threads[numThreads - 1] = new Steppable()
            {
            public void step(SimState state) 
                {
                HeatBugs heatbugs = (HeatBugs)state;
                int _gridWidth = heatbugs.valgrid.getWidth();  // read-only, so threadsafe
                diffuse(heatbugs, _gridWidth / numThreads * (numThreads - 1), _gridWidth);
                }
            };
        diffusers = new ParallelSequence(threads);
        }
        
        
    public void step(SimState state)
        {
        diffusers.step(state);
                
        // copy HeatBugs.this.valgrid2 to HeatBugs.this.valgrid
        HeatBugs heatbugs = (HeatBugs)state;
        
        // We COULD just copy all thre results into heatbugs like this;
        //heatbugs.valgrid.setTo(heatbugs.valgrid2);

		// But we can get a big improvement in speed (almost 2x at 8 processors!)
		// if we just swap the field pointers internally.  This is a hack and there
		// are obviously other ways to do it but it will work fine here.
        double[][] temp = heatbugs.valgrid.field;
        heatbugs.valgrid.field = heatbugs.valgrid2.field;
        heatbugs.valgrid2.field = temp;
        }
        
        
    /** We'll have HeatBugs call this in its stop() method to give US a chance to
        call cleanup on our underlying ParallelSequence so we don't leak threads. */
    public void cleanup()
        {
        diffusers.cleanup();
        }
                
    /** Diffuse heatbugs.valgrid.field[start...end] not including end */
        
    // this code is confusing.  See Diffuser.java for other less confusing examples
    // and for an explanation for why the code looks the way it does.
        
    void diffuse(HeatBugs heatbugs, int start, int end)
        {
        // locals are faster than instance variables
        final DoubleGrid2D _valgrid = heatbugs.valgrid;
        final double[][] _valgrid_field = heatbugs.valgrid.field;
        final double[][] _valgrid2_field = heatbugs.valgrid2.field;
        final int _gridHeight = _valgrid.getHeight();
        final double _evaporationRate = heatbugs.evaporationRate;
        final double _diffusionRate = heatbugs.diffusionRate;

        double average;
        
        double[] _past = _valgrid_field[_valgrid.stx(start-1)];
        double[] _current = _valgrid_field[start];
        double[] _next;
        double[] _put;
        
        int yminus1;
        int yplus1;
        
        // for each x and y position
        for(int x=start ; x< end ;x++)
            {
            _next = _valgrid_field[_valgrid.stx(x+1)];
            _put = _valgrid2_field[_valgrid.stx(x)];
            
            yminus1 = _valgrid.sty(-1);  // initialized
            for(int y=0;y< _gridHeight;y++)
                {
                // for each neighbor of that position
                // go across top
                yplus1 = _valgrid.sty(y+1);
                average = (_past[yminus1] + _past[y] + _past[yplus1] +
                    _current[yminus1] + _current[y] + _current[yplus1] +
                    _next[yminus1] + _next[y] + _next[yplus1]) / 9.0;

                // load the new value into HeatBugs.this.valgrid2
                _put[y] = _evaporationRate * 
                    (_current[y] + _diffusionRate * 
                    (average - _current[y]));

                // set y-1 to what y was "last time around"
                yminus1 = y;
                }
                
            // swap elements
            _past = _current;
            _current = _next;
            }
        }
    }