Question

Working on some matrix code, I’m concerned of performance issues.

here’s how it works : I’ve a IMatrix abstract class (with all matrices operations etc), implemented by a ColumnMatrix class.

abstract class IMatrix
{
    public int Rows {get;set;}
    public int Columns {get;set;}
    public abstract float At(int row, int column);
}

class ColumnMatrix : IMatrix
{
    private data[];

    public override float At(int row, int column)
    {
        return data[row + columns * this.Rows];
    }
}

This class is used a lot across my application, but I’m concerned with performance issues.
Testing only read for a 2000000×15 matrix against a jagged array of the same size, I get 1359ms for array access agains 9234ms for matrix access :

public void TestAccess()
{
    int iterations = 10;

    int rows = 2000000;
    int columns = 15;
    ColumnMatrix matrix = new ColumnMatrix(rows, columns);
    for (int i = 0; i < rows; i++)
        for (int j = 0; j < columns; j++)
            matrix[i, j] = i + j;

    float[][] equivalentArray = matrix.ToRowsArray();

    TimeSpan totalMatrix = new TimeSpan(0);
    TimeSpan totalArray = new TimeSpan(0);

    float total = 0f;
    for (int iteration = 0; iteration < iterations; iteration++)
    {
        total = 0f;
        DateTime start = DateTime.Now;
        for (int i = 0; i < rows; i++)
            for (int j = 0; j < columns; j++)
                total = matrix.At(i, j);
        totalMatrix += (DateTime.Now - start);

        total += 1f; //Ensure total is read at least once.
        total = total > 0 ? 0f : 0f;

        start = DateTime.Now;
        for (int i = 0; i < rows; i++)
            for (int j = 0; j < columns; j++)
                total = equivalentArray[i][j];
        totalArray += (DateTime.Now - start);
    }

    if (total < 0f)
        logger.Info("Nothing here, just make sure we read total at least once.");

    logger.InfoFormat("Average time for a {0}x{1} access, matrix : {2}ms", rows, columns, totalMatrix.TotalMilliseconds);
    logger.InfoFormat("Average time for a {0}x{1} access, array : {2}ms", rows, columns, totalArray.TotalMilliseconds);
    Assert.IsTrue(true);
}

So my question : how can I make this thing faster ? Is there any way I can make my ColumnMatrix.At faster ?
Cheers !

Answer 1

The array code you’ve written can be optimized easily enough as it’s clear that you’re accessing memory sequentially. This means the JIT compiler will probably do a better job at converting it to native code and that will result in better performance.
Another thing you’re not considering is that inlining is still hit and miss so if your At method (why not using an indexer property, by the way?) is not inlined you’ll suffer a huge performance hit due to the use of call and stack manipulation. Finally you should consider sealing the ColumnMatrix class because that will make the optimization much easier for the JIT compiler (call is definitely better than callvirt).