linux.conf.au hackfest: the solution, part two

In the last article we finished with a SPE-based fractal renderer, but with a limited maximum fractal size of 64 × 64 pixels:

We'd like to generate full-size fractals, but the DMAs (which we use to transfer the fractal image out of the SPE) have a maximum size of 64kB. The solution is to perform multiple DMAs each containing a subset of the image's rows.

Each invocation of render_fractal() should render a DMA-able chunk of fractal data, then we perform the DMA. We do this until the SPE has processed the entire image:

We just need to modify the spe-fractal code (spe-fractal.c) a little. At present, we just render the whole fractal in one pass and DMA the data in the main() function:

        render_fractal(&args.fractal);

        mfc_put(args.fractal.imgbuf, ppe_buf,
                args.fractal.rows * args.fractal.cols * sizeof(struct pixel),
                0, 0, 0);

        /* Wait for the DMA to complete */
        mfc_write_tag_mask(1 << 0);
        mfc_read_tag_status_all();

First, we need to modify our render_fractal() fuction to take a starting row, and a number of rows to render. This is the new prototype of render_fractal():

static void render_fractal(struct fractal_params *params,
                int start_row, int n_rows)

In the SPE program's main(), we just need to set up some convenience variables:

        bytes_per_row = sizeof(*buf) * args.fractal.cols;
        rows_per_dma = sizeof(buf) / bytes_per_row;

And do the rendering and DMAs in a loop:

        for (row = 0; row < args.fractal.rows; row += rows_per_dma) {

                render_fractal(&args.fractal, row, rows_per_dma);

                mfc_put(buf, ppe_buf + row * bytes_per_dma,
                                rows_per_dma * bytes_per_row,
                                0, 0, 0);

                /* Wait for the DMA to complete */
                mfc_write_tag_mask(1 << 0);
                mfc_read_tag_status_all();
        }

This loop will render as many image rows as will fit into a single DMA, then DMA the rendered data back to main memory.

Now, we're able to render fractals larger than 64 × 64 pixels:

The source for the updated fractal renderer is available in fractal.2.tar.gz.

performance

Now that we can generate full-size fractals, we can compare the running times with the PPE-based fractal renderer. The following table shows running times with a standard fractal (using these fractal parameters).

Running times of fractal renderer

Implementation	Time (sec)
PPE	55.7
1 SPE	40.7

So, we get a 27% speedup by moving the fractal generation code to run on a SPE. We're still a way behind the optimal performance though, and benchmarking on other systems gives better times (for example, generating the same fractal on an Intel Core 2 Duo @ 2.4GHz takes 13.8 seconds).

We can improve the Cell performance by a large amount - stay tuned for the next article to see how.