XNA significant loss in fps by just adding a few hundred thousand more triangles?

Question

I've got a minecraftish game that's running nice and smooth with a total of 9.3 million triangles, about 50-70fps average. Then if I try to render 9.9 million triangles I get hell and it drops to < 1 fps. I'm drawing them exactly the same way, it's definitely the drawing as I have a vertex buffer for each chunk, then I run through them all, check if they're in sight and if they are I draw them. It works fine at high fps until I have just those few extra triangles which destroy the FPS.

Edit: Added drawing code:

First I loop through all the chunks. I check if they're visible and if they are I mark them as visible then draw them.

        foreach (Chunk chunk in GlobalWorld.LoadedChunks)
        {
            if (chunk == null || !chunk.DrawThis)
                continue;

            if (!player.ViewFrustrum.Intersects(chunk.Bounding))
            {
                chunk.IsInSight = false;
                continue;
            }
            chunk.IsInSight = true;

            GlobalGame.graphicsDevice.SetVertexBuffer(chunk.VertexBuffer);
            GlobalGame.graphicsDevice.Indices = chunk.IndexBuffer;

            ChunkShader.Parameters["xWorld"].SetValue(chunk.WorldMatrix);
            ChunkShader.CurrentTechnique.Passes[0].Apply();
            GlobalGame.graphicsDevice.DrawIndexedPrimitives(PrimitiveType.TriangleList, 0, 0, chunk.VertexBuffer.VertexCount, 0, chunk.IndexBuffer.IndexCount / 3);
        }

Then after that I draw all the transparent parts of the chunk (so they don't conflict with other chunks). I already made clear if they're visible above so I just check if that bool is true.

        foreach (Chunk chunk in GlobalWorld.LoadedChunks)
        {
            if (chunk == null || !chunk.IsInSight || !chunk.DrawThisTransparent)
                continue;
            GlobalGame.graphicsDevice.SetVertexBuffer(chunk.TransparentVertexBuffer);
            GlobalGame.graphicsDevice.Indices = chunk.TransparentIndexBuffer;

            ChunkTransparencyShader.Parameters["xWorld"].SetValue(chunk.WorldMatrix);
            ChunkTransparencyShader.CurrentTechnique.Passes[0].Apply();
            GlobalGame.graphicsDevice.DrawIndexedPrimitives(PrimitiveType.TriangleList, 0, 0, chunk.TransparentVertexBuffer.VertexCount, 0, chunk.TransparentIndexBuffer.IndexCount / 3);
        }

With 3721 chunks loaded it runs fine with them all being drawn at the same time. With 3844 chunks being loaded it runs fine unless you view all of them. Now that's only just over 100 more chunks yet it's dropping from 60fps to < 1 fps.

Answer 1

You may be simply running out of VRAM, causing degenerate behavior paging into/out of main RAM when rendering your scene. How many verticies/indicies are you wrangling here? What's your vertex format like? Is this on an xbox 360 or do you have a specific GPU you can point us at?

10 million triangles x (0.5-3) verts per triangle x 5 floats (pos + 1 uv) per vert x 4 bytes per float x = 100-600MB, and that's ignoring fragmentation, indicies, and whatever else you might have loaded (textures)

Answer 2

Mauling Monkey's answer is very right, but to solve your problem, I'll add my response.

I guess you aren't optimizing the cube faces away...
9 300 000 triangles means 4 650 000 faces. If each cube has 6 visible faces, this is 775 000 visible cubes. That number is too high. A correct number should be something like 20 000 cubes (that is how many visible cubes I have in my CraftMania, with a viewing distance of 100 meters).

Two cubes touching each other are having each one invisible face.

  +-----+-----+
 /     /     /|
+-----+-----+ |
|     |     | +
|     |     |/
+-----+-----+
      ^
   (They have this face in common, which is invisible.)

Try to implement this and your results will improve a lot!