2 frames per second

2 frames of what ? FPS is not the best measurement, 2 fps of 4 fps is much more than 2 fps of 10000 fps.

From what you describes I would say, that vertex perfomance is not your bottleneck and a drop of 2 fps of X is not a proof of interlaeving arrays are slower than not interleaved ones.

so if I interleaved the data for every vertex then this should provide the benefit of locality of reference like in a cache, and things should speed up.

Did you make sure that each vertex is a power-of-2 size? If your vertex is an odd size it will cross multiple cache lines - 16, 32 or 64 bytes are typically good sizes.

As a general rule interleaving should be faster, but there is one case in which it may be slower, and that's if the per-vertex pipeline is either implemented in or being emulated by software. In that case splitting off the position component of your arrays, and padding it to 4 floats to better allow for SIMD optimizations in your driver, may be the optimal solution.

You haven't really given much info about the kind of workload you're running or what kind of hardware you've got, but some circumstances that can cause per-vertex to run in software would include having older graphics hardware that doesn't support hardware T&L (some Intel integrateds from about 5/6 years or more ago would fall under this category), or exceeding a hardware limit/trying to do something that's allowed by the GL spec but is not actually supported in hardware (that may include using GL_UNSIGNED_INT for your GL_ELEMENT_ARRAY_BUFFER on older hardware - again, you have to go back at least 6 years for this - or using GL_UNSIGNED_BYTE on any hardware). If any of this sounds familiar you may have just identified your cause.

Did you make sure that each vertex is a power-of-2 size? If your vertex is an odd size it will cross multiple cache lines - 16, 32 or 64 bytes are typically good sizes.

Aha! I had very odd vertex sizes, 12 bytes for a position, 12 bytes for a normal, 8 bytes for a texture or just 12 bytes for a position, 8 bytes for a texture. So I will pad the vertices, textures and normals out to 16 bytes each but make each stride between a whole bunch of vertex data 64 bytes as 48 bytes isn't a power of two. Is this a good strategy?

Actually it isn’t about it being a power of 2, but a multiple of the cache size, which is usually 16 or 32. MSDN quotes 32 explicitly for certain cases of vertex buffers but my testing reveals this to improve the performance in all cases, not just what they list. Of course, their documentation is for Direct3D, but cache issues and friends are a universal issue.

Performance Optimizations

Also, we are not talking about padding each vertex-buffer element. 12-byte normals and positions (etc.) are completely normal.

This padding is between each vertex in the buffer, not each element of each vertex.

Normally once you interleave position, normals, UV’s, tangents, and bitangents, you have 56-byte vertices. This should be padded out to 64.

L. Spiro

Actually it isn’t about it being a power of 2, but a multiple of the cache size, which is usually 16 or 32.

Aha! I had very odd vertex sizes, 12 bytes for a position, 12 bytes for a normal, 8 bytes for a texture