在DirectX9的文档里面发现一篇文章(Accurately Profiling Direct3D API Calls)，讲述了D3D和驱动对于Command Buffer的处理。澄清了一些原来疑惑的地方。
D3D和驱动运行在两个不同的模式下user mode和kernel mode，也就是说D3D传递Command Buffer到驱动和真正执行返回需要在这两个模式切换，这个是一个比较耗时的操作。在下面的情况下会发生这样的切换:
When one of the lock methods (IDirect3DVertexBuffer9::Lock) is called on a vertex buffer, index buffer, or texture (under certain conditions with certain flags).
When a device or vertex buffer, index buffer, or texture is created.
When a device or vertex buffer, index buffer, or texture is destroyed by the last release.
When IDirect3DDevice9::ValidateDevice is called.
When IDirect3DDevice9::Present is called.
When the command buffer fills up.
When IDirect3DQuery9::GetData is called with D3DGETDATA_FLUSH.