On 8/25/06, Jon Smirl <[EMAIL PROTECTED]> wrote:
On 8/25/06, Timothy Miller <[EMAIL PROTECTED]> wrote:
> I've done all this before, and I can assure you that MANY commands
> take less time to submit than they do to execute, especially if PCI is
> the bus. With 3D, we'll get lots of tiny triangles that are also
> faster than the bus. Packing is vital for throughput.
Your experience should be better. From the outside it is hard to tell
how long the GPU takes to execute an instruction. That suggests a
feature, an on-board timer that tracks how long the command stream
runs and reflects pipelining. Then you could run various combinations
of op codes and see which one is the fastest. Other overhead getting
in the way makes this hard to measure.
Here's the approach I have in mind (which I think I may be copying
from somewhere):
Each packet has a 32-bit header. Some bits are reserved for the
packet type, some are reserved for the packet length (not everything
submitted--just this one command), and the rest depend on the packet
type. For a drawing command packet, those remaining bits will usually
be used as flags to indicate which of the many possible attributes are
also contained in subsequent words of the packet header.
Consider a 2D solid rectangle fill. Upper left corner (32 bits) and
width/height (32 bits) are mandatory. But there are attributes, like
foreground color, that are optional because it's common to draw a
bunch of rects all of the same color. Futhermore, the packet length
is used to indicate how many rectangles (all with the same set of
attributes) are in the packet.
You might define another packet type that lets you specify multiple
rectangles, but all with their own attributes (which ones being
specified again by the flags in the header).
We'll define a set that makes sense to us. Some of the packet types
will exist just to set some of the more obscure attributes in an
efficient way.
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