Saturday, March 25, 2006


3D add-on Alpha 4 released..


Well, it's done: Alpha 4 is outthere. I had to do a lot more work than I anticipated, hence the delay. But it's well worth it I think: a lot more bugs were solved. In other words, Quake 1 and 2 showed more rendering errors after all, just less easy to see for someone not much running games (yet :).

The driver entry on bebits contains a list of all errors solved, so just have a look there for the nifty details, or just download Alpha 4 and read the included HTML file (which contains the same list plus updated application running status info).

What can I add to that info?
Well, rendering speed is higher than ever before, although still slow compared to windows and linux closed source drivers. But I mentioned that already as well. I'll try to do a new benchmark using Alpha 4 to give you the current detailed results: that way you can (finally) compare it to old Alpha 2 speeds I once gave.

GPU and RAM speed (overclocking and bottlenecks)
Let's talk a bit about rendering speed and bottlenecks. I spent a lot of time to find out why the speed is so much slower than on Windows and Linux closed-source drivers. I also tried to get NV20 and higher going once more. I did not find the solution to either problem, but I learned more about the cards and windows drivers in the meantime: who knows, it might help one day.

Anyway, one of the things I did was add tweaking options for GPU and RAM clocking speed in the 2D driver. Of course the 3D driver also benefits from this, and that was the intended result.
I did a test on my P4 at 2.8Ghz/533Mhz fsb, using the GeForce 4MX 440. This card has BIOS settings 275Mhz clock for GPU and 400Mhz for RAM. Coldstarting the card revealed that these speeds are actually programmed.
Here's the result of testing GPU speed with RAM speed at default (400Mhz):
GPU speed, Q2 timedemo1 fps in 800x600x16 mode (Alpha 4)
50, 38.8
100, 66.2
150, 78.8
200, 82.8
275, 86.0

I find this interesting, doubing the GPU speed did NOT double the rendering speed. Now look at RAM testing with GPU at default (275Mhz):
RAM speed, Q2 fps
100, engine hang
150, engine hang
200, engine hang
250, 55.7
300, 66.4
350, 76.4
400, 86.1
450, 92.9 (overclocking 12%!)

Interesting here is the fact that increasing the RAM with a certain percentage, increases fps with the same percentage! When we combine both tables, we can conclude that the RAM access speed is the bottleneck, not the GPU speed. When you benchmark Q2 some more using different settings for texture filtering this conclusion remains intact: fps is not influenced one bit depending on filtering. At least, the GPU doesn't care.

About the engine hangs at low speeds: most RAM used on graphics cards is of the dynamic type: it must be refreshed within a certain amount of time to keep it's content. When it's done too slow the content gets damaged: hence trouble.

So, why is RAM access the bottleneck?

background: RAM bandwidth considerations
So how much data can be transferred with RAM anyway? Well, raw speeds look like about this.
the NV18 (most cards as a fact still), have a 128bit wide path between GPU and RAM. This means that per clock-cycle (SD-RAM) 128/8=16 bytes data are transferred. If we have a clock of 400Mhz, that means 400.000.000 * 16 bytes = 6,4Gbytes/second can be transferred.
We need to deduct some room for refreshcycles, so let's say for argument sake we keep about 6Gbytes/sec bandwidth for our card's functions.

So which functions are running? Well, we need to send RAM content to the screen (monitor). In 1024x768x32 mode at 75Hz refresh that means 1024*768*4*75 = 225Mb/sec are transferred.

This leaves some 4.8Gbytes/sec bandwidth for the GPU accesses and CPU accesses combined.
Note that when you run for instance Quake2 timedemo, that at first the textures are loaded into the cardRAM, and then the demo starts running. Running the demo nolonger transfers data between the host system (CPU) and cardRAM: everything needed is already there. Apart from the actual rendering commands that is, but these are a relatively small amount of data: which resides in main system RAM, and are fetched by the GPU directly (AGP DMA accesses). So these commands don't load the RAM bandwidth. Just the GPU.

Bottleneck identification?
One serious 'problem' with these calculations is the fact that the GPU not always needs chunks of 16 bytes (128bits, the width of the datapath, data transferred in one clock-cycle). If you render using a 16-bit Z-buffer, and some serious hardware access optimisation doesn't exist, those two bytes will cost 16 bytes worth of bandwidth. In other words: these accesses run at 2/16 = 12,5% of maximum speed. For a 32bit colorbuffer, this would be 4/16 = 25% speed.

This could be what we are looking at. Unfortunately, I don't have a clue how to engage optimisation in the GPU for this kind of stuff: the crossbar memory controller (if it exists in those cards, this I should check in the coarse specs from nVidia). This piece of hardware is capable of splitting up those 16 bytes in seperate smaller lanes so to speak.

On the other hand, this same problem should exist on TNT class cards: but we are running at relatively high speeds there already compared to GeForce class cards. If you look at the windows driver results. But then again, there might be completely other reasons for that. It remains sort of guessing.

So, how are those speeds again? I'll sum a bit up once more (for the P4 2.8Ghz system at 1024x768x32 mode):
card, windows (blit-swapbuffer function forced, 16-bit textures), beos ALpha 4
TNT2 (ASUS V3800) , 41.3, 15.6
GF2MX400, 86.0, ---
GF2Ti, 165, ---
GF4MX440, 119, 26.3
(--- means: not tested)

So, the TNT2 works at 15.6/41.3 = 38% of max. speed. The GF4MX440 at 26.3/119 = 22% of max speed. Both cards have 128 bit wide buses by the way.
Note to self: so geforce class cards seem to be running at relatively 50% speed of the TNT class cards. Do we need to enable DDR (double data rate) explicitly?? At least more cards should be compared for this. I seem to remember the GF2Ti running at some 23fps on BeOS, which would be relatively much slower than the GF4MX440.

Indications for actual RAM access bandwidth on BeOS
I had in mind to give you the result of an interesting delta-speed test I did on BeOS, but that will have to wait until another day. Time is up for now. But I'll post it as next item, I promise. It's very much related to the above story after all!

In the meantime: have fun with 3D! It seems we can run Quake1,2 and 3 all on BeOS now. With acceleration...

Signing off. Good night :-)

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