Insane performance results or not?

[Roland Scheidegger]

On 10.02.2010 15:12, Pauli Nieminen wrote:
> Hi!
> 
> I made some testing how fast my system can move data to VRAM/GTT and I
> got very interestig results:
> 
> (II) RADEON(0): BENCH: copy 3129344 bytes to vram took 78595us,
> resulting in 39Mbps
"Mbps" is a bit confusing I can only read that as Megabit per second
which is even more pathetic :-)

> (II) RADEON(0): BENCH: copy 3129344 bytes to gtt took 11411us,
> resulting in 274Mbps
> (II) RADEON(0): BENCH: copy 3129344 bytes to gtt took 8431us,
> resulting in 371Mbps
> (II) RADEON(0): BENCH: copy 3129344 bytes to vram took 75773us,
> resulting in 41Mbps
> (II) RADEON(0): BENCH: copy 3129344 gtt to vram took 3143us, resulting
> in 995Mbps
> 
> 
> So direct write to VRAM operates only at 40 mega bytes per second.
> That is insanely slow. I hope we won't hit that kind of limit anywhere
> in any code.
> 
> I did check that VRAM is WC cached in /proc/mtrr. But still it is
> surprising slow.
Isn't that something which fast writes should help with, an option we
never really got to work? I agree though it's really bad.
In any case I think it would be interesting to repeat those tests on
pci/pcie cards.

> 
> But most insane result is that CPU can only write to GTT max 371 Mbps
> while GPU can do gtt to vram at 995Mbps. More insane in that results
> is that I was nearly sure that my memory can't operate that fast but
> still when code to check vram content runs everything is correctly in
> vram! What did GPU/AGP did to cheat that much? Is there some error in
> my test case?
> 
> System is:
> Athlon mobility XP at 2.1Ghz
> 333MHz ddr memory
> AGP 8x bus to mobility radeon 9200.

Why is that surprsing? AGP 8x is good for ~2GB/s. Your memory is good
for ~2.7GB/s. Granted, those are theoretical peak values, but getting
1GB/s out of it looks very reasonable. Looks to me like CPU->GTT is slow
rather than GTT->VRAM insanely fast...

Roland