Insane performance results or not?

[Pauli Nieminen]

On Wed, Feb 10, 2010 at 9:51 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> Am Wednesday 10 February 2010 15:12:27 schrieb Pauli Nieminen:
>> 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
>> (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
>
> Why do the two copies to GTT differ by such a huge amount performance-wise? Is
> this simply caching? What does the picture look like for smaller / larger
> transfers?
>
>>
>>
>> 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.
>>
>> 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?
>
> Try putting the gettimeofday() *after* the radeon_bo_map(). I wouldn't be
> surprised if the seemingly fast GTT -> VRAM copy is simply because the
> transfer hasn't actually finished. Mapping VRAM forces a wait for the transfer
> to finish.
>

benchmrk_accel function is mapping and unmapping the vram bo before
return. This should make sure that bo is free to be used.

> In any case, this is a very interesting experiment that I'll try to replicate
> if I can get around to it.
>
> cu,
> Nicolai
>
>
>>
>> System is:
>> Athlon mobility XP at 2.1Ghz
>> 333MHz ddr memory
>> AGP 8x bus to mobility radeon 9200.
>>
>> Of course the most important part of message is the attachments. diff
>> file contains benchmark code. Also another attachment is my shell
>> scriptthat I used to run the test. It sets my cpu to performance mode
>> to make sure that cpu frequency changes won't affect the results
>>
>> Pauli
>
>
>
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