[Mesa-dev] [PATCH 0/5] radeon: Write-combined CPU mappings of BOs in GTT

[Marek Olšák]

On Fri, Jul 18, 2014 at 7:47 PM, Marek Olšák <maraeo at gmail.com> wrote:
> On Fri, Jul 18, 2014 at 5:47 PM, Christian König
> <deathsimple at vodafone.de> wrote:
>> Am 18.07.2014 05:07, schrieb Michel Dänzer:
>>>>>
>>>>> [PATCH 5/5] drm/radeon: Use VRAM for indirect buffers on >= SI
>>>>
>>>> I'm still not very keen with this change since I still don't understand
>>>> the reason why it's faster than with GTT. Definitely needs more testing
>>>> on a wider range of systems.
>>>
>>> Sure. If anyone wants to give this patch a spin and see if they can
>>> measure any performance difference, good or bad, that would be
>>> interesting.
>>>
>>>> Maybe limit it to APUs for now?
>>>
>>> But IIRC, CPU writes to VRAM vs. write-combined GTT are actually an even
>>> bigger win with dedicated GPUs than with the Kaveri built-in GPU on my
>>> system. I suspect it may depend on the bandwidth available for PCIe vs.
>>> system memory though.
>>
>>
>> I've made a few tests today with the kernel part of the patches running
>> Xonotic on Ultra in 1920 x 1080.
>>
>> Without any patches I get around ~47.0fps on average with my dedicated
>> HD7870.
>>
>> Adding only "drm/radeon: Use write-combined CPU mappings of rings and IBs on
>>>= SI" and that goes down to ~45.3fps.
>>
>> Adding on to off that "drm/radeon: Use VRAM for indirect buffers on >= SI"
>> and the frame rate goes down to ~27.74fps.
>>
>> So enabling this unconditionally is definitely not a good idea. What I don't
>> understand yet is why using USWC reduces the fps on SI as well. It looks
>> like the reads from the IB buffer for command stream validation on SI affect
>> that more than thought.
>
> Yes, there is a CS parser with SI, but shouldn't the parser read from
> the CPU copy that came with the ioctl instead? Anyway, I recommend
> only using VRAM for IBs which are not parsed and patched by the CPU
> (which reduces it down to CIK graphics and DMA IBs, right?)

Oh, sorry. There is no CPU copy, just the IB. My recommendation still stands.

Marek