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

Michel Dänzer michel at daenzer.net
Tue Jul 22 20:54:35 PDT 2014

On 21.07.2014 17:07, Christian König wrote:
> Am 19.07.2014 03:15, schrieb Michel Dänzer:
>> On 19.07.2014 00:47, Christian König 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.
>> Hmm, looks like I'll need to do more benchmarking of 3D workloads as
>> well.

I haven't been able to consistently[0] measure any significant
difference between all placements of the rings and IBs with Xonotic or
Reaction Quake with my Bonaire. I'd expect Xonotic to be shader / GPU
memory bandwidth bound rather than CS bound anyway, so a ~40% hit from
that kernel patch alone is very surprising. Are you sure it wasn't just
the same kind of variation as described below?

[0] There were slightly different results sometimes, but next time I
tried the same setup again, it was back to the same as always. So it
seemed to depend more on the particular system boot / test run / moon
phase / ... than the kernel patches themselves.

>> Alex, given those numbers, it's probably best if you remove the "Use
>> write-combined CPU mappings of rings and IBs on >= SI" change from your
>> tree as well for now.
> I wouldn't go as far as reverting the patch. It just needs a bit more
> fine tuning and that can happen in the 3.17rc cycle.

There's no need to revert it, just drop it from the tree. I'd still
prefer that for now.

> My tests clearly show that we still can use USWC for the ring buffer on
> SI and probably earlier chips as well.

Yeah, that might be the safest approach for now.

Earthling Michel Dänzer            |                  http://www.amd.com
Libre software enthusiast          |                Mesa and X developer

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