[Mesa-dev] [PATCH 1/7] gallium: add pipe_blend_state::srgb_enable and the CAP

[Samuel Pitoiset]

On 06/14/2017 10:07 PM, Marek Olšák wrote:
> On Wed, Jun 14, 2017 at 9:45 PM, Jose Fonseca <jfonseca at vmware.com> wrote:
>> On 14/06/17 17:12, Marek Olšák wrote:
>>>
>>> On Tue, Jun 13, 2017 at 3:43 PM, Marek Olšák <maraeo at gmail.com> wrote:
>>>>
>>>> On Tue, Jun 13, 2017 at 1:40 PM, Jose Fonseca <jfonseca at vmware.com>
>>>> wrote:
>>>>>
>>>>> On 12/06/17 22:56, Marek Olšák wrote:
>>>>>>
>>>>>>
>>>>>> On Mon, Jun 12, 2017 at 10:43 PM, Jose Fonseca <jfonseca at vmware.com>
>>>>>> wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 12/06/17 21:25, Marek Olšák wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Mon, Jun 12, 2017 at 9:51 PM, Jose Fonseca <jfonseca at vmware.com>
>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> How does this help exactly?
>>>>>>>>>
>>>>>>>>> Are applications actually rendering to the same FBO w/ and w/o SRGB
>>>>>>>>> decoding?
>>>>>>>>>
>>>>>>>>> Or is the problem here GL_SRGB_WRITE state getting spuriously
>>>>>>>>> dirtied
>>>>>>>>> by
>>>>>>>>> the
>>>>>>>>> application?
>>>>>>>>>
>>>>>>>>> And even if they do, why is toggling surface views in framebuffer
>>>>>>>>> state
>>>>>>>>> so
>>>>>>>>> expensive?
>>>>>>>>>
>>>>>>>>> I don't object per se, but it looks like an unusual thing to
>>>>>>>>> optimize
>>>>>>>>> for.
>>>>>>>>>
>>>>>>>>
>>>>>>>> set_framebuffer_state is basically a memory barrier. We have
>>>>>>>> different
>>>>>>>> caches between FB and textures and we have to flush them when a
>>>>>>>> texture is unbound from the framebuffer and set as a sampler view. To
>>>>>>>> keep thing simple, set_framebuffer_state is the barrier. When we
>>>>>>>> change the blend state, the barrier is avoided. Note that the barrier
>>>>>>>> makes set_framebuffer_state a function that is always GPU-bound.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I see.
>>>>>>>
>>>>>>> And you're sure that the incoming set_framebuffer_state are not
>>>>>>> spurious?
>>>>>>>
>>>>>>> I know cso_context always eliminates redundant
>>>>>>> pipe_context::set_framebuffer_state calls, but it is perhaps possible
>>>>>>> that
>>>>>>> Mesa state tracker is reseting the framebuffer state with different
>>>>>>> surface
>>>>>>> views, but that in practice are exactly the same as the previous one?
>>>>>>>
>>>>>>> Like I said, it seems odd apps are doing this: it doesn't make much
>>>>>>> sense
>>>>>>> to
>>>>>>> me to change colorspace of the fragments between draws. (Unless some
>>>>>>> of
>>>>>>> the
>>>>>>> assets are already in SRGB and the app is trying to be too smart for
>>>>>>> its
>>>>>>> own
>>>>>>> good to avoid the sRGB->RGB->sRGB.)  It seems much more likely that
>>>>>>> these
>>>>>>> framebuffer state changes are self-inflicted some where in our stack,
>>>>>>> than
>>>>>>> something truly demanded by the app.
>>>>>>>
>>>>>>> And if that's the case and we can fix it, then it would be a better
>>>>>>> solution
>>>>>>> all around.
>>>>>>
>>>>>>
>>>>>>
>>>>>> Yeah the funny part and the reason is that we have a microbenchmark in
>>>>>> piglit (drawoverhead) changing this state between draw calls. :)
>>>>>>
>>>>>> Marek
>>>>>>
>>>>>
>>>>> I couldn't find that piglit microbenchmark.  mesademos has
>>>>> src/perf/drawoverhead.c but it doesn't set GL_SRGB_WRITE.  So if fbo is
>>>>> changing internally, then it's a perf bug in Mesa state tracker.
>>>>>
>>>>> Unless it's mimicking something that real apps do, then it's probably
>>>>> better
>>>>> to fix the microbenchmark to use a more realistic tests.
>>>>
>>>>
>>>> If you build piglit, it's in bin/drawoverhead.
>>>>
>>>> You're right that this subtest (switching GL_FRAMEBUFFER_SRGB) is
>>>> rather artificial and fairly unlikely to occur with real apps.
>>>
>>>
>>> FYI, I'm dropping this series and I don't have it in my repo anymore.
>>> piglit/drawoverhead will be updated not to test this state change.
>>>
>>> Marek
>>
>>
>> Great.
>>
>> BTW, I'm not sure what's a good state to change in such microbenchmark.
>>
>> There is of course, a myriad of states to pick, but they are not all the
>> same: performance can vary wildly depending on the choice.   I'm not sure
>> what's a good representative state change in such circumstances Perhaps
>> toggling between two texture objects? Or some sampler state?
> 
> If you've ever run the microbenchmark, you know there are plenty of
> state changes tested. I think there are like 15 state changes tested
> in about 60 subtests at the moment. I'm adding more tests into it.
> Currently I have 100 subtests in there locally. At the moment the
> missing subtests are mostly just shader resources: immutable textures
> (mutable textures i.e. not TexStorage-based are already tested), TBOs,
> images, image buffers, SSBOs (maybe), atomic counters (maybe). The
> methodology is 1 state change followed by 1 draw call in a loop,
> measuring the number of draw calls per second for that case, and
> comparing with the baseline draw rate (which is without the state
> change).

And I plan to add bindless support to it. :)

> 
> Marek
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