[Mesa-dev] [PATCH 0/4] RadeonSI: Multithreaded shader compilation

[Marek Olšák]

On Sat, Jul 9, 2016 at 11:02 PM, Grazvydas Ignotas <notasas at gmail.com> wrote:
> On Sat, Jul 9, 2016 at 6:49 PM, Marek Olšák <maraeo at gmail.com> wrote:
>> On Fri, Jul 8, 2016 at 3:20 AM, Timothy Arceri
>> <timothy.arceri at collabora.com> wrote:
>>> On Wed, 2016-06-29 at 18:32 +0200, Marek Olšák wrote:
>>>> Hi,
>>>>
>>>> This series implements basic multithreaded LLVM shader compilation
>>>> in a minimally invasive way. (+51 lines of code in the main patch)
>>>>
>>>> It doesn't help on-demand shader compilation, but it does improve
>>>> loading and startup times by being able to saturate up to 4 CPU cores
>>>> if given enough shaders to compile. A proper shader cache might make
>>>> this redundant, but we don't have that now.
>>>
>>> Have you had a chance to take a look at my recent shader cache work
>>> [1]? The glsl work is mostly done I'm now just cleaning up and fixing
>>> up the fallback paths for when we have a cache miss. I'm not sure if
>>> you guys will need the fallback path or not since you have a way
>>> dealing with varients, you might be ok which would make things much
>>> simpler.
>>>
>>> Anyway I don't think that getting something up and running would be a
>>> large amount of work. Most of your time would likely be spent tweaking
>>> the glsl to tgsi path to haveit to skip over the IR conversion and just
>>> grab the required state.
>>>
>>> The two files you would find most interesting would be:
>>>
>>> src/compiler/glsl/shader_cache.cpp
>>> src/mesa/drivers/dri/i965/brw_shader_cache.c
>>>
>>> Everything else (besided the cache code itself) is pretty much just
>>> wiring things up to be called at the right time, or skipped.
>>>
>>>
>>> [1] https://github.com/tarceri/Mesa_arrays_of_arrays.git shader-cache20
>>
>> No, I haven't looked at it.
>>
>> An on-disk shader cache would be nice to have, but it's not a pressing
>> thing since we don't really get many apps compiling shaders before
>> drawing. UE4 was doing that with GL 4.1 but not GL4.3, so that's cool.
>> The only app where a shader cache would help a lot is Borderlands 2.
>> Sadly, the same game would benefit from OpenGL driver multithreading
>> even more, because once all shaders are compiled, the game is
>> CPU-bound most of the time with frame rates as low as 20 on Core i5
>> 3570.
>>
>> All in all, I need more user feedback to be sure if the on-disk shader
>> cache would make any difference outside of Borderlands 2.
>
> Having played with Timothy's i965 cache I can tell you there are
> certainly more games affected. One of them is The Talos Principle
> where difference is quite large (Serious Sam 3 probably too as the
> engine is the same). Another one is Left 4 Dead 2 where it solves "do
> something the first time" stalls. There are some bugs about older
> Unreal engine games:
> https://bugs.freedesktop.org/show_bug.cgi?id=96790
> https://bugs.freedesktop.org/show_bug.cgi?id=92806
>
> I do note that from subjective user's point of view, i965's shader
> compilation seems to be a lot slower than r600g's. I don't know how
> that all compares to radeonsi though as I have no way to test that.

Yeah, Talos looks like it compiles shaders on demand, but it's pretty
fast and the majority of shaders are compiled during the first frame,
which is like part of the loading. Any later compilations are barely
noticeable even though our shader compiler is quite slow. I'd say the
problem is that i965 and r600 have shader variants and do a lot of
recompilations while radeonsi doesn't. Even though Talos apparently
compiles shaders on demand, there are only 324 unique GLSL program
objects during the first few puzzles, which is pretty much nothing
compared Borderlands 2 with its ~4000 unique program objects.The
recent radeonsi shader improvements (no recompilations + in-memory
shader cache) made shader compilation in Borderlands 2 seem so much
faster that an on-disk shader cache is no longer an absolute
necessity.

I'm open to having the on-disk shader cache for radeonsi just because
of Borderlands 2. I have yet to discover other games that would
benefit from it.

Marek