[Mesa-dev] Time to merge threaded GL dispatch? (aka glthread)

[Marek Olšák]

On Thu, Feb 9, 2017 at 1:52 PM, Ian Romanick <idr at freedesktop.org> wrote:
> On 02/08/2017 10:26 AM, Nicolai Hähnle wrote:
>> On 07.02.2017 23:54, Matt Turner wrote:
>>> On Tue, Feb 7, 2017 at 10:56 AM, Marek Olšák <maraeo at gmail.com> wrote:
>>>> On Tue, Feb 7, 2017 at 2:57 AM, Kenneth Graunke
>>>> <kenneth at whitecape.org> wrote:
>>>>> On Monday, February 6, 2017 8:54:40 PM PST Marek Olšák wrote:
>>>>>> On Mon, Feb 6, 2017 at 8:20 PM, Ernst Sjöstrand <ernstp at gmail.com>
>>>>>> wrote:
>>>>>>> FYI glmark2 segfaults with mesa_glthread=true. Expected that some
>>>>>>> programs
>>>>>>> will segfault?
>>>>>>
>>>>>> Yes, even segfaults are expected with mesa_glthread=true.
>>>>>>
>>>>>> Marek
>>>>>
>>>>> Would it make sense to be crash-free or even regression-free on at
>>>>> least Piglit, before merging?  (Or are we there already?)
>>>>
>>>> It's not necessary. glthread is disabled by default. Nobody has tested
>>>> piglit with glthread. That will follow after it's been merged, or
>>>> never if it's never merged.
>>>
>>> I don't understand why you're so concerned about merging untested
>>> code. That violates some pretty fundamental development practices of
>>> the project.
>>>
>>> It's exactly unfinished projects like this that cause problems and
>>> inevitably have to be deleted later (ilo, openvg, d3d1x, etc). I don't
>>> think it's a burden to develop something out of the master branch
>>> until it's somewhat useful.
>>
>> The code is already somewhat useful. Actually, make that _very_ useful
>> (big performance improvement) for _some_ cases.
>>
>> I suspect most of the people in this discussion haven't really looked at
>> the code in detail (myself included). We should probably do some of that
>> before it is merged, since the code isn't just a new driver that is
>> isolated in its own directory. So I agree with Emil that it makes sense
>> to let the patches go over the mailing list once.
>>
>> However, it's fine to merge this without passing piglit.
>
> No, it absolutely is not fine to merge.  We have never allowed such a
> thing, and I'll be damned if I'll allow this project to start.  Things
> that land that are known to be broken never actually get fixed.  Then we
> have to waste time fielding bug reports and Phoronix threads because
> users turn on the performance features and everything breaks.  It's just
> a terrible idea.

It does pass piglit, but only when it's disabled.

We have to ask the question of how long it will take to reach the
level of perfection that some people here demand. 1 year? 2? 4 years
even? Are we willing to wait that long? Is there a sufficient minimum
requirement on merging this project that's possible to reach within 2
weeks? Instead of saying "absolutely not" and "terrible idea", why not
just say "yes if X gets done"?

If you only expect absolutism and perfectionism, you'll be very
disappointed with this project. Doing a threaded GL dispatch is hard.
NVIDIA had had performance issues with it for a very long time. AMD
doesn't even have a solution that can be enabled by default.

I'm not heavily invested in this project (yet), so it won't bother me
much if this doesn't get merged before mid-2017. If people wanna
contribute, they can send me pull requests. (please do it early &
often, don't wait until you have something that's good enough)

I've been contemplating doing threaded gallium dispatch for a while
now. It should be very easy if we copy some sync prevention tricks
from radeonsi, and we might be able to enable it by default for all GL
apps (core & compat) from day 1. That can be our short-term goal for
gallium, while threaded GL can be our long-term goal and only limited
to whitelisted apps for quite a while.

Marek