[Mesa-dev] Path to optimize (moving from create/bind/delete paradgim to set only ?)

[Jerome Glisse]

On Tue, Nov 16, 2010 at 2:38 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> On 16.11.2010 20:21, Jerome Glisse wrote:
>> Hi,
>>
>> So i looked a bit more at what path we should try to optimize in the
>> mesa/gallium/pipe infrastructure. Here are some number gathers from
>> games :
>> drawcall /     ps constant   vs constant     ps sampler    vs sampler
>> doom3            1.45             1.39               9.24              9.86
>> nexuiz             6.27             5.98               6.84              7.30
>> openarena  2805.64             1.38               1.51              1.54
>>
>> (value of 1 mean there is a call of this function for every draw call,
>> while value of 10 means there is a call to this function every 10 draw
>> call, average)
>>
>> Note that openarena ps constant number is understable as it's fixed GL
>> pipeline which is in use here and the pixel shader constant doesn't
>> need much change in those case.
>>
>> So i think clear trend is that there is a lot of constant upload and
>> sampler changing (allmost at each draw call for some games) Thus i
>> think we want to make sure that we have real fast path for uploading
>> constant or changing sampler. I think those path should be change and
>> should avoid using some of the gallium infrastructure. For shader
>> constant i think best solution is to provide the ptr to program
>> constant buffer directly to the pipe driver and let the driver choose
>> how it wants to upload constant to the GPU (GPU have different
>> capabilities, some can stream constant buffer inside their command
>> stream, other can just keep around a pool of buffer into which they
>> can memcpy, ...) As there is no common denominator i don't think we
>> should go through the pipe buffer allocation and providing a new pipe
>> buffer each time.
>>
>> Optimizing this for r600g allow ~7% increase in games (when draw is
>> nop) ~5% (when not submitting to gpu) ~3% when no part of the driver
>> is commented. r600g have others bottleneck that tends to minimize the
>> gain we can get from such optimization. Patch at
>> http://people.freedesktop.org/~glisse/gallium_const_path/
>>
>> For sampler i don't think we want to create persistant object, we are
>> spending precious time building, hashing, searching for similar
>> sampler each time in the gallium code, i think best would be to think
>> state as use once and forget. That said we can provide helper function
>> to pipe driver that wants to be cache sampler (but even for virtual hw
>> i don't think this makes sense). I haven't yet implemented a fast path
>> for sampler to see how much we can win from that but i will report
>> back once i do.
>>
>> So a more fundamental question here is should we move away from
>> persistant state and consider all states (except shader and texture)
>> as being too much volatile so that caching any of them doesn't make
>> sense from performance point of view. That would mean change lot of
>> create/bind/delete interface to simply set interface for the pipe
>> driver. This could be seen as a simplification. Anyway i think we
>> should really consider moving more toward set than create/bind/delete
>> (i loved a lot the create/bind/delete paradigm but it doesn't seems to
>> be the one you want with GL, at least from number i gather with some
>> games).
>
> Why do you think it's faster to create and use a new state rather than
> search in the hash cache and reuse this? I was under the impression
> (this being a dx10 paradigm) even hw is quite optimized for this (that
> is, you just keep all the state objects on the hw somewhere and switch
> between them). Also, what functions did you really see? If things work
> as expected, it should be mostly bind, not create/delete.
> Now it is certainly possible a driver doesn't make good use of this
> (i.e. it really does all the time consuming stuff on bind), but this is
> outside the scope of the infrastructure.
> It is possible hashing is insufficient (could for instance cause too
> many collisions hence need to recreate state object) but the principle
> mechanism looks quite sound to me.
>
> Roland
>

The create/bin & reuse paradgim is likely good for a directx like api
where api put incentive on application to create  and manage
efficiently the states it wants to use. But GL, which is i believe the
API we should focus on, is a completely different business. From what
i am seeing from games, we repeatly see change to shader constant and
we repeatly see change to sampler. We might be using a tool small hash
or missing opportunity of reuse, i can totaly believe in that. But
nonetheless from what i see it's counter productive to try to hash all
those states and hope for reuse simply because cost of creating state
is too high and the reuse opportunity (even if we improve it) looks
too small. Here you have to think about hundred of thousand call per
frame and wasting time to try to to find a GL states pattern in
application looks doom to failure to me. From what i have seen, the
closed source driver directly translate GL state into GPU register
value and directly submit a whole register to the GPU, i guess GPU are
good at changing state and so that it's useless to burn CPU to try to
save GPU time.

So yes, it seems there is no reuse and no i don't think we can or it's
even worth of trying to fix that.

That being said, i was believer in the create/bind & reuse state
paradigm, but from what i am seing it's just not what you want to do
for GL API. We are just trying to match DX intermediate representation
with GL and i think it's a bad match. Dunno if there is GL on DX open
source implementation that one can benchmark to get a better idea on
that...

Cheers,
Jerome Glisse