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

[Roland Scheidegger]

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