[Mesa-dev] [PATCH 1/2] i965: Don't bother trying to extend the current vertex buffers.

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> This essentially reverts the following:
>
>   commit c625aa19cb53ed27f91bfd16fea6ea727e9a5bbd
>   Author: Chris Wilson <chris at chris-wilson.co.uk>
>   Date:   Fri Feb 18 10:37:43 2011 +0000
>
>       intel: extend current vertex buffers
>
> While working on optimizing an upcoming Steam title, I broke this code.
> Eric expressed his doubts about this optimization, and noted that the
> original commit offered no performance data.
>
> I ran before and after benchmarks on Xonotic and Citybench, and found
> that this code made no difference.  So, remove it to reduce complexity
> and make future work simpler.

I think both of those are VBO-only, so I don't think they would show
anything.  The way I think this path could work is if you're doing
interleaved upload.  Openarena would be an example of that.

That said, while I'd like numbers, I dislike this code even more.  When
the code was written, iirc the kernel was doing a spinlocked lookup per
relocation to find the target buffer, and that's what the optimization
was trying to avoid.  Now, the lookups are done once into a temporary
chaining hash table and that's used in the relocations.  So I would
expect that, even if this code mattered back then, it matters less now,
to the point of hopefully being unnecessary.

If the kernel is still too expensive, I think there are things we could
do in the kernel to improve lookup in general.

Reviewed-by: Eric Anholt <eric at anholt.net>
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