[Mesa-dev] [Bug 97549] [SNB, BXT] up to 40% perf drop from "loader/dri3: Overhaul dri3_update_num_back" commit

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97549

--- Comment #5 from Eero Tamminen <eero.t.tamminen at intel.com> ---
(In reply to Michel Dänzer from comment #4)
> Created attachment 126214 [details] [review]
> Always use at least two buffers
> 
> Does this patch fix the problem?

Yes, but that's not all...

BXT performance raised a lot in the affected programs end of last week (some
more than the drop, some less than the drop).  This was due Chris' commit
adding BXT & KBL PCI IDs to xf86-video-intel. I assume with this, X will be
using SNA also on BXT instead of the "default acceleration" that uses (slow)
legacy blitter.

So, I did few tests:
* BXT: reverting the "Overhaul dri3_update_num_back" with the new X DDX didn't
affect the performance (same good one)
* BXT: with old X DDX attached patch fixes the drops
* BXT: with new X DDX attached patch doesn't give additional improvement
* SNB: attached patch fixes the drops

Comments:
- This isn't necessarily Mesa issue, but X DDX one
- Something fishy with SNB for which it would be good to get comment from Chris

Still TODO:
-Check modesetting driver perf with the before and after the commit
- Provide requested info from GDB

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