[Bug 102272] [BXT] up to 6% performance drop with "i965: Set "Subslice Hashing Mode" to 16x16 on Apollolake"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=102272

Eero Tamminen <eero.t.tamminen at intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #2 from Eero Tamminen <eero.t.tamminen at intel.com> ---
Bisect verified that the memory bandwidth improvements on 18 EU BXT:
- 5% GLB 2.7 Fill, SynMark ZBuffer, GpuTest Triangle (fullscreen)
- 3% SynMark TexMem* & TexFilterTri

were also from the same Hashing Mode change that caused the perf drops.

When investigating the Fill case with performance counters, 16x16 hashing mode
improves the cache hit rate as expected, which explains the performance
improvement.

-> Marking as wontfix, the perf regressions from the commit are acceptable
compromise compared to improvements.

If there are important use-cases with huge amounts of really small triangles
where 16x16 can regress performance, it may make sense to have this as DRI conf
option for the context creation.

(If terrain tessellation perf dropping, but geom instancing not with 16x16 mode
is because of their different triangle ordering, that raises a question why
tessellation in GPU side produces triangle order that's aligned worse for the
16x16 hashing mode.)

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