[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

            Bug ID: 102272
           Summary: [BXT] up to 6% performance drop with "i965: Set
                    "Subslice Hashing Mode" to 16x16 on Apollolake"
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: eero.t.tamminen at intel.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Bisecting on BXT (J4205, 18 EUs) revealed following commit:
----------------------------------------------------------
    i965: Set "Subslice Hashing Mode" to 16x16 on Apollolake.

    As of 4.11, the kernel isn't bothering to set the subslice hashing mode
    on Apollolake, leaving it at the default of 8x8.  (It initializes it to
    16x4 on most platforms.)

    Performance data for GPUTest Triangle on Apollolake at 1024x640:

... <max ~1% perf improvement> ...

    Based on this, we choose 16x16 for Apollolake.

    Skylake GT2 with X-tiled buffers appears to be a toss-up between 16x4
    and 16x16, and with Y-tiled buffers it doesn't seem to really matter.
    So we'll leave Skylake alone for now.

    The hashing mode doesn't seem to make a measurable impact on more
    complex benchmarks.

    Acked-by: Matt Turner <mattst88 at gmail.com>
----------------------------------------------------------

To drop performance in several test-cases:
- 6% in SynMark v7 TerrainPanTess
- 3% in GpuTest v0.7 GiMark
- 3% in GpuTest v0.7 FurMark
- 2% in SynMark v7 TerrainFlyTess

And cause additional drop in max sampling rate, on top of bug 102258.

(Unigine Valley had also dropped by 1% somewhere around this time, but that was
too small change to be bisected reliably.)

GpuTest tests were run in HalfHD window, SynMark ones in FullHD fullscreen.


I don't have yet reliable data on the potential improvements from this commit,
but around same time:
- GpuTest Triangle seems indeed to have improved marginally
- raw GPU texture read, copy & blend bandwidth has improved slightly
- Bug 102258 perf drop in SynMark TexMem*, TexFilterTri & GLB 2.7 Fill cases
gets mostly compensated
- SynMark ZBuffer test, which does lot of depth buffer reads, also improves
slightly

-> I will bisect also these (when the BXT machine is free again), to verify
they come from the same commit and to see how much their exact impact is.

Note: Above data is for 18 EU BXT, if commit was tested with 12 EU variant,
it's possible that hashing mode has less impact on 12 EU one.

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