[Bug 87726] [BDW Bisected] OglDrvCtx performance reduced by ~30% after use true PPGTT in Gen8+

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=87726

--- Comment #16 from Chris Wilson <chris at chris-wilson.co.uk> ---
You would need to measure current mesa with execlists=0 and ppgtt=1 to get the
current baseline. 

Baseline: i915.enable_execlists=0 i915.enable_ppgtt=1
# Not sure if i915.enable_execlists=0 i915.enable_ppgtt=2 is functional enough
to # test
1: i915.enable_execlists=1 i915.enable_ppgtt=1
2: i915.enable_execlists=1 i915.enable_ppgtt=2

The biggest improvement in the kernel for this bug was from using an rbtree for
vma lookup, and improving the mechanisms for writing the GTT. (Almost)
Everything I had planned for this bug (since ~30 months ago) is almost in the
kernel, the last dregs can be found here:
https://cgit.freedesktop.org/~ickle/linux-2.6/log/?h=prescheduler (down to 37
patches!!!).

The remaining features are O(1) lookup for vma from a context (i.e. excellent
scaling of execbuf to very large number of contexts), context state caching.

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