[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 #8 from Chris Wilson <chris at chris-wilson.co.uk> ---
One step forward, several back in the meantime:

commit db6c2b4151f2915fe1695cdcac43b32e73d1ad32
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Tue Nov 1 11:54:00 2016 +0000

    drm/i915: Store the vma in an rbtree under the object

    With full-ppgtt one of the main bottlenecks is the lookup of the VMA
    underneath the object. For execbuf there is merit in having a very fast
    direct lookup of ctx:handle to the vma using a hashtree, but that still
    leaves a large number of other lookups. One way to speed up the lookup
    would be to use a rhashtable, but that requires extra allocations and
    may exhibit poor worse case behaviour. An alternative is to use an
    embedded rbtree, i.e. no extra allocations and deterministic behaviour,
    but at the slight cost of O(lgN) lookups (instead of O(1) for
    rhashtable). The major of such tree will be very shallow and so not much
    slower, and still scales much, much better than the current unsorted
    list.

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