[Bug 87725] [BDW Bisected] OglBatch7 performance reduced by ~7% after enable execlists

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=87725

--- Comment #14 from Chris Wilson <chris at chris-wilson.co.uk> ---
It's more or less a fundamental difference in hw implementation between
execlists and legacy submission. For whatever reason, execution latency from
idle is much worse under execlists, and OglBatch7 hits that path continuously
(since the overhead in the driver is sufficient that mesa cannot keep the GPU
busy). However, some of this is self inflicted as execlists incurs greater
execution overhead, both from per-bb/per-ctx instructions and from being irq
driven. Those we can improve and have been improving, but that execution
latency is hard to hide.

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