[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 #12 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to Eero Tamminen from comment #11)
> Wendy provided me data with execlists enabled (default) and disabled
> (i915.enable_execlists=0 kernel command line option), for full SynMark
> test-set with recent 3D stack.
> 
> Execlists is in no case faster.
> 
> It's ~50% slower in GL context re-creation test, and in other CPU bound

Hmm, not looked at that. The cause is fairly obvious since execlists implies
even more state to create per context. I think I can safely eliminate most of
the extra overhead. Worth filing that as a separate bug (I guess the report got
overlooked due to the first regression from enabling ppgtt).

> tests it's 5-10% slower:
> - Batch5-7 (batching huge triangle strip with ever increasing number of draw
> calls)

These are what I've been looking at. There is a lot of low hanging fruit we can
eliminate in execlist submission that brings us back on par. However, looking
at the profile of igt/gem_exec_nop and igt/gem_exec_blt, I came to the
conclusion that the largest effect is when the GPU is idle (i.e. the GPU takes
longer to respond to ELSP than a TAIL update). Therefore the biggest impact of
execlists will be different on different machines depending on just how
frequently the GPU stalls waiting for the driver.

> - DrvState (mostly CPU bound GL state change test)
> - Multithread (GL calls from multiple threads)

I presume these will be more or less the same effect as above for execlist
submission overheads. Except we will be exercising context switching (and thus
ring switching) much more. In the driver we can amalgamate consecutive batches
from the same context into a single submission, with more contexts active we
will not only have to do more ELSP submissions (i.e. more irq handling and
general driver overheads) but also more latency from hw context switches.

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