[Bug 102336] [BYT] Offscreen tests performance drops to <1/3 due to power management

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=102336

--- Comment #9 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to Eero Tamminen from comment #8)
> (In reply to Chris Wilson from comment #7)
> > Now fixed in x86/tip.
> 
> Verified, all the regressions are no fixed, thanks!
> 
> 
> > If you want to report some details of the hang, otherwise there's not much I
> > can do.
> 
> Was the issue BYT specific, like the related perf drop was?

The hangs? Haven't seen any, so I don't yet know what's going on there. The
perf issue here is for byt only (as it is the only one that cannot use the HW
EI thresholds and so we calculate those manually).

> > The reason why it suddenly became worse is all due to switching to
> > modesetting, which only used rcs and so we end up interesting behaviour with
> > X getting stuck behind extra frames, and not reporting the buffer as idle as
> > early (so the client would be waiting on the CPU fence being signaled and
> > not waiting on the GPU, so being invisible to the waitboost mechanism).
> > Interesting effect.
> 
> Modesetting drop isn't BYT specific.
> 
> When Martin tested modesetting ~1 year ago against Intel DDX, modesetting
> was same speed or slightly better in 3D cases (and lost in 2D ones).
> 
> However, now switching from Intel DDX to modesetting drops onscreen 3D
> test-cases perf on all platforms for Unigine demos (on anything after BYT),
> GfxBench test-cases and high FPS cases.
> 
> Any idea why?

Hmm, from my inspection on e.g. Unigine -modesetting was and still is around
10% slower. As you are very aware, there are so many different factors at play
-- but under ideal circumstances, the ddx is irrelevant to game
throughput/latency.

Do you have a specific workload that I can use as an example to see what
changes may have impacted it?

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