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Fredrum
Up and Coming
August 11, 2017

SlateUI crippling my games' performance, only in VR.


Hello,

I am currently spending some time looking at my games' performance and optimizing both cpu and gpu based areas.
One thing that is currently driving me crazy is this SlateUI item from the GPU profiler output.

I am testing this with a packaged/cooked version of my Unreal game. I am not using any slatey 2d ui things.
When I test the same build in 2d desktop mode I don't even get a SlateUI item in the list.

It seems to just happen when rendering to the Rift.

Here's an example output,
[2017.08.11-23.18.47:140][391]LogRHI: 38.1% 3.86ms   SlateUI 1 draws 2 prims 4 verts

So since it seems to be VR only related I was hoping someone here know what causes this and how I can get rid of it.
It's way too many ms from the 11ms budget.

Cheers
Fred



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    4 replies

    FredrumAuthor
    Up and Coming
    August 14, 2017

    I would love to get the thought of someone at Oculus about this one.

    After testing some more it seems like this is tied to the mirroring window output.

    I created a new Unreal default VR blueprint project. Didn't change anything except a toggle to turn on and off vr mirroring with a blueprint command 'vr.spectatorScreenMode 0'.
    I then ran the same cooked build eight times, alternating the on/off state of the vr mirroring output window, and getting a unreal editor GPU profiling output dump to the log file using Shift+Ctrl+comma.
    I restarted the game inbetween each take, ie 8 times.

    This is what I got and you can clearly see that the mirrored runs consistently get an ~3ms penalty.

    (the runs without mirroring don't get a 'slate ui' item in the profiler output)

    This is the same result as I am seeing with my main game project.
    It's not good as it means that I can't have the mirroring it's just too crazy expensive. I don't see why it even should be it should just be using already an rendered image and displaying that in a window.

    Cheers
    Fredrik



    FredrumAuthor
    Up and Coming
    August 17, 2017

    Hi imperativity,

    did you manage to find anyting out?

    Cheers
    Fredrik
    FredrumAuthor
    Up and Coming
    August 17, 2017

    Hi I have just recently started to profile more seriously so not sure if the same thing was happening in early versions. It should be easy to reproduce for anyone looking into this just follow the steps I mention as it was done with Unreals default VR project initiation. My test was done using UE4.17.



    FredrumAuthor
    Up and Coming
    September 5, 2017

    imperativity said:

    @Fredrum

    I've made an internal task to review this issue and may have to request a copy of your project in the near future. If I don't have an update for you by end of week, ping me again next week and I will bring you up to speed with regards to where we are at in diagnosing this issue.

    One question though: does this behavior continue in earlier versions of UE4 integrations?




    Hi again,  just wanted to see if you had any updates on this one?

    Cheers
    Fred

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