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[mattkhan]

I have a couple of UHD discs; Star Wars The Last Jedi and Dunkirk. The former plays without any issue, the latter stutters quite badly so is unwatchable.  Playback is to a 1080p pj so renderer has to downscale.

I'm using madvr and have tried both latest (92.14) and the shipped version. This does not appear to be a madvr issue as

a) the gpu is not under particular load
b) no setting in madvr makes any difference to the result, and
c) render times are well within normal range (~25ms which is well inside the ~40ms budget)

madvr stats show see the assorted queues are not filling, cpu stats show MC is using ~60% of cpu (core i7 4770)

I guess this might indicate an MC issue or perhaps LAV?

RO standard can't cope either (though not sure if it is meant to be able to deal with 4k playback) and reports an avg frame rate of around 18fps.

Any ideas?

[Hendrik]

The high CPU usage would indicate you're not using your GPU for decoding, and that generation of CPU might not manage to decode a 4K movie fast enough.
Using hardware accelerated decoding could help.

[mattkhan]

thanks, that did the trick.

any downsides to using hardware decoding? IIRC it was more prone to glitches than software (would explain why I have it turned off) but not sure if that is still the case.

[Hendrik]

There is that chance, but with 4K you don't get much of a choice anymore.

[jmone]

I too used to just use SW Decoding (years) ago but from what I've seen HW decoding seems to be equally competent these days.  FYI - tested UHD Dunkirk downscaling to a 1080p monitor with and without HW Decoding using madVR:
- HW Decode Off :  CPU 50%, GPU 50%
- HW Decode On : CPU 13%, GPU 60%

CPU i7-6700K / GPU : GTX960

So a strong CPU can handle it but all 6 of my cores were running at 50%, but more importantly even the old GTX960 has no issues.