4k LG ultrawide latency issue when mirroring over BetterDummy

[Amidone97]

Hey,

I'm using BetterDummy to get better scaling with hidpi mode, It works great except I feel that the rendering of the screen has about 50-100ms latency by eyeballing it, as I'm used to 1-5ms, this is noticeable. Any idea if it could be fixed somehow? I've tried almost all of the resolutions and it happens on all of them, it doesn't happen when there is no mirroring going on though.

Thanks.

[waydabber]

Hi @Amidone97 - what config is it?

[Amidone97]

Sorry, should have added. I'm using 21.5:9 QHD dummy with 1978x828 resolution atm on 2021 M1 Macbook Pro, Big Sur 11.4 I'm also using switchresX to manipulate the mirrors and resolutions. I've tested all of the viable ones and they all seem to have lag, some more, some less. The monitor is LG 34WL85C-B.

[waydabber]

Well, I am afraid there is not much to do with this. We did some performance tests and there was no readily noticeable lag, but for sure, the additional latency might not be optimal for gaming or people sensitive to it will notice. The actual mirroring is handled by macOS. On M1 the kind of mirroring used is classified as hardware mirror, so it is as fast as the graphics hardware is able to produce a mirror and do the required scaling in general (it would be interesting however to try mirroring the internal screen to the external display using a comparable resolution with the dummy and see if there is a difference).

[waydabber]

(on a side note, you could try updating to a newer OS. M1 is a relatively fresh architecture and Apple still constantly optimizes how things work, including the graphics subsystem, so there might be some performance beneftis).

[bconway]

I'm currently running a MacBook Air M1 (16 GB, 7-core GPU) with HP X27q (27" IPS, 1440p, 165Hz, 1ms) connected via DP 1.2 on Monterey 12.2. Obviously I'm using the lowest end graphics available on Apple Silicon. My resolution is set to 2240x1260 and things are functional, including display sleep. Like the OP, I'm seeing very significant latency compared to native, i.e. the mouse lag is noticeable and ugly.

@waydabber Is it possible to lower the overhead in any way for such a graphics configuration? Could the dummy be configured to run as the lowest resolution that still supports HiDPI, if it isn't already as such?

Also, perhaps changing the discussion title to something more generic and latency-related?

[waydabber]

Hi @bconway - thanks for the feedback!

There should not be much latency beyond what is inherently given in a mirroring situation in macOS. But the dummy works only at 60Hz so compared to 165Hz you'll experience a serious decrease in refresh rate and it in itself causes a perceived latency. Unfortunately this is a macOS limitation at the moment, this might change once Apple starts supporting ProMotion for virtual displays (like Sidecar).

For lag and performance we might continue this discussion as well: #53

[jacksongoode]

I feel that there may be some slight latency (on M1 Pro 14' with a LG 34WN80C-B). Just wanted to add to this discussion - I feel that some actual tests (for those that have the equipment) would be interesting to see on the dummy mirroring.