As I understand it, most kernel operations can’t be interrupted (i.e., they’re non-preemptible). But PREEMPT_RT allows high-priority tasks to interrupt lower-priority ones near-instantly. For specific types of tasks this improves response times and thus performance.

I never looked into the details of realtime Kernel. I know it is or was used for professional realtime audio mixing and recording and such. Besides that, if this improves response times, would gaming benefit from this? What are downsides for using a realtime Kernel for gaming?

[–] CameronDev 34 points 2 days ago* (last edited 2 days ago) (11 children)

Realtime doesn't necessarily mean low latency, it means consistent latency.

So if the latency from and input takes 1s, that is realtime, as long as its always 1s.

Typically for gaming you want the lowest latency possible, and at least historically, that meant not realtime.

Edit: Some examples with made up numbers:

Airbag: you want an airbag to go off EVERY time, and if that means it takes 10ms, thats usually OK. RT guarantees that your airbag will go off 10ms after a crash every time.

Games: you want your inputs handled ASAP, ideally <5ms, but if one or two happen after 100ms, you'll likely not notice. If you enable RT, maybe all your inputs get handled after 10ms consistently, which ends up feeling sluggish.

Unless you know you need RT, you probably dont actually want it.

[–] [email protected] 9 points 2 days ago (4 children)

Games: you want your inputs handled ASAP, ideally <5ms, but if one or two happen after 100ms, you’ll likely not notice. If you enable RT, maybe all your inputs get handled after 10ms consistently, which ends up feeling sluggish.

Actually I think its the other way for gaming: If you have consistent input delay, it will not feel sluggish. Same why consistent 30 fps feels better than varying 31 to 39 fps. Similar for gaming, especially if you play speedrun or 1vs1 fighting games, you would want to have consistent delay. However, if that adds too much delay its probably counterproductive. But for single player games, a consistent delay is the opposite of sluggish.

[–] CameronDev 4 points 2 days ago (1 children)

At low numbers, it doesnt matter. If you exqgerate the numbers the effect is more clear.

Eg. if the latency was 100ms, it would feel your movments are behind by 100ms, which would be unplayable.

But if you had a typical latency of 10ms, with rare spikes to 1s, the spikes would be considered lag, and annoying, but most of the time its good and playable.

[–] [email protected] 6 points 2 days ago (1 children)

@CameronDev @thingsiplay I refer you to this: https://www.pubnub.com/blog/how-fast-is-realtime-human-perception-and-technology/

That said, we did an experiment in a physics class many years ago where by there was a beeper and an electromagnet that were powered by the same source. The electromagnet held a yard stick in place. When the beeper went off we were supposed to push a button in response. The button stopped the fall of the yardstick. Then by calculating how far the yard stick fell using the 32m/s^2 speed of gravitational acceleration we calculated how long response was, average was about 200ms, I responsed in 30ms, however this only works for me for auditory queues, visual is more delayed for me, and I can't detect any change in under 20ms and just barely at that, let alone respond to it. But what I learned in that class was that reaction times varied individual to individual by a factor of about ten, so what is true for one person may not be for another.

[–] [email protected] 1 points 2 days ago

Yeah it definitely varies per person, and I think you can even train for it somewhat.

I used to play a lot of guitar hero, and I think this improved my visual "latency" a lot, to the point where I could definitely tell when something was visually 5-10ms out of sync (in the case of guitar hero, when you strum the bar vs when the note lines up with the strikeline).

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