How does run ahead work regarding save states? Does it use the hdd/ssd in any way or is everything done in RAM?
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Everything stays in RAM.
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No. If someone would use that in a run, that run can only be classified as tool-assisted.
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Let’s see if I understood what is runahead correctly: the game on the original console plugged in a real CRT tv would draw the image instantly, but when it comes to computer, all depends on how much powerful the hardware is to render the image the software sent to it before this is sent to display, so, the runahead uses savestate to save the game in the exactly frame the player is looking at, but NOT the actual frame he is because the display is currently receiving the rendering from the hardware and didn’t displayed to monitor yet, so it back some miliseconds to the past (saved on RAM memory) to let the player’s input to work on that very moment. This is it?
I’m a big fan of some hard games, but sometimes I feel the game was made for a better hardware than the console it was released, and I wonder if latency was added accidentaly in this transition, so maybe the better the latency, more like developer intended, so I guess the best is to set runahead in every game you can, but can’t let it stutter, or it will ruin the experience. I like games as hard as Contra and F-Zero, and I can swear lot of times that I pressed the button at the time to dodge something but I get dead anyways, I never stopped playing games so this can’t be lack of practing, but on real console I was better, emulating it, on the other hand, I feel more depressed in those hard games, other games like Mario or Donkey Kong is the same experience it was back them because they are not that intensive (unless you want to play a hard rom hack).
But now I’m interested in something: how do you set your runeahead settings to make it exactly the same as the original hardware? Not better, neither worse, but exactly the same latency, for every game?
The way it works is: states are being made constantly and held in memory, and any time you change your inputs (e.g., you press a button or let go of a button that’s currently held), it rolls back X frames (where X is your runahead value) and applies your input change in the past and then emulates the frames between then and the current frame with your input change applied, all within the time it takes to show the next frame.
So, depending on your perspective, it’s either showing you the future based on your current inputs (i.e., running ahead) or sending your inputs into the past and then retroactively applying them to the present.
As long as the runahead frames do not exceed the game’s internal latency, you will not see any of this. It all happens invisibly, under the hood. However, if you exceed the game’s internal latency, you will see rollback artifacts in the form of skipped animation frames, choppy motion, etc., just like you would see during rollback-based netplay on a dodgy connection.
To make the latency match the original hardware, you first need to measure the total latency of the original hardware setup (we’ll call this A), then measure the total latency of your emulation setup (we’ll call this B). Subtract A from B and the remainder is what you need to shave off, either with runahead or whatever other mitigation strategy (frame delay, hard gpu sync, etc). Since latency is experienced as a total of all contributing factors, reducing it via any of the available means feels the same. That is, you reduce the time between your finger pressing a button and your eyeballs seeing the effects produced on-screen.
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In addition to everything hunterk said, you can check this too:
https://docs.libretro.com/guides/runahead/
It teaches you the correct way to find how many frames of internal latency a game has.
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An interesting game to try out and totally feel the difference is King of Dragons for the arcade, it probably has an intended input delay to give your character more “weight”. In my current setup, it’s running on Vulkan, max swapchains 2, if runahead is OFF, the character will react in the 4th frame (tested using the frame-to-frame function with the K key, the core used is FBNEO since MAME doesn’t support runahead so far, but still have even less natural input latency in some cases. Turning runhead and setting it to 3 makes the character react in the next frame, the difference is huge, but Capcom games often have this delay, maybe one frame less on real hardware, which I suppose is intended. If you run this test yourself, I’d say that that once you get used to the more responsive controls these functions provide, it will be a bit hard to play like we used to back then, or even on older hardware that can’t afford such features.
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The feeling of weight could be given to the characters with longer animations if desired. If those 4 frames of delay are present for all actions and moves in the game, it’s probably an indication of a technical limitation on the game engine and not intentional. The fact that multiple games from the same company have the same amount of frames of latency probably means those games use the same engine.
Your PC and RetroArch settings (other than runahead) don’t change anything in the game’s internal latency, which is what you’re measuring with K. Even with the worst settings for latency, you will always measure the same 4 frames of internal latency on that game.
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Yes, Capcom arcade games, at least in emulation has 3 to 4 frames in my experience. Interesting about your points, so basically only runahead really does make the difference there. Still, with it ON and at 3, most Capcom games feel great to play. The only downside is that once you play even official compilations such as the Genesis Collection on the Xbox SX, it’s clear when playing Shadow Dancer and basically any other fast-paced game, in the PC using Retroarch or even standalone emulators that are now incorporating runahead natively, the games are much more responsive. So it can make us a bit spoiled, specially for action oriented games.
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This explanation was just perfect! I got it interely.
Thank you! I’ll check this, that’s what I needed.
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Not only official compilations lack run-ahead support (that I know of), but they’re also typically terrible when it comes to input lag.
Most often than not those compilations use emulation with no attention paid to latency at all. I don’t ever buy any of those compilations because of those and other issues.
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If I can have a better experience playing on retroarch instead of original console, I guess I’ll go for it, I’m crazy enough to try to maximize competitiveness and fun at the same time, so, I should buy some refurbished or FPGA based consoles if I want to record some authentic gameplay, and use retroarch functions to make some really hard games less frustrating as possible.
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I agree, and that’s why I said the only downside is that we can’t unnotice that. I tried it in my brothers Xbox and I told him that it was hard for me to play Shadow Dancer, a game I’m used to playing, so for him, the games were probably good enough, but since I’m more into PC emulation than he is, I’m spoiled or just a bit more aware about this difference. About runahead, do you notice if it adds sound delay compared to the same test with it off, I’m pretty sure that some games add a bit of delay in that regard.
Absolutely, specially if you’re into shooters and action games in general, it’s that much better. Be aware that FPGA, while an interesting approach isn’t better by default just because someone said it is. It’s still emulation and it still requires good coding, or like the article from below states, any inherent input delay is from the OS and not from the code itself, but then again, I’m not an expert to really explain why that is.
Near wrote this article (archived from 2018) which explains it better than I could. I really like both approaches, I just think this should be clarified, though.
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Yeah, after one is spoiled by low-latency, going for another system with higher latency is immediately noticeable. There’s this feeling of disconnection with the actions in the game.
Not sure as I don’t use runahead myself, but you can test sound synchronization on the 240p test suit for the SNES, for example.
This. It’s very important to note that an FPGA might, or might not, beat the latency of a beefy PC. It depends.
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Has anyone noticed that by overclocking, depending on the game, input lag is halved or can be even be lower? I was always fascinated by PC to console ports and I just tried the Quake II release for the PS1, the game runs fine mostly at 30FPS and looks great, specially for the hardware. I noticed that the game feels a bit heavy on the controls but what I wanted to do was to see how far I could smooth the gameplay, so I started overclocking from 200% up to 1000%, from 300% onwards, the game runs at 60FPS most of the time but I suddenly noticed the controls were much more responsive too and I measured 7 frames of delay in the stock CPU speed, 4 frames of delay at 300% OCed, 3 frames from 500%+, the sweet spot for this game is at 500~, I couldn’t notice any improvements beyond that. By activating Swanstation’s internal runahead to 1 (Core Options), I could reduce the input lag even further and the character reacts at the second frame, where 3 by the OCis already quite impressive, but I didn’t fine tuned it extensively, as the OC and the runahead together started to make the game hiccup a bit as I was pushing it too hard for my old i7-2600, still I found it very interesting to see how input lag can be inherent to the game’s own performance limitations, depending on how it was coded and how it can be basically eliminated with modern enhancements, while making the game look stunning with a tweaked shader (which still needs more time to reach that goal, by the way) and even in widescreen, very impressive stuff. If you try this approach using other cores/games to see improvements on input lag across different systems, I’m sure the info will be useful to others as well. Here’s a short one showing the overclocked performance: Edit: I’ve uploaded a much better video:
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higher frame rates always bring lower input lag, thats natural.
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I was going to say that since you’re doubling the framerate from 30 to 60 fps, you would naturally get lower latency. But the expected latency reduction should be only half a frame of 30fps (~16ms). But you got a 3 frames at 60fps (50ms) reduction.
It’s impressive how that game benefits so much from overclocking.
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That makes sense, the benefits from getting less latency were unexpected, though, as I was focused in framerate performance, you’d expect the game to control better just by making it run consistently, that’s for sure.
True, I wasn’t really even thinking about going that far but the way the game is designed really benefits from this. I’ll say that even in its stock performance, if I still had a PS1 console around, the game itself is very enjoyable. I’m used to how games worked and actually find them fascinating in a way or another, I don’t recall once thinking SNES games were slow or any drops in framerate being an annoyance, as a kid and a teen, hooking a Power Base on the Mega Drive to play Master System games was fascinating, it still is, also, each hardware behaved so differently and the same games across these systems also did.
Don’t take my word for it, though, if you can test yourself and confirm the benefits from this game or others that can benefit from increased internal performance, that would be very interesting.
PS: Another improvement shown in the video above I forgot mentioning is the ability of the core/frontend to preload the CHD into memory, and the added increased CD reading speeds/seek time, I probably could go even further to eliminate loading times as the game is split into small sections that needs to load here and there. One might say “are you still playing a PS or 'insert retro game console/game here”? Yes, it’s still a PS1 game, the design is so good it shows how much more they can offer when enhanced, it’s like they have so much potential to be discovered and enjoyed.
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There were definitely some games with lots of slowdown on the 8 and 16 bit consoles, but yeah, from my experience, framerate consistency started going down from the PS1 onwards.
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