Latency — Near's Respite

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## Latency2019-10-07

There are many sources of latency involved in emulating game consoles. In this article, I'll go over the one input latency source we have direct control over as emulator authors, and propose my method for minimizing it.

## Input Polling

An emulated game system typically reads from controllers once per emulated frame during the non-maskable vertical blank interrupt.

Technically, games can poll controllers at any time, and as many times as they want, but 99.9% of games will follow the above strategy.

Usually, inputs will be available from a memory-mapped I/O register that is polled. Emulators then must convert the emulated mapping to a physical keyboard, mouse, or gamepad, and return the current button state to be returned from reading said I/O register.

## Typical Workflow

Most emulators will be designed to poll the real hardware devices once per frame, in a run loop that looks something like this:

    void Program::run() {
      while(stopped() == false) {
        hardware.pollInputs();
        emulator.runFrame();
        video.drawFrame();
      }
    }

In this case, `hardware.pollInputs()` will query DirectInput, XInput2, SDL, or whatever hardware input APIs are available, and cache the results for later use:

    void Hardware::pollInputs() {
      keyStates = directInput.pollKeyboard();
    }

Next, `emulator.runFrame()` will run the emulator continuously until one frame of video data is ready, at which point the function will return control back to the program.

Inside the emulation core, when the memory-mapped I/O register for gamepad states is read, the cached states are returned:

    uint8_t Emulator::pollGamepad() {
      uint8_t data = 0;
      data |= program.readInput(GAMEPAD_UP  ) << 0;
      data |= program.readInput(GAMEPAD_DOWN) << 1;
      ...
      return data;
    }

This is done by the program translating the emulated inputs to mapped hardware inputs:

    bool Program::readInput(uint inputID) {
      if(inputID == GAMEPAD_UP  ) return hardware.keyStates[KEY_UP];
      if(inputID == GAMEPAD_DOWN) return hardware.keyStates[KEY_DOWN];
      ...
    }

Finally, `video.drawFrame()` will take the emulated video frame and output it to the onscreen window for the program, using Direct3D, OpenGL, SDL, etc:

    void Video::drawFrame() {
      direct3D.draw(program.window, emulator.frame, emulator.width, emulator.height);
    }

This is how the multi-emulator frontend RetroArch is written.

## Missed Frames

For the purposes of this article, let's assume we are talking about the Super Nintendo, which has 262 scanlines per frame. Let's also declare "V" to mean the vertical scanline the emulator is currently generating, from V = 0-261.

If `emulator.runFrame()` emulates an entire frame, including the vertical blanking period, then we end up at V=0 when `hardware.pollInputs()` is called. This state remains until V=225 where it is read by the game, so by the time we actually use the polled results, almost an entire frame, or 16ms, has passed.

(Tangent: if the emulator is extremely performant and synchronizes itself to video, it could reach this state faster, and then sleep the program until the host machine reaches its own vertical blanking period. Synchronizing an emulator to audio, or simply being a demanding emulator, would negate this benefit.)

## Avoiding Missed Frames

Games typically draw the screen, and then enter into a vertical blanking period. Games typically poll the emulated controllers during vertical blanking.

Again assuming this is the Super Nintendo, the screen is rendered between V = 1-224 for NTSC mode, and V = 1-239 for PAL (overscan) mode.

One strategy is for the emulator to exit right after the screen has rendered, but before the inputs are polled. In other words, right at the start of the emulated vertical blanking period.

If `emulator.runFrame()` returns at V=225 (for NTSC) or V=240 (for PAL), then we will poll the host machine inputs right before returning to the emulated machine's vertical blank handler that then polls the inputs.

This is how lag-fix patches are made for RetroArch, which seek to remove this potential one-frame lag penalty.

## Complications

Overscan is actually a setting that tells the SNES to render an additional 15 scanlines, which can be readily seen on a PAL display. However, there are NTSC games that do use overscan, and PAL games that do not use overscan.

In fact, it's even possible, if pathological, to toggle the overscan setting during the vertical blanking period. The Titan Overdrive 2 demoscene software for the Sega Genesis abuses the Genesis' graphics chip to eke out additional scanlines beyond what the original hardware was capable of even.

But even for the SNES, if we reach V=225 with overscan disabled, that doesn't guarantee the entire frame has been rendered: a game may turn on overscan and happily start drawing more scanlines. That's a big problem for trying to exit `emulator.runFrame()` at V=225.

Also consider that games may not necessarily choose to poll the controllers at exactly V=225. Some games may choose to poll inputs at V=220, before the screen ends, or at V=261, right at the end of the vertical blanking routine.

A pathological game might even choose to poll inputs whenever it has available cycles to do it, and the polling location may change every single frame as a result.

The above optimization is too coarse. We can do better.

## Polling Every Scanline

The PC Engine emulator Ootake tries to improve upon this by polling the real hardware inputs once every emulated scanline. That certainly works, but that's 262 DirectInput polling events per emulated frame. At a 60hz refresh rate, that works out to 15,720 calls per second to the DirectInput API.

This is just wasteful as even the fastest USB devices only poll 1,000 times per second. And usually it's only 100 times in default OS configurations.

## Hardware Polling Overhead

Calling `hardware.pollInputs()` is expensive: we have to query hardware APIs that likely require kernel transitions, get entire keyboard states, map those states to our emulated inputs, etc.

This is why emulators try to only do this once per frame. If not for this overhead, the easy solution would just be for `Program::readInput()` to call out to `hardware.pollInputs()` itself.

But there is a simple solution to this.

## Proposal: Just-in-Time Polling

For lack of a better term, I'll just call this JIT polling.

By keeping a timestamp of the last time the host hardware inputs were polled, we can short-circuit doing the actual polling if called too frequently.

This new design would look something like this:

    void Hardware::pollInputs() {
      static uint64_t lastPollTime = 0;
      uint64_t thisPollTime = getHostMachineCurrentTimestampInMilliseconds();
      if(thisPollTime - lastPollTime >= 5) {  //latency timeout: 5 milliseconds
        keyStates = directInput.pollKeyboard();
        lastPollTime = thisPollTime;
      }
    }

    void Program::run() {
      while(stopped() == false) {
      //we no longer have to call hardware.pollInputs() here
        emulator.runFrame();
        video.drawFrame();
      }
    }

    bool Program::readInput(uint inputID) {
    //we call hardware.pollInputs() here instead
      hardware.pollInputs();
      if(inputID == GAMEPAD_UP  ) return hardware.keyStates[KEY_UP];
      if(inputID == GAMEPAD_DOWN) return hardware.keyStates[KEY_DOWN];
      ...
    }

What the above code does is remove the need to care about when `emulator.runFrame()` returns: it simply doesn't matter anymore.

Whenever the emulated system tries to poll the inputs, we poll the host machine inputs at that time. And because we have the 5 millisecond timeout, that means `Emulator::pollGamepad()`'s second call to `Program::readInput()` to get the state of `GAMEPAD_DOWN` will not call `hardware.pollInputs()` a second time.

The hardware is polled immediately before the inputs are read, and then all of the inputs are almost always read together in one cluster by the emulation core, and so `hardware.pollInputs()` is only invoked once per frame.

Now that we've polled the hardware, the emulator continues running for another frame, and so another 16-20 milliseconds have passed, and so once again, the host hardware is polled immediately, no matter when the emulated game has decided to poll the inputs, presuming a typical game that polls inputs once per frame.

And in the event you run into a pathological game that does something like continually poll the input every scanline, we have guaranteed that the maximum latency is 5 milliseconds on the host machine.

In 99.9% of cases, JIT polling will only poll DirectInput 60 times per second, the same as the original flawed technique first discussed in this article.

In the pathological case, the 5 millisecond timeout sets an upper-bound on the maximum overhead, and also the maximum input latency: 5 milliseconds, or 200 DirectInput calls per second.

This is how bsnes and higan emulate input polling. And this is why I did not merge the RetroArch lag-fix patch into bsnes in the past: it was needed for RetroArch's design, but not for bsnes' design. And it would have interfered with my emulation of SNES overscan being possible to toggle during vertical blanking periods.

## Variadic JIT Polling

A cute side-effect of JIT polling is that the 5 millisecond delay can be turned into a user-configurable variable. Set as low as 1 millisecond, you can keep up with the best 1000hz USB polling gamepads and drivers on the market. And if you set the value higher than it takes to render one frame, it becomes a lag simulator! I have no idea why anyone sane would want to do this, but ... input latency is usually a rather abstract concept to users: it's not directly measurable without high-speed 240fps+ cameras and careful analysis of side-by-side video.

JIT polling with an adjustable latency timeout allows users to simulate what it's like to have more input lag. By setting it to 50ms, they introduce approximately two additional frames of input lag, and they can see by direct hands-on experience how two additional frames of latency affects the playability and feel of the game.

I suppose it could also make for a fun party trick: an easy way to greatly increase the difficulty of a game could be to raise the input latency. An easy handicap in a fighting game would be to have a larger latency period for one player than the other. Practical or useful? Probably not so much.

## Closing

I believe that by applying this technique to other emulators such as RetroArch, we can effectively lag-fix every emulation core in one fell swoop, without requiring any patches to upstream emulators.

I further believe that even already lagfixed emulators have the potential to receive very slight further input latency reductions (as in the case of my hypothetical rare game that might poll inputs at the end of vertical blank rather than at the start.)

I'm not claiming to have invented this technique: there are thousands of emulators out there. I'm only stating that at the time of writing, I'm not aware of this being done elsewhere, and I think it would help emulators if this technique were to become more widespread.

I am also not claiming it's a particular stroke of brilliance to come up with: it was a rather simple idea that worked well for me.

I am not asking for credit for this idea. Consider it public domain.

And most importantly of all, I am not criticizing anyone else's approaches. For many years, my own emulators used the first approach as well. Lately there's been a huge push in the emulator development community to mitigate latency, and I couldn't be happier about this. I'll have more to say on this topic in the future, such as an article on a very neat time-shifting latency reduction implemented in RetroArch known as run-ahead. Stay tuned for that.

Emulation development isn't a competition, and everyone benefits from sharing ideas and improving things. That's all I seek to do with this article.

Thanks for reading, I hope this will be of some use to folks out there.

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