Discord causing Osu to lag.

[tpcstld]

Hey! I'm a software engineer at Discord.

Yesterday, we saw https://www.reddit.com/r/discordapp/comments/1tk4mut/discord_may_causes_systemwide_input_lag/ on Reddit about how Discord is causing input lag for games. Our desktop team is investigating and we thought it might be a good idea to engage here.

First of all, we're gonna work on making sure our hooks are not active if Overlay/Keybinds/whatever is not used. This should hopefully alleviate problems at the root.

But furthermore, we're also wondering if you have a pretty good reproduction of the issue? Or if you can share more details/hypothesis in your own investigation? We've been trying out SDL2 vs SDL3 in toy apps and we're not able to see much difference w.r.t input lag. We're wondering if this is some specific corner case (hardware? setup? load?) where this issue arises.

It would be really helpful if someone can point us at the specific APIs that were used before and after. It could be that the devil is in the implementation details and it can really help us narrow down what's happening on our side.

[Susko3]

There are two ways discord could cause osu! to lag: delaying input or delaying rendering to the display. osu-framework has a multi-threaded design, so lag spikes on the draw (render) thread won't cause lag spikes on the input thread and vice-versa. If there is substantial lag, you should be able to tell whether it's input or draw lag by listening to audio feedback. The hypothesis is that misbehaving keyboard hooks cause input latency, and screen overlays draw latency.

I've personally never had problems with input latency, even though I have a few programs with global hotkeys (including discord). I always disable the discord overlay, so can't have problem there.

Input lag

On SDL3, we are using relative mouse mode which enables raw input, so input delay shouldn't happen there. Input delay can still happen with keyboards, as we're not yet using SDL_HINT_WINDOWS_RAW_KEYBOARD. We have very recently moved away from SDL2, so I won't focus on it (but it's worth mentioning that reported issues likely happened on SDL2).

When a input lag spike happens, it's usually a call to SDL_PumpEvents taking a lot longer than usual, this can be detected in-game by looking at the input thread frame statistics overlay, specifically, white bars (for WndProc) appearing the the frame graph (see details by holding Ctrl).

I assume discord is using SetWindowsHookEx with WH_KEYBOARD or WH_KEYBOARD_LL to implement global hotkeys, if you're calling non-C/C++ code from there (eg. JavaScript) that would be an immediate red flag for input latency. If I were you I'd first check if the hook procedure can cause the observed lag. Try wasting CPU time or do a Sleep(100 ms) in your hook procedure and check if that is manifested in osu! as keyboard lag. Maybe you can sleep if Shift is held to have an immediate A/B.

Draw lag

I know much less about this one, but we've had reports of users having bad latency, and disabling screen overlays usually fixes it. Not just with discord's, but also nvidia, overwolf, etc.

Same idea goes here as well, try wasting CPU or GPU time when drawing the overlay to see if that makes osu! lag.

[tpcstld]

Thanks for the details.

Draw lag is regrettable and we hope to improve overlay rendering performance in the long-term. But the cost will never reach 0 and disabling overlay is an appropriate remediation for those users. In particular, we know there is a list of Windows optimizations that can no longer happen when overlay is running and we know that sucks. Make sure those users are disabling both the legacy and new overlay haha.

I passed along the information for input lag. FWIW, I've been told that we actually don't use SetWindowHookEx to implement global hotkeys. On Windows (which I assume where most of the reports are) hotkeys are implemented just by reading the Raw Input events which "in theory" shouldn't be blocking the other consumers.

Regardless, we're probably gonna audit our usage of Windows APIs and see if there some weird stuff happening. If SDL3 seems to have solved your issues, that's great, although we're concerned about the many non-Osus out there still on SDL2 generally.

For the future, if you're getting reports of this nature feel free to add more information in this discussion.