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Ask HN: Why does my Node.js multiplayer game lag at 500 players with low CPU?

15 points jbryu | 1 comments | 26 Jun 25 17:27 UTC | HN request time: 0.207s | source

I’m hosting a turn-based multiplayer browser game on a single Hetzner CCX23 x86 cloud server (4 vCPU, 16GB RAM, 80GB disk). The backend is built with Node.js and Socket.IO and is run via Docker Swarm. I use also use Traefik for load balancing.

Matchmaking uses a round-robin sharding approach: each room is always handled by the same backend instance, letting me keep game state in memory and scale horizontally without Redis.

Here’s the issue: At ~500 concurrent players across ~60 rooms (max 8 players/room), I see low CPU usage but high event loop lag. One feature in my game is typing during a player's turn - each throttled keystroke is broadcast to the other players in real-time. If I remove this logic, I can handle 1000+ players without issue.

Scaling out backend instances on my single-server doesn't help. I expected less load per backend instance to help, but I still hit the same limit around 500 players. This suggests to me that the bottleneck isn’t CPU or app logic, but something deeper in the stack. But I’m not sure what.

Some server metrics at 500 players:

- CPU: 25% per core (according to htop)

- PPS: ~3000 in / ~3000 out

- Bandwidth: ~100KBps in / ~800KBps out

Could 500 concurrent players just be a realistic upper bound for my single-server setup, or is something misconfigured? I know scaling out with new servers should fix the issue, but I wanted to check in with the internet first to see if I'm missing anything. I’m new to multiplayer architecture so any insight would be greatly appreciated.

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pvg ◴[26 Jun 25 17:33 UTC] No.44389469[source]▶
It sounds like you want to coalesce the outbound updates otherwise everyone typing is accidentally quadratic.
replies(1): >>44389558 #
jbryu ◴[26 Jun 25 17:41 UTC] No.44389558[source]▶
I thought this might've been the issue too, but because the game is turn-based there should only ever be 1 person typing at once (in a given room).
replies(2): >>44390462 #>>44392350 #
pvg ◴[26 Jun 25 23:10 UTC] No.44392350[source]▶
60 * 7 is not all that great either if you get cascading and clumping as people type at the same time- coalescing the outbound updates still seems like a good idea and since the game is turn based you know it's not really going to affect gameplay. You've basically made yourself a first person shooter networking problem for a game that's slower than WoW. That feels like overkill in terms of self-imposed obstacles.
replies(1): >>44392641 #
1. jbryu ◴[27 Jun 25 00:02 UTC] No.44392641[source]▶
Ahhhh I see what you mean now. You just gave me some good ideas. Alas because of the nature of my game, it will always have first person shooter esque networking problems despite it being turn-based. But it's good to know that I'm dealing with a non-trivial level of throughput.