Multiplayer / Integrated servers have very poor performace and cannot keep up even with one player on a high-end PCs / Servers / Machines

Description

Vanilla server platforms (integrated and external) are very poorly optimized and can barely run smoothly, even on a high-end hardware with higher amounts of memory allocated.

Typical, small multiplayer server drops to critically low TPS and can crash / be forcibly shut down by the watchdog (if enabled) with just a few players normally generating new chunks (in survival gamemode).

A server, using the "Default GUI" is almost impossible to run stable. When launching it via command line and allocating sufficient amount of RAM, the performance is a little bit better (and the server is not running out of memory), but the playing experience is still very rough and there is quite a lot of rubber-banding, freezes, lag spikes, etc.

How to reproduce the issues & Testing methods

Singleplayer (Integrated Server)

• Launch the game with plenty of memory allocated (4GB is usually more than enough)
• Enter into a world or create a new one
• If creating a new world, initially there are extreme framerate drops (probably client-side issue), the integrated server is rubber-banding and skipping ticks
• Start exploring the terrain around and perform the usual things (generating / loading new chunks, dropping item entities, etc.) - There will be noticable stuttering, lag and rubber-banding when punching mobs or making redstone circuits, for example
• If the player is in creative mode (or spectator mode) then everything becomes much worse

If you are playing in a survival world for long enough (assuming there are animal pens / farms, some mob grinders, chest rooms with item frames and similar) the world will run much slower and this can cause issues with redstone circuits, influence on the gameplay and so on.

Multiplayer (Vanilla platform, without any mods / tweaks / changes)

• Start a server on a relatively powerful hardware with sufficient amount of memory allocated (4GB again, would be probably more than enough)
• Join the server, along with a few other players (2-4 players at the same time is normal, 5+ will be a bit heavier)
• Start playing as usual (exploring around, interacting with entities, mobs, items) - There will be clearly noticable rubber-banding and lag spikes, which usually result in stuttering entities, desyncing issues between the clients and the server and other

If some of the players start to generate new chunks on creative mode, the server will most likely crash.

Testing methods & RAM allocation

Tested and experimented with many combinations of JVM heap sizes (amount of memory allocated), using the default, "recommended" start-up command line:

java -Xms<Initial Memory>M -Xmx<Max Memory>M -jar <Server Jar>.jar nogui

(Xms = Starting Memory / Xmx = Maximum Memory)

• Xms1024M / Xmx1024M
• Xms2048M / Xmx2048M
• Xms3072M / Xmx3072M
• Xms4096M / Xms4096M
• Xms6144M / Xmx6144M
• Xms8192M / Xmx8192M
• Xms1024M / Xmx2048M
• Xms1024M / Xmx3072M
• Xms1024M / Xmx4096M
• Xms1024M / Xmx6144M
• Xms1024M / Xmx8192M

• Players at the same time: 1-3, 3-5, 5-10

Usual activities during the tests:

• Exploring the terrain around in survival mode, creative mode (for a while), spectator mode (just for the experiments, not expecting actual results)
• Interacting with mobs, entities, armor stands, droppping and picking up item entities
• Placing water and lava (creates many block updates)
• Small, medium and advanced redstone circuits (light, piston, hopper updates)
• Small, medium and large mob farms / grinders
• Updating gravity-affected blocks (sand, gravel, concrete powder)
• Entering different dimensions and teleporting items, mobs, entities through portals
• Transfering small and large amounts of items through hoppers
• Loading previously generated and alredy saved chunks

The RAM & CPU usage (especially) was abnormally high for very small and short activities or sometimes none at all. Not only chunk generation is causing the more serious issues (but mainly it is).

Conclusion

The main reason for most of the issues is not exactly the TPS drop / loss / poor server performance, but client-side ticking, updates and synchronization.
Other games (clients) handle the updates, coming from the server in different way.
If the server can't keep up with the consistend update rate / tick rate, then the client should enable some sort of "interpolation" and increase slightly the delay between the changes (updates).

The Minecraft client runs at hardcoded 20 TPS and waits for the next tick from the server. If it is synchronized (matches with the incoming client update / tick) then it runs normally.
But even if the server drops to 19.99 TPS - There is desynchronization once in a while and this can create issues (mainly client-side).

Overall, the server performance should be definetly optimized more and some things can be tweaked.

Related issues: MC-117611, MC-44385, MC-4890, MC-342, MC-54026