Mobs move incredibly slowly when multiple people join private server

[javid74]

I didn't know where else to post this since it's not the problem of one single mod, and not necessarily forge either. I'm just looking for someone knowledgable. My friend in California hosts a server for himself, me, and one other person. We're all a fair distance apart but still in North America, think 50-60ms ping apart. When only one of us is on the server (it doesn't matter who it is) the mobs move at normal speed. But as soon as anyone else joins they slow to about 20-30% of their normal movement speed. I presume this is a TPS issue but I'm not really sure. The only huge detriment is the mob movement speed.

 

Symptoms:

-When looking up at the sky the stars have rubberband lag which I hear is a classic low TPS symptom.

-My friend increased the threshold tps, not sure if that's the actual name of it but he apparently made it so if the tps drops below a certain point, the server just closes. And of course it closed every 5 minutes after that implementation. So we took that restriction off quite soon.

-When breaking blocks they experience some rubberband lag too when falling to the floor, and it takes a little bit of time to pick them up. But this is basically a given when dealing with large modpacks so I thought quite little of that.

-When hitting mobs they get knocked back very slowly and take some time to fall back to the ground. 

-When hitting mobs you have to wait a considerable amount of time between strikes, or it seems the hit isn't registered. Like all mob related interactions are slowed down.

-When we restarted the world to see how it would run on a newly created world (although same seed) it ran very close to how it runs while alone. Since we've been playing on this server for quite some time now (months) and have explored much of the world, this led me to believe the considerable world size we've generated to be part of the problem. How this would be an issue when render distance is a thing I have no idea, but it's something to think about.

 

Information: 

The server is run off the California friend's PC.

My friend's PC has 32 GB of RAM and he's dedicating a good amount of it to the server so unless we've got a herculean case of Trogoldor the Mole Man I don't see how that would be a problem. Fairly good i7 processor too, and a 1070ti. I do think it runs off a hard drive though so perhaps the read/write speed of that guy isn't up to code?

Minecraft Version 1.12.2

Forge Version 14.23.5.2772

We're running 73 mods so yes a considerable amount but yet again, while alone, this modpack is buttery smooth.

This server hasn't been running for months straight, we've closed and restarted it at the California friend's discretion. 

We heard something about being able to clear the server's cache or something similar to that but haven't found sufficient documentation on how to actually do that. It seems promising that would help.

 

I find this issue so odd because when only one person is on the server it runs buttery smooth, but the moment another joins it turns into molasses. We're both beat as to what the problem is and I'd love any and all feedback to possible fixes. We don't want to always restart the world due to the server getting bogged down over time since we've done so much over the months we've had this open. Thanks for reading!

 

 

[Ugdhar]

Get ahold of the server logs for a period of time that the problems are happening, and post them to github/pastebin or something, and post the link here.

[javid74]

So we looked at the server while it was running and even though we have 16 gb of ram dedicated to the server, and our minimum is set to 8gb, the server tends to only use like 4. Any ideas?

Also i'll see about getting those logs.

[javid74]

We recently tested it again and it has been revealed that when the two people not hosting the server are joined, but my friend in california is away, it seems to run almost as smoothly as alone. My California friend is running the server off the same computer he is playing on, could there be a bit of conflict there? Yet again 32gb RAM so that's not the problem but idk perhaps some other conflict.

[javid74]

He has an i7-6700HQ

[StitchNChill]

If it's only while he's online, it's probably that his CPU is getting overtaxed, especially if you've got a lot of intense mods (as diesieben07 already pointed out). The 6700hq is 2.6GHz base, 3.5GHz turbo... it's also 4 double-cores, so possibly an issue trying to run on a single thread and not using the extra cores. I don't know, I'm not an expert on how MC runs (though I vaguely remember it being optimized for 2 cores.. if i'm wrong, feel free to correct me  )

https://ark.intel.com/content/www/us/en/ark/products/88967/intel-core-i7-6700hq-processor-6m-cache-up-to-3-50-ghz.html

 

[StitchNChill]

https://www.minecraftforum.net/forums/minecraft-java-edition/discussion/2937717-minecrafts-multicore-singlecore-argument

yeah, it looks like MC likes strong single-core, maybe strong 2-core, and although it "uses" multiple cores the term should be loose. In this case, 2.6GHz may not be enough to prevent the lag you guys are seeing.