City no longer growing at 940k population

[Arikadesu]

I have a map that was at around 940k population until a few days ago when it started to fall. The population dropped down to 650k over the course of 8 hours and is now back to around 820k. Despite the population screen showing more births than deaths the population has been falling and climbing back to around this 820k mark for the last 72 hours without going beyond this number. In fact it took me less time to go from 0-940k people that it has for it to climb back from 640-820k population. As you can imagine the game is incredibly slow at this point and there is nothing I can do in game unless the population decides to start growing again.

After investing so much into this map it would be really frustrating to have to abandon if because of some messed up mechanic I can't see. Is there anything I can do? I had a map over a year ago at 1.3million pop with a similar issue where even if I let the game run continuously for days the population just would not increase anymore.

 

[Promethian]

Limits of the system and death waves.

There are literal hard limits to how many of everything can exist. Some things won't hold you back too much (like the tree limit) but others definitely do (like the actor limit). Incidentally 1 million population is about when you start butting up against the limits. In fact a lot of people start hitting the limits before that but that is because they make really inefficient cities.

The population swings are known as death waves. Its a side effect of an interaction between the cim lifecycle and the way the system determines where they live. What happens is elderly cims still take up a spot in a residential building. However they live for quite a while, don't reproduce and there isn't much of a randomization on when they die. This leads to a large build up of elderly cims with no new children being born. They start dying off and the open residential slots get filled by immigrants, who all arrive at the same age. Thus the cycle repeats. There is a lifecycle mod that mostly clears this up on its own. I forget the exact name but if you search lifecycle you will find it. There is also a mod that lets you plop residential buildings that house the elderly. It draws them in gradually from nearby freeing up the residential slots for younger cims. This mod also mostly solves the problem. Together they hit the death wave problem from both sides and solve it completely.

 

[MarkJohnson]

Sounds like a mod issue. For starters, the game is hard coded for 1million pop, so if you made 1.3 million it sounds like a mod interfering somehow.

Other than that, you may have balancing issues and you are reaching the vehicle and/or citizen limits, which can cause your city to crash from not spawning any more agents. i.e. it won't spawn a hearse to pick up the dead, a freight trick to pick up raw materials, etc. then things start crashing.

Check to make sure your mods are up to date.