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HOI4 Dev Diary - Performance and AI

Hello! I’ve been drafted from EU4 to fight in the trenches together with the HOI4 team for a month and give some support on AI and performance. So I’m here to talk about those.


I’ve been mostly devoting my time into improving garrison orders to solve some really wonky cases that could happen. Like it placing two units on Magdeburg instead of Berlin letting the capital be completely exposed. But also focused on making the garrison work a lot better when you don’t have enough troops to cover every point in the order that needs it.

First I implemented a new tool for HOI4 to let us programmers visually inspect what the unit controller is up to.


Province Weights


State Weights

After that came the big task, rewriting garrison order and how it assigns units. There’s been a lot of issues here and you’ve probably experienced them from time to time. Shuffling, units prioritizing the wrong states or provinces, the list goes on.

So what did I do that is new. Instead of having the old logic setting up rules for where units should go I went with a thing I hate. Mathematics. The main difference is that each state will sort of calculate how “painful” it is to remove a unit from it now. This means as each state goes through this, they will be trying to find the least painful way to perform the garrison order and will eventually settle on a sort of equilibrium.

It’s given us a lot easier way to control where units should go in a garrison and where they end up will match a lot more in style of the weights you can see on the pictures above.

A thing that we have been planning to do for a long time and finally got around to is to allow a more flexible way to control how the AI plays. We use a system we call “Ai Strategic Plans” for this. Its essentially telling Ai to act a certain way when it comes to national focuses and priorities based on scripted conditions. To put that in english I am going to hand over to @Bratyn for some examples:

Some situations in which the new Strategy Plans have been useful is to ensure ‘compatible’ AI behaviour in the case of an ahistorical Germany. A lot of the AI behaviours for the major European powers has been based on the assumption that Germany remains fascist and causes mayhem in Europe. This means that AI Italy would usually see Germany as an ally, and that AI France would either always go with the United Kingdom (if historic mode was turned on), or be so unpredictable that it was problematic for planning out and predicting alternate history scenarios.

Using strategy plans, we have made these AI nations partly behave more logical to the changing world scene, and partly ensure that the player will always have a challenging playthrough. Going democratic Germany, only to get your only enemy (Soviet Union) end up at war with both the Allies and yourself, is not a very fulfilling playthrough. Instead, a new strategy plan will now load for France upon German completion of the “Oppose Hitler” focus. This delays their pick of which political direction to head into until we have more information from the German player, essentially a ‘holding pattern’.

  • If Germany revives the Kaiserreich and chooses “Focus on the True Enemy”, France is released from the holding pattern and will go with Britain, forming a counter-reaction to rising German colonialism. Essentially, in this playthrough not much will change other than Germany being unaligned rather than fascist. The war will likely develop in a similar matter (though not on a similar timeline), with one war with the Allies, and one war with the Soviet Union.

  • If Germany revives the Kaiserreich and chooses “Expatriate the Communists”, these communists will head to France and give them a communist drift national spirit. France is released from the holding pattern and will go communist, choosing to side closer with the USSR so that both powers can stand against the new Anglo-German alliance.
  • If Germany goes democratic, France is released from this holding pattern and will choose to Revise Versailles. Which direction they pick next is up to the AI, ensuring some unpredictability. France may go Communist and start their own faction, or join the Soviet Union, ensuring a two-front war if Germany goes to war with the USSR. Alternatively, the French may go fascist. They then will go for “Woo Italy”, creating a new semi-powerful Fascist faction in south-west Europe. Similar strategy plans for Italy (and liberal application of AI strategy values…) ensure this alliance is stable. A new focus for france “Dismantle the Democracies” becomes available if Germany is democratic, and the AI has a high likelihood of taking this when the Germans are already at war with the USSR. This makes gameplay more dynamic, and ensures Germany will have ample enemies to fight against even if they elected to go democratic.
We also used AI strategy plans to teach the Chinese AI how to handle the Inflation mechanic and do things like making the PRC AI keep the peace in China when they are going for the peaceful takeover through the political struggle. All in all, to myself and @Archangel85, this has been (and will continue to be) an invaluable tool for ensuring believable and, most importantly, fun playthroughs for each ahistorical path that one can take. We hope to continue building upon this basis and make nations ‘smarter’ in dealing with alternate history situations, or just general oddities that arise in the state of the world over the course of a playthrough.

Modders I am sure are going to like this a lot, so for you guys check out the example below for how it can be set up (regular humans without modding superpowers can safely ignore ;) ).




Late game performance have always been a tough nut to crack for us on our games. For Cornflakes we rolled up our sleeves and got elbow deep into the code to improve it the best we could.

In order to measure performance and see where it goes bad to better pinpoint what causes the slowdown I write a little script that generates this super sexy graph for us.


We did tons of neat little tricks to improve our performance. AI when it micromanages does a lot less pathfinding now. The game will automatically balance workload between cores depending on how long the hourly update took for a country, The fronts system has been a lot more streamlined to make it not hog performance

And we’ve done quite A good job at it as well. Here’s the result of our hard work just this week


Between finding ways to make things more efficient and utilizing the cores on the CPU better we’ve gotten an overall improvement late game. We won’t rest there though, there’s tons more that we can do to make late game an even more enjoyable experience!

To conclude this Dev Diary, we have a reminder for you. Tune in for the first day of the Three Day War later tonight, 20:00 until 22:00 CET. Since the release of Waking the Tiger is approaching we have invited some of the grandest streamers to battle it out in multiplayer and show off the expansion as well as put the build to the test.

Quill18, Alex the Rambler, Marbozir, Enter Elysium, Solar Gamer, Spiffing Brit, Drew Durnil, The Prussian Prince, Bratyn, Da9L, Shen Plays and Jay’s Gaming will all be streaming their point of view in their own channels, and we’ll be hosting a main observer stream at the Paradox Twitch channel: https://www.twitch.tv/paradoxinteractive

Next week we are going to try and do a big recap of all the features and changes and also talked about a bunch of small stuff that hasn't fit into diaries so far. See you then!

Rejected Titles:
Droogies Quest For The Missing Core
Hoi4 Dev diary: Reverse the polarity of the deflector dish
Graphs are hot!
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haha, Groogy posted this in EU4 forum ;D
ok, in right forum now ;D
Hahaha whoops, I went into the EU4 forum by habbit
Really excited about the performance improvements. Should this make players lagging out in multiplayer less of a problem as a side-benefit?

What are the real-world impacts to performance - eg. at Speed 5 how long does it take to complete a single year in 1940 on 1.4.2 vs 1.5.0?

What if you canned some of the irrelevant micro-nations like Haiti, Nicaragua, Nepal etc? (eg. simplifying what they can do - no research, no production, units only given by focuses - turning them into 'micronations') Do they chew up a lot of CPU? (Even though their impact on the world/gameplay is minimal)
Why did Groogy listen to the hobbits?
What if you canned some of the irrelevant micro-nations like Haiti, Nicaragua, Nepal etc? (eg. simplifying what they can do - no research, no production, units only given by focuses - turning them into 'micronations') Do they chew up a lot of CPU for nothing?

Nah they basically do nothing.
Will France be more likely to pick "Little Entente" in the new patch, with the new AI scripts? Because it never happens now, and it makes for a dull game.

Also, sorry to keep banging on this drum, but will A-H get more territorial demand events to go with their focuses?
This dev diary is just the best news ever. Performance increase will improve the gameplay so much. The “Strategy Plans” which make the A.I. react better to the different Strategies of other countries also sounds very very good. I was very exited about todays dev diary and it feels like my biggest HOI hopes and dreams are becoming true in the next patch.
Nice map change i see there in Wielkopolskie, but theres just 2 more things i'd like to mention
the province highlighted below should belong to Austria, not Slovenia (I believe it was part of the Carithian plebiscite, but ultimately it remained with Austria)

also, please add upper silesia state (the area in red), then the eastern borders of german empire can be truly restored.
Right on, I have been looking forward to this thread :).