UPDATE 6/15/2021: This method no longer works because Valheim updated the game code. New method is here: https://jamesachambers.com/revisiting-fixing-valheim-lag-modifying-send-receive-limits/
Valheim is a really fun game that recently entered early access where you are basically a Viking in purgatory trying to get to Valhalla. It honestly works pretty well (I would say especially compared to recently ‘AAA’ titles like Cyberpunk) for an early access title. Unsurprisingly though there are definitely still some issues and a very common one is lag on a dedicated server.
This guide will show some methods you can use to help reduce lag on your Valheim dedicated server!
If you are looking for how to back up your local game client’s world or characters, check out my guide here!
Symptoms of lag on Valheim dedicated servers
The symptoms you are experiencing on the server can help narrow down what type of lag you are experiencing and what may be causing it. Lag caused by your CPU being too overloaded for example will have different symptoms than lag that is caused by your server hitting the maximum send/receive limit as an example.
By far the most common type of dedicated server lag seen in the early access Valheim release so far has been mainly due to the low send/receive limit if 64KB/s that is hard coded into the server. This means that when you do something like take your character a heavily populated area on the server it has to send all that information about all the buildings/players through an artificially limited very narrow 64KB/s pipe.
Symptoms of the send/receive limit type of lag include things like chests taking a very long time to open, sometimes 10-30 seconds or never opening while at the same time being able to see players walking around normally. This is because the other people’s X / Y / Z positions is a very small amount of data compared to all the items/names/attributes that are stored in the chest. The other players moving easily fits through the narrow pipe while the chest full of loot’s data chokes since it is much larger.
I’ll specifically lay out how to modify your server and increase this limit in the “Modifying your server’s send/receive limit” section below. Before that though let’s cover a more specific type of lag to watch out for (mostly by being avoided) related to terrain manipulation.
Avoid extensive terrain manipulation (for now)
Another commonly observed source of lag has to do with terrain manipulation. Some players have found that if they take a big chunk out of the ground in Valheim and make extensive terrain modifications that the area gets filled with separate “instances” of terrain for each modification people make. Think of the original ground as one big piece in the game files. Valheim has the very cool feature of letting you modify terrain by raising it/lower it/level it/etc. Each time you do this though the game has to store what you did as a new “instance” or a modification.
The way Valheim seems to work now is it renders that original piece and then the modifications afterward. Early reports suggest having a whole bunch of them in one spot seems to sometimes cause everyone’s in-game FPS on the server to drop as low as 40 when the clients normally are much much higher than that.
The best way to avoid this type of lag at the moment is to limit how much terrain modification you are doing in areas near your base. It is likely this will be addressed/eased in future patches. There should be some things the developer team can do to clean up/merge these instances or at least reduce their effect on the rendering process so it doesn’t drop the in-game FPS so dramatically.
Modifying your server’s send/receive limit
At this time there is no options file for Valheim unfortunately. The only way to change it is to modify the server’s code and recompile the module that controls this.
Fortunately this is a lot easier than it sounds because the module we need to change is in .NET instead of something like C++ which would be much more difficult to do this with. I did not come up with this method, a very clever user on reddit (maximgame) shared it here and I want to give credit where credit is due!
Despite it not being too difficult these modifications are at your own risk. I haven’t experienced any problems so far but technically we are modifying the server’s code with the following procedure so understand that this isn’t an officially supported method and could have side effects or if you make a mistake could cause serious problems. Make sure you have backed up everything (your game world files especially) before you try anything!
Another important note is that it’s very likely you will have to make the changes again each update since the file we are going to modify is going to get replaced by updates. Until they add either a launch option or options file to officially change/control this property that is unfortunately unavoidable.
With all that being said if you understand the above and still feel comfortable let’s proceed!
Get dnSpy utility
We are going to use the utility dnSpy to modify the server code and recompile the module. The utility is free and is available here (download the Win64 build or dnSpy-net-win64.zip): https://github.com/dnSpy/dnSpy/releases
Extract the archive and run the program dnSpy.exe from inside the folder you extracted. The dnSpy application will open.
Modifying server with dnSpy
We are going to go to the “File” menu in the top left and choose “Open” like this:
Now you need to navigate to the Valheim dedicated server folder which is located in your “steamapps” folder. If you left all the options default when you installed it’s typically “C:\Program Files (x86)\Steam\steamapps\common\Valheim dedicated server\valheim_server_Data\Managed”.
We are looking for the file “assembly_valheim.dll” in that folder:
Select the file “assembly_valheim.dll” and click “Open” and the server’s assembly will load into dnSpy.
Once it has finished loading we are going to go to the “Edit” menu and click “Search Assemblies” like this:
For the search criteria enter “ZDOMan” like this:
After the search completes you should see a list of results like the one above. Double click on one of these entries on the list and it should place you in the ZDOMan section of the assembly (seen on the left hand side of the screen):
Perfect. We are now in the right place to make the changes. We just need to locate the line that has the send/receive limitation. To find that press Ctrl+F to bring up the find box (or choose “Edit” -> Find from the menu) and search for “m_dataPerSec =” which should bring up the following line:
Now right click on the m_dataPerSec line and click “Edit Class (C#)…”. Another window will open that will let you make changes to the file. We are going to modify ‘61440’ to a variable of our choosing. Common choices are to double it or triple it or to add a 0 to the end. I added a ‘0’ to the end of the existing number for my server making it 614400.
Theoretically setting a limit too high could cause the server to swamp your internet connection if you have a large number of players on and there’s essentially no limit but there shouldn’t be any other effects of a very high number.
Once you’ve changed the number to the desired value we need to press the “Compile” button in the bottom right corner of this new window. This window will close and take you back to the previous one.
Saving the new assembly
It’s time to save our changes into a new .dll file. Choose “File” -> “Save Module…”:
Change the file name to assembly_valheim_modded.dll. Don’t overwrite the original file just yet as we will want to take a backup and make sure that the server is closed before we do that. Press “OK” to write the new assembly_valheim_modded.dll file.
Backing up original file and replacing with our new one
Now it’s time to back up the original file and replace it with our new modded file. First make sure your dedicated server is closed otherwise the dll file will be in use. It is also a good time to make a backup of your world file just in case (see my Valheim Dedicated Server Backup Location / Guide for instructions)
Navigate in a file explorer window to your Valheim server directory where we saved the new assembly file (usually C:\Program Files (x86)\Steam\steamapps\common\Valheim dedicated server\valheim_server_Data\Managed).
First make a copy of the original assembly_valheim.dll file. You can just select the file and “copy” then paste a copy of it in that same folder and Windows will make a “valheim_assembly Copy(1).dll” file for you or you can back it up to a separate folder. Now remove the original file and rename your “valheim_assembly_modded.dll” file to the original file name of “valheim_assembly.dll”.
That’s it, you’ve now replaced the old assembly with the new one! Go ahead and start your server normally and the changes will have taken effect.
(Optional) Modifying Clients (not recommended unless absolutely necessary)
Note that the game client also has rate limits so the server is only half the picture. It’s in the exact same file only instead of the dedicated server folder it is the client. The path on an install with default paths would be something like:
C:\Program Files (x86)\steamapps\common\Valheim\valheim_Data\Managed\assembly_valheim.dll
and each player would make the same modification as you did with the server to this file.
I did not do this on my server as I found the performance gained from lifting the limits on the server side to be enough of an improvement. It alleviated the congestion / eliminated the chest lag problem completely on my server. It would also be a royal pain to have every single player have to do this (and remember, again for every single update since those files will get replaced). However, if you know you are doing something “extreme” like having full server with 10 people building a massive fortress that takes up an entire valley / a crazy giant farm with lots of livestock / anything like that with a huge amount of entities it may actually be worth the effort to modify the clients in that case.
I haven’t heard of modifying game clients triggering VAC (Valve Anti-Cheat) on Valheim yet but remember you are in fact modding the game client when you run this procedure on the client side and this is often not allowed and actively enforced with frequent ban-waves on many games today such as competitive multiplayer titles. I really recommend against doing it because of this risk (if not now then possibly in the future) unless you’ve tried the server modifications and it isn’t enough or you’re doing something really extreme and know you need it and understand the risk!
It’s also important to back up your local game’s characters and local worlds if you are using them. I have written a guide for the local game side that is available here!
Other Valheim Lag Issues
If you know of other Valheim lag issues that I didn’t cover here or are still having problems let me know in the comments and I will update this guide with other tips/tricks and try to answer any questions. It’s still a very new game and there isn’t very much out there for it yet. Have fun and enjoy!