UE5.7 source build is not incremental building?

[profoundsolitude]

Specs at the bottom of the post.

Transitioning to a source build of the engine has taken my 5-10 second incremental builds and turned them into 5-7 minute builds. When it’s working. The problem is, Unreal keeps trying to rebuild pieces of the engine when I make changes to my game code. Say I create a template project and compile it. It’ll take forever to build because it has to run through all 55 projects in the engine source, and then when it finally builds the game code, the actual build process is hung and frozen for multiple minutes, whereas this identical process would have already been done with a binary build. Now if I were to change the movement speed in the character class for example, instead of 5-7 actions to build, I now have over 3,500 actions to build, so about half the size of the engines source build I already did.

I recognize a couple of solutions. I can exclude most of the engines projects from my build so only UE5 and the game project build, but I cannot get past the massive rebuild that only pops up when I make changes to my code. I’m a solo programmer who doesn’t have the support of massive CI/CD pipelines, so sitting around for over an hour for a every build isn’t realistic. I did not modify the build in any way or do anything that went away from the guides and tutorials.

TLDR; Extremely slow across the board compared to binary builds.

Here is what I’ve done to try and fix it:

• Multiple IDEs. The problem is source build specific, and doesn’t change between VS2022, VS2026 and Rider.
• Clean reinstall of tooling (Visual Studio)
• I’ve triple checked the Epic guides on how to setup a development environment with Visual Studio and how to build the engine from source. I followed everything to the letter
• Tried building different versions of the engine from source
• Followed video tutorials step-for-step
• Cleaned up files like the BuildConfiguration.xml in AppData/Roaming
• Swapped between source and binary builds and tried to build each for the same project. Binary builds do not have this problem, it’s only for source builds of the engine.

For a comparison on the last point:
UE5.7.4 binary build with minimal change: 4.78 seconds, 8 actions to build.

UE5.7.4 source build with identical minimal change: Multiple minutes of sitting still while the UHT tries to run the accelerator on over 3500 actions. Takes about five minutes of sitting here before the actions start building.

One of the tutorials I followed had the programmer compiling game changes in about 7 seconds. None of them ever showed me having to build an extra 3500 actions after already having built the engine, and the internet has shockingly little information on this issue other than this one post:

500×500 2.84 KB

Hi, I’m modifying UE and the modification itself works. However, VS keeps triggering full rebuild which takes super long to finish. Here’s the first N lines of the log: 1>Using bundled DotNet SDK version: 6.0.302 1>Running UnrealBuildTool: dotnet "..\..\Engine\Binaries\DotNET\UnrealBuildTool\UnrealBuildTool.dll" -Target="UnrealEditor Win64 Development" -Target="ShaderCompileWorker Win64 Development -Quiet" -WaitMutex -FromMsBuild 1>Log file: D:\code\UnrealEngine\Engine\Programs\UnrealBuildTo…

Basically, nobody seems to know. I’m chopping this up to me being new to using source builds of Unreal Engine, and I’d like for this to be a case of me just forgetting something. I’m interested in modifying the engine source, but it would be a waste of time to try and do so when the environment is this slow. Has anyone else encountered this issue? Why are my problems exclusively with the source build and not the binary releases?

Specs:

CPU: Intel i7 9700k
RAM: 32gb
SSD1: 500gb SSD used for storing the engine and only the engine
SSD2: 2tb nvme to store game projects
GPU: NVIDIA RTX 4060

[profoundsolitude]

While I don’t know specifically why Unreal keeps trying to rebuild a huge chunk of the engine, I assume it has something to do with the UBT detecting a mismatch of some kind in Unreals unity-build file that is causing a huge blob of stuff to be out of date.

My current approach is to, in Visual Studio, go to Build->Configuration Manager and uncheck every module except for my game. That seems to work well enough. I’m not rebuilding huge parts of the engine, and game code compiles in about 10 seconds, which is what I was hoping for. I also made a slight modification to the engine, and was able to build and run my game code using the modified engine. I’m not sure if this is the ‘correct’ way of doing it, but it worked for me. If anyone actually knows what causes this, I’d like to know. Until then, I’ll mark this as completed.