Incremental Garbage Collection

Be aware

It is 5.4, and it is experimental feature. So learn about it, try it. But remember that feature is not production ready.

Problem

Unreal Engine uses Garbage Collector to manage memory usage of UObject. But there is one major issue with it.

Garbage Collection runs every X seconds. But before “collect” memory, GC should define which objects should be collected. It is called reachability analysis. The main idea is check references, and mark objects which should be cleaned. But if you have a lot of objects loaded, this process may take a while, or even longer. This leads to lags or stutters.

There are few ways to avoid this problem, disabling Garbage Collection during complex gameplay, pooling objects, etc.

Incremental Reachability Analysis

But 5.4 introduces new technique. Now we can split reachability analysis across multiple frames, instead of running analysis in single frame.

First of all, it doesn’t work with UPROPERTY and raw pointers. If you want use this feature, you should use TObjectPtr (if you don’t know what is it, read here)

As soon as you mark TObjectPtr as UPROPERTY, it becomes trackable by engine. Unreal’s source code is almost full compatible with TObjectPtr, so you should prefer them to raw pointers.

Secondly, to enable IGC, add to your DefaultEngine.ini:

[ConsoleVariables]
gc.AllowIncrementalReachability=1 ; enables Incremental Reachability Analysis
gc.AllowIncrementalGather=1 ; enables Incremental Gather Unreachable Objects
gc.IncrementalReachabilityTimeLimit=0.002 ; sets the soft time limit to 2ms;

Performance

On this screen from Unreal Insight, we see how analysis is split across few frames, and there is no spike. And that’s how we could avoid hitches with this feature.