When working with Horde for build automation in Unreal Engine, ensuring efficient parallelization is crucial for reducing build times. One of the key ways to improve task distribution is by utilizing the RunEarly property in BuildGraph scripts.

Understanding RunEarly

The RunEarly property allows a node to start execution as soon as its dependencies are met, without waiting for all dependencies of its assigned agent to be completed. This can significantly enhance parallelization, especially when dealing with multiple tasks assigned to a single agent.

Default Behavior vs. RunEarly Optimization

By default, Horde waits for all dependencies of an agent to be resolved before executing any assigned nodes. This can lead to inefficient scheduling where tasks that could have started earlier are blocked by unrelated dependencies of the same agent.

With RunEarly="true", a node can start executing as soon as its specific dependencies are resolved, leading to better resource utilization and reduced idle time for build agents.

Example: Implementing RunEarly

Below is an example of a BuildGraph script that utilizes RunEarly to optimize execution order:

<Agent Name="$(CompileNodeName)" Type="$(CompileAgentType)">
    <Node Name="$(CompileNodeName)" Requires="$(ToolsNodeName)" Produces="#$(CompileNodeName) Binaries" RunEarly="true">
        <Compile Target="$(ProjectName)" Platform="$(Platform)" Configuration="$(Configuration)" />
    </Node>
    
    <Node Name="$(ExtraNodeName)" Requires="$(AnotherNodeName)">
        <!-- AnotherAction -->
    </Node>
</Agent>

Breakdown of the Script

• The CompileNodeName node requires ToolsNodeName and produces binary output.
• RunEarly="true" ensures that as soon as ToolsNodeName is available, compilation can begin, rather than waiting for all of the agent’s dependencies.
• Another node, ExtraNodeName, has a different dependency (AnotherNodeName) and does not use RunEarly, meaning it will follow the standard scheduling behavior.