# Compiling Sysimages

The Julia VS Code extension makes it easy to compile a custom sysimage for your Julia environments. The extension will also automatically use sysimages for the current environment when it starts a new Julia REPL in VS Code.

Custom sysimages can significantly reduce load times for packages.

## Creating a sysimage for the active environment

The Julia extension adds a build task for custom sysimages to the Visual Studio Code task system. To run this build task, select the command Tasks: Run Build Task and then select Julia: Build custom sysimage for current environment.

The build task will create a sysimage that includes all packages in the current Julia environment. This sysimage will be saved in the same folder where the Project.toml and Manifest.toml of the current Julia environment are stored. The name of the sysimage file will be JuliaSysimage.dll (Windows) or JuliaSysimage.so.

## Using custom sysimages

Whenever the Julia extension starts a new Julia REPL process, it will first look whether a custom sysimage file exists for the current Julia environment, and whether this custom sysimage file is up-to-date (by comparing the last modified date of the sysimage file with the last modified date of the Manifest.toml file). If an up-to-date sysimage file exists, the Julia extension will automatically start the Julia REPL process with this custom sysimage loaded.