Apertis has a fairly typical Linux boot process:
- A platform specific bootloader performs initial low-level configuration of the system
- The Linux kernel performs higher level configuration and creates a standardised environment
- The kernel passes execution to the user space which enables services based on the functionality provided by the kernel and sets up the system to accept user input
Critical sequences during startup
The phases during startup generally proceed from most- to least-critical. For instance, an early deadlock in the kernel would mean the system would never complete its boot process and thus would be non-responsive to user input. On the other hand, if the networking system (near the end of the boot process) fails to start, specific system functionality (for example, in the case of a car infotainment unit, driving directions) may be unavailable, but the user may still be able to use other functionality (such as play music and other local media).
Any failure during boot should not result in a worsened state. Failures such as filesystem corruption should be automatically repaired upon subsequent boot. Due to Apertis’ focus on making the core system storage read-only for the majority of the system's uptime, there should be few opportunities for defects to adversely affect the boot process. More serious issues resulting from updates will result in the system returning to the pre-update configuration. For more details, please refer to the System Updates and Rollback Proposal.
Systemd and its role in startup
Systemd starts system services on-demand, serving as a replacement for the traditional SysV init system. In this duty, systemd is the first process started by the kernel. It starts all processes required to bring the system up to the point where it is ready for user input.
Unlike the traditional init system, systemd can also handle automated activation of services, after the system has settled, based upon requirements of applications run due to user interaction. Further, systemd can stop services when they are no longer needed.
As an implementation detail, systemd starts services in a highly-parallel fashion, which allows them to make the most use of the processor while they wait on their dependencies. This reduces total start-up time by preventing services blocking input/output delays from blocking their dependent services (which would otherwise need to wait until the first service is done starting).
More information about systemd can be found on its manual pages.
For guidance on improving systemd boot time, see Boot Optimisation and Profiling.
Boot splash screen
Apertis provides functionality to show a boot animation using Plymouth. Plymouth can be themed to show different boot animations, see video animation on boot for information on how to modify the boot animation theme.