boottime

Boottime, or boot time, is the duration from power-on or system reset until a device is ready to accept user input and its main services are available. It is a key measure of how quickly a platform transitions from a powered-down state to a usable state and is relevant for personal computers, servers, and embedded systems.

The boot process typically consists of several stages. First, hardware initialization and a power-on self-test or firmware phase (BIOS/UEFI) prepare the system hardware. Next, a bootloader loads the operating system kernel, after which the kernel initializes core subsystems and starts the init or onboarding services. Finally, user-space services, background tasks, and login prompts are brought up, at which point the system is considered ready. The portion of time spent before the system reaches a usable state is what is commonly referred to as boottime.

Boottime is measured with various metrics and tools. Common distinctions include cold boot (from a powered-off state) and warm boot (restarting without full power cycle). Tools such as systemd-analyze, dmesg, and bootchart can help quantify the time spent in each phase and identify bottlenecks. Important metrics include time to early userspace, time to login, and total time to interactive readiness.

Factors influencing boottime include hardware speed (processor, memory, storage media), firmware and bootloader efficiency, the number and order of startup services, kernel and driver load times, and network-dependent initialization. Optimization strategies include reducing or parallelizing startup services, enabling fast boot options, using solid-state drives, keeping firmware and drivers up to date, and disabling unnecessary background tasks. In embedded and real-time contexts, boottime is often a critical constraint to meet specific deadlines.

See also: boot sequence, bootloader, init system, startup performance.