Boot-up Time Definition Of Terms

Definition of Terms for Bootup Times working group

 * Boot up: The time from power on to user start (the start of user init).
 * Busy wait: Using a timed loop to create a delay. This is often used with probing. Because the device is executing instructions in the loop, no other work can be done until the loop ends.
 * Cold start: The time from power on to first available use.
 * Deferred: An operation, which would occur at a specific point in the unmodified Linux boot up processing, is changed so that it can occur later in the processing.
 * De-serialized: A set of operations, which would be serialized in an unmodified Linux environment, is changed so that the operations can be done concurrently.
 * Disk spinup: The time required for a hard disk to reach operational speed after application of power.
 * Firmware: The first set of machine instructions to run on the hardware after the application of power. The firmware may consist of several stages that bootstrap from one stage to the next. On an x86 machine, for example, the firmware is the BIOS.  It is the responsibility of the firmware to do initial preparation of the hardware, and to load and execute the OS. This typically includes probing the hardware to discover attached devices and setting up configuration information that the OS will read during boot up.
 * First available use: The first opportunity for actual use of the product, after a boot up, resume, or unhibernate operation.
 * First user experience: The first visual or audio indication of activity to the user of the product, after a boot up, resume or unhibernate operation. (same as "splash")
 * Flash: Flash memory
 * Hibernate: The action of transitioning the machine into a no power state, in a way such that the current operating state can be restored in the future without a full initialization sequence.
 * Kernel decompression: The action of decompressing a compressed kernel image. This stage usually includes simultaneously transferring the kernel image from persistent storage to RAM.
 * Kernel init: The kernel initialization sequence, consisting of the period of time from kernel start to start of user init.
 * Kernel start: The point in the boot up when the product executes the first instruction of start_kernel routine in the kernel.
 * Power lost: The moment that the product can no longer function because there is no more power applied to it.
 * Power off: The moment that the machine transitions to a "losing power" state (this is not the same as the time when no more power is being applied to the CPU).
 * Power on: The moment of application of power to the device.
 * Probing: The act of discovering the configuration or attributes of the machine or its operating state. A probe typically consists of sending a hardware signal and waiting, for a given time, for a response from an attached bus or device.
 * Resume: The action of transitioning the machine into a usable state from a low power state (suspend).
 * ROM: Read-only-memory
 * Shutdown: The action of turning the product off. The time from user power off to power lost.
 * Suspend: The action of transitioning the product into a low power state, wherein the device cannot be used, but can be resumed quickly.
 * Serialized: A set of operations is serialized if the operations are performed in order, without overlap or concurrency.
 * Splash: The first visual or audio indication of activity to the user of the product, after a boot up, resume or unhibernate operation. (same as "first user experience")
 * Time-to-splash: The time from power on to splash (or first user experience).
 * Unhibernate: The action of transitioning the machine from hibernation into a usable state. Specifically, the usable state is the same or as close as possible to the state when hibernation was entered (eg., the same applications are running, the same files are open, etc.)
 * User init: The time from the first instruction of the first user space program (usually /sbin/init) until the product is ready for first available use.
 * User power off: The action or moment that the user initiates terminating the power to the device. Some devices do not use "user power off" as the primary means of "turning off" the device.  Instead, a press of the "off" button may result in a suspend operation.
 * Warm start: The time from start of resume to first available use.
 * XIP: Execute-in-place - a method of executing code directly from ROM or Flash, without first loading it into RAM. This affects startup time, RAM and ROM footprint (size), and execution performance.

The terms used to describe boot up can refer to events (things that occur or start at a specific moment in time) or time periods (the time interval from one event to another).

Here is a list showing the sequence of main events during cold start: Note: * means this event is optional.

1.   power on 2.    firmware starts 2.1 * firmware splash 3. * kernel decompression starts 4.   kernel start 4.1 * kernel splash 5.   user start 5.1 * user splash 6. * RC script start 7.   application start 8.   first available use

Note that there may be multiple "splash" events in the system, but that the terminology here refers only to the first splash. This is also referred to in the terminology definitions as "first user experience". Splash events are highly important to diminish the apparent time to boot of the product. Note that the first splash event might occur at different locations in this sequence, depending on how the system is configured and what software performs the first user output. Also, even though user splash presentation is listed as event 5.1, it could occur anywhere between events 5 and 8, depending on the user-space software which performs the splash.

Here are bootup time periods (intervals), defined according to the above sequence of events:
 * cold start: time from 1 to 8
 * bootup: time from 1 to 5
 * kernel init: time from 4 to 5
 * user init: time from 5 to 8
 * time-to-splash: time from 1 to either 2.1, 3.1, or 5.1 (depending on which is used)