Reducing Kernel BootUp Time

The kernel boots rather quickly, even with no attempts to make it go faster. In the example later in this chapter, an unmodified kernel for a 400Mhz arm board boots in about seven seconds from power-up, and three of those seconds are spent by the boot loader waiting for the user to interrupt the boot process (the measurement was done on a device still used for development). This means the device is ready to run user programs in about four seconds. This also means speeding up the kernel boot process is a series of changes that reduce the speed in the quarter- to half-second range; with effort, you can reduce the time to about two or three seconds, which is in the range of a digital camera.

Also notice that the choice of root file system is the largest predictor of boot time for the kernel. In the example, using a JFFS2 file system takes a breathtaking six seconds to mount for 50MB flash storage. When using a read-only file system, the mount time is in the subsecond range. If you need to have a large, solid-state drive, consider getting hardware that has flash memory that appears as though it's a standard block-based device, similar to a USB memory stick.

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