Reducing Boot Time in Embedded Linux Systems

Using some reasonably simple techniques, you may be able to reduce dramatically the boot time of your embedded Linux system.
Calibration Routine

You may have seen the interesting “BogoMIPS” message plastered on your screen or terminal during boot. Linux calibrates its internal software timing loops to your processor system clock on each boot, arriving at a constant value used by the loops_per_jiffy variable (lpj). Although the technique varies across different architectures, this can be a time-consuming routine. It is easy to bypass this dynamic calibration routine by “hard-coding” the time constant calculated by this routine. This is quite easily passed to the kernel through the kernel command line. Simply add lpj=xxxxx to your kernel command line, where xxxxx is the lpj value printed to your boot log during boot. This is what the boot message looks like on the Intel Atom-based Netbook on which this article is being drafted:

Calibrating delay using timer specific routine..
    3194.85 BogoMIPS (lpj=6389712)

From this information, simply add the string lpj=6389712to your kernel command line. This will bypass the often lengthy calibration routine and instead use the fixed value for loops_per_jiffy.

Filesystem Selection

One of the keys to achieving single-digit boot times is your choice of root filesystems. Some filesystems designed for Flash use, for example the ubiquitous JFFS2, can get into a state that requires a significant and noticeable time delay while the kernel reads the sequential journal entries and reconstructs the files and directories on the filesystem. Consider using a small, compact and fast root filesystem for your initial system boot, and then mount a more general-purpose filesystem later in the initialization sequence.

CRAMFS is a read-only, compressed filesystem that is perfectly suited for this purpose. Configure a preliminary root filesystem using CRAMFS, which contains all the executables and libraries you need to get your system into a preliminary operational state. Later, while other less critical tasks are being executed, you can mount a writable JFFS2 partition when time is not so critical. Also consider the liberal use of tmpfs for volatile data such as /tmp, /var and others. Tmpfs is fast and efficient, and dynamically resizes itself to meet storage requirements. Remember, the contents of all tmpfs filesystems are lost on power-down, so if there are any files (log files, configuration data and so on) that must be saved, it will be up to your application to save this data periodically to nonvolatile storage.

Udev Considerations

Udev has become an efficient and powerful system configuration tool. Its primary role is to create device nodes for devices that the kernel discovers. Virtually every modern Linux distribution uses udev coupled with a set of rules for device naming. Udev also has the capability to run external programs in response to device detection. The most common example of this is to run modprobe to install a device driver upon device detection. For example, if you plug an SD card into an appropriate socket, a properly configured udev-based Linux system will perform all the actions required to enable the device. This includes loading device drivers and creating the device nodes associated with the device and driver.

This powerful and flexible scheme has one drawback. Although udev itself is fast and efficient, some of the external programs it runs may require significant time to complete.

When a Linux system is booted and reaches userland, udev basically “plays back” all the device notification events generated by the kernel and performs the required actions (primarily device node creation and module loading). This can take a significant amount of time. One solution to this problem is to configure your Linux system with statically generated device nodes for critical system devices (those that you need to be operational immediately) and defer the running of udev until your fast-path boot chores are complete. For each device you need to have immediately available at boot time, create a static device node in /dev as part of your root filesystem. Later, when udev takes over, your udev startup script can merge these static devices with those devices that udev creates dynamically.

Measuring Boot Time

Several tools are available to help you identify the long paths in your system boot. They vary in complexity and ease of use, but most can be mastered quickly. The simplest tool (and perhaps a good starting point) is to configure your kernel to add timestamps to the kernel messages that are displayed on boot. Select CONFIG_PRINTK_TIME in the Kernel Hacking section of your kernel configuration to enable this feature. This lets you see at a glance where significant time is being spent during the actual kernel boot sequence.

Another easy tool to use is to enable printout of each kernel initcall. An initcall is a special type of kernel function call specifically related to subsystem initialization. This is accomplished by adding the single parameter initcall_debug to your kernel command line. When enabled, the kernel will display a line that lists the kernel virtual address of each initcall together with return data and call duration. While you must “decode” the kernel virtual address into its symbolic function name, this data is readily available in the System.map file in your Linux kernel source tree. If you have CONFIG_KALLSYMS enabled (found under General setup→Configure standard kernel features for small systems), the initcall line will be decoded for you. A sample of the output from the system boot log with initcall_debug enabled is displayed in Listing 1.

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