Tips and tweaks for reducing Linux startup time

GRUB Delay

By default, GRUB gives the user five seconds to interrupt the automatic boot process. If you want to shave that five second wait down to one second, change the GRUB timeout variable in /etc/default/grub to GRUB_TIMEOUT=1. Then enter sudo update-grub to transfer this value by recreating /boot/grub/grub.cfg. If you make this change, you'd better memorize the boot menu, because you'll only have one second to switch from the default. You can also set the value to  , which would mean you wouldn't see the boot menu at all and would need to use a Live system for repairs in case of an error.

A Brief Introduction to initrd

Initrd, which is short for "Initial RAM disk," is a temporary filesystem that is copied into memory to support the Linux startup process. One of the first goals of the startup process is to gain access to the root partition to access the files and start the init system. However, the root partition could be on a network drive, a USB device with one filesystem, or an NVMe disc with another filesystem. Initrd therefore contains a large number of drivers to ensure that the system will start regardless of where the root partition resides.

When the startup process unpacks initrd into memory, it spends a lot of time unpacking drivers that might not be needed for your hardware.

In the Linux messages, which you can view by typing sudo dmesg or journalctl -k, you will find output like Listing 2. After almost 400ms, the Linux kernel unpacks the initrd image, taking about 419ms to do so.

Listing 2

Log Messages for initrd

$ sudo dmesg | grep -A1 initramfs
[ 0.398506] Trying to unpack rootfs image as initramfs...
[ 0.817686] Freeing initrd memory: 25484K

Analysis Made Easy

The systemd-bootchart tool displays the kernel's metrics in graphical form. If systemd-bootchart isn't present on your system, enter

sudo apt install systemd-bootchart

to install the package. Then add the string from Listing 3 to the GRUB_CMDLINE_LINUX_DEFAULT variable in /etc/default/grub. After running sudo update-grub and a subsequent reboot, you will have an SVG file in the /run/log/ directory that you can view in the browser or an image viewer (Figure 1). The chart will help you identify which routines have the longest runtime and therefore the greatest optimization potential.

Listing 3

Supplementing the GRUB Command Line

*n*itcall_debug log_buf_len=2M init=/lib/systemd/systemd-bootchart
Figure 1: The SVG file stored in /run/log/ graphically displays the resource utilization, execution times, and durations.

The Linux initcall_debug parameter in Listing 3 tells Linux to print timing information for each initcall. initcalls are used to load statically linked kernel drivers and subsystems.

As you can see in Figure 2, the populate_rootfs() method is one of the most time consuming, with a runtime of 419ms (Figure 2). My challenge is to reduce this time to 10ms.

Figure 2: Among the kernel init threads, the populate_rootfs function takes up the most time during the boot process, clocking in at 419ms.

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