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
The Linux initcall_debug
parameter in Listing 3 tells Linux to print timing information for each initcall
. initcall
s 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.
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