Improving performance/Boot process
Improving the boot performance of a system can provide reduced boot wait times and a means to learn more about how certain system files and scripts interact with one another. This article attempts to aggregate methods on how to improve the boot performance of an Arch Linux system.
systemd-analyze command can be used to show timing details about the boot process, including an svg plot showing units waiting for their dependencies. See
man systemd-analyze for details.
Compiling a Custom Kernel
To decrease boot time, a stripped kernel is a must. Read more about compiling a kernel.
Early start for services
One central feature of systemd is D-Bus and socket activation. This causes services to be started when they are first accessed and is generally a good thing. However, if you know that a service (like UPower) will always be started during boot, then the overall boot time might be reduced by starting it as early as possible. This can be achieved (if the service file is set up for it, which in most cases it is) by issuing:
# systemctl enable upower
This will cause systemd to start UPower as soon as possible, without causing races with the socket or D-Bus activation.
Some hardware implements staggered spin-up, which causes the OS to probe ATA interfaces serially, which can spin up the drives one-by-one and reduce the peak power usage. This slows down the boot speed, and on most consumer hardware provides no benefits at all since the drives will already spin-up immediately when the power is turned on. To check if SSS is being used:
$ dmesg | grep SSS
If it wasn't used during boot, there will be no output.
To disable it, add
libahci.ignore_sss=1 to the kernel line.
Thanks to mkinitcpio's
fsck hook, you can avoid a possibly costly remount of the root partition by changing
rw on the kernel line and removing it from
/etc/fstab. Options can be set with
rootflags=mount options... on the kernel line.
You can also remove API filesystems from
/etc/fstab, as systemd will mount them itself (see
pacman -Ql systemd | grep '\.mount$' for a list). It is not uncommon for users to have a /tmp entry carried over from sysvinit, but you may have noticed from the command above that systemd already takes care of this. Ergo, it may be safely removed.
Other filesystems like
/home can be mounted with custom mount units. Adding "noauto,x-systemd.automount" will buffer all access to that partition, and will fsck and mount it on first access, reducing the number of filesystems it must fsck/mount during the boot process.
As mentioned above, boot time can be decreased by slimming the kernel, thereby reducing the amount of data that must be loaded. This is also true for your initramfs (result of mkinitpcio), as this is loaded immediately after the kernel, and takes care of recognizing your root filesystem and mounting it. To boot, very little is actually needed and includes the storage bus, block device, and filesystem. Falconindy (Dave Reisner) has begrudgingly created a short tutorial on how to achieve this on his blog.
Less output during boot
quiet on the bootloader's kernel line. For some systems, particularly those with an SSD, the slow performance of the TTY is actually a bottleneck, and so less output means faster booting.