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.
Analyzing the boot process
systemd provides a tool called
systemd-analyze that can be used to show timing details about the boot process, including an svg plot showing units waiting for their dependencies. You can see which unit files are causing your boot process to slow down. You can then optimize your system accordingly.
To see how much time was spent in kernelspace and userspace on boot, simply use:
To list the started unit files, sorted by the time each of them took to start up:
$ systemd-analyze blame
At some points of the boot process, things can not proceed until a given unit succeeds. To see which units find themselves at these critical points in the startup chain, do:
$ systemd-analyze critical-chain
You can also create a SVG file which describes your boot process graphically, similiar to Bootchart:
$ systemd-analyze plot > plot.svg
man systemd-analyze for details.
Bootchart has been merged into systemd since Oct. 17, and you can use it to boot just as you would with the original bootchart. Add this to your kernel line:
initcall_debug printk.time=y init=/usr/lib/systemd/systemd-bootchart
You could also use a version of bootchart to visualize the boot sequence. Since you are not able to put a second init into the kernel command line you won't be able to use any of the standard bootchart setups. However the AUR comes with an undocumented systemd service. After you've installed bootchart2 do:AUR package from
# systemctl enable bootchart
Read the bootchart documentation for further details on using this version of bootchart.
Systemd comes with its own readahead implementation, this should in principle improve boot time. However, depending on your kernel version and the type of your hard drive, your mileage may vary (i.e. it might be slower). To enable, do:
# systemctl enable systemd-readahead-collect systemd-readahead-replay
Remember that in order for the readahead to work its magic, you should reboot a couple of times.
Compiling a Custom Kernel
Compiling a custom kernel can reduce boot time and memory usage. Though with the standardization of the 64 bit architecture and the modular nature of the Linux kernel, these benefits may not be as great as expected. 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. Remember to remove the entry from your /etc/fstab file, else the systemd-remount-fs.service will continue to try to apply those settings. Alternatively, one could try to mask that unit.
If btrfs is in use for the root filesystem, there is no need for a fsck on every boot like other filesystems. If this is the case, mkinitcpio's
fsck hook can be removed. You may also want to mask the
systemd-fsck-root.service, or tell it not to fsck the root filesystem from the kernel command line using
fsck.mode=skip. Without mkinitcpio's
fsck hook, systemd will still fsck any relevant filesystems with the
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 mkinitcpio), 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.