Boot Time

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Boot Time includes topics such as measurement, analysis, human factors, initialization techniques, and reduction techniques. The time that a product takes to boot directly impacts the first perception an end user has of the product. Regardless of how attractive or well designed a consumer electronic device is, the time required to move the device from off to an interactive, usable state is critical to obtaining a positive end user experience. Turning on a device is Use Case #1.

Booting up a device involves numerous steps and sequences of events. In order to use consistent terminology, the Bootup Time Working Group of the CE Linux Forum came up with a list of terms and their widely accepted definitions for this functionality area. See the following page for these terms:

Technology/Project Pages

The following are individual pages with information about various technologies relevant to improving Boot Time for Linux. Some of these describe local patches available on this site. Others point to projects or patches maintained elsewhere.

Measuring Boot-up Time

  • Printk Times - simple system for showing timing information for each printk.
  • Kernel Function Trace - system for reporting function timings in the kernel.
  • Linux Trace Toolkit - system for reporting timing data for certain kernel and process events.
  • Oprofile - system-wide profiler for Linux.
  • Bootchart - a tool for performance analysis and visualization of the Linux boot process. Resource utilization and process information are collected during the user-space portion of the boot process and are later rendered in a PNG, SVG or EPS encoded chart.
  • Bootprobe - a set of System Tap scripts for analyzing system bootup.
  • and, let us not forget: "cat /proc/uptime"
  • grabserial - a nice utility from Tim Bird to log and timestamp console output
  • process trace - a simple patch from Tim Bird to log exec, fork and exit system calls.
  • ninja-ts - wsa's TimeStamper: A small filter prepending timestamps to STDOUT; a bit similar to grabserial but not limited to serial ports
  • Initcall Debug - a kernel command line option to show time taken for initcalls.
  • See also: Kernel Instrumentation which lists some known kernel instrumentation tools. These are of interest for measuring kernel startup time.

Technologies and Techniques for Reducing Boot Time

Bootloader speedups

Kernel speedups

  • Disable Console - Avoid overhead of console output during system startup.
  • Disable bug and printk - Avoid the overhead of bug and printk. Disadvantage is that you lose a lot of info.
  • RTC No Sync - Avoid delay to synchronize system time with RTC clock edge on startup.
  • Short IDE Delays - Reduce duration of IDE startup delays (this is effective but possibly dangerous).
  • Hardcode kernel module info - Reduce the overhead of loading a module, by hardcoding some information used for loading the relocation information
  • IDE No Probe - Force kernel to observe the ide<x>=noprobe option.
  • Preset LPJ - Allow the use of a preset loops_per_jiffy value.
  • Asynchronous function calls - Allow probing or other functions to proceed in parallel, to overlap time-consuming boot-up activities.
  • Reordering of driver initialization - Allow driver bus probing to start as soon as possible.
  • Deferred Initcalls - defer non-essential module initialization routines to after primary boot
  • NAND ECC improvement - The pre 2.6.28 nand_ecc.c has room for improvement. You can find an improved version in the mtd git at;f=drivers/mtd/nand/nand_ecc.c;hb=HEAD. Documentation for this is in;f=Documentation/mtd/nand_ecc.txt;hb=HEAD. This is only interesting if your system uses software ECC correction.
  • Check what kernel memory allocator you use. Slob or slub might be better than slab (which is the default in older kernels)
  • If your system does not need it, you can remove SYSFS and even PROCFS from the kernel. In one test removing sysfs saved 20 ms.
  • Carefully investigate all kernel configuration options on whether they are applicable or not. Even if you select an option that is not used in the end, it contributes to the kernel size and therefore to the kernel load time (assuming you are not doing kernel XIP). Often this will require some trial and measure! E.g. selecting CONFIG_CC_OPTIMIZE_FOR_SIZE (found under general setup) gave in one case a boot improvement of 20 ms. Not dramatic, but when reducing boot time every penny counts!
  • Moving to a different compiler version might lead to shorter and/or faster code. Most often newer compilers produce better code. You might also want to play with compiler options to see what works best.
  • If you use initramfs in your kernel and a compressed kernel it is better to have an uncompressed initramfs image. This is to avoid having to uncompress data twice. A patch for this has been submitted to LKML. See
File System issues

Different file systems have different initialization (mounting) times, for the same data sets. This is a function of whether meta-data must be read from storage into RAM or not, and what algorithms are used during the mount procedure.

  • Filesystem Information - has information about boot-up times of various file systems
  • File Systems - has information on various file systems that are interesting for embedded systems. Also includes some improvement suggestions.
  • Avoid Initramfs - explains on why initramfs should be avoided if you want to minimize boot time
  • Split partitions. If mounting a file system takes long, you can consider splitting that filesystem in two parts, one with the info that is needed during or immediately after boot, and one which can be mounted later on.
  • Ramdisks demasked - explains why using a ram disk generally results in a longer boot time, not a shorter one.

User-space and application speedups

  • Optimize RC Scripts - Reduce overhead of running RC scripts
  • Parallel RC Scripts - Run RC scripts in parallel instead of sequentially
  • Application XIP - Allow programs and libraries to be executed in-place in ROM or FLASH
  • Pre Linking - Avoid cost of runtime linking on first program load
  • Statically link applications. This avoids the costs of runtime linking. Useful if you have only a few applications. In that case it could also reduce the size of your image as no dynamic libraries are needed
  • GNU_HASH: ~ 50% speed improvement in dynamic linking
  • Application Init Optimizations - Improvements in program load and init time via:
    • use of mmap vs. read
    • control over page mapping characteristics.
  • Include modules in kernel image - Avoid extra overhead of module loading by adding the modules to the kernel image
  • Speed up module loading - Use Alessio Igor Bogani's kernel patches to improve module loading time by "Speed up the symbols' resolution process" (Patch 1, Patch 2, Patch 3, Patch 4, Patch 5).
  • Avoid udev, it takes quite some time to populate the /dev directory. In an embedded system it is often known what devices are present and in any case you know what drivers are available, so you know what device entries might be needed in /dev. These should be created statically, not dynamically. mknod is your friend, udev is your enemy.
  • If you still like udev and also like fast boot-up's, you might go this way: start your system with udev enabled and make kind of a backup of the created device nodes. Now, modify your init script like this: instead running udev, copy the device nodes that you just made a backup of into the device tree. Now, install the hotplug-daemon like you always do. That trick avoids the device node creation at startup but stills lets your system create device nodes later on.
  • If your device has a network connection, preferably use static IP addresses. Getting an address from a DHCP server takes additional time and has extra overhead associated with it.
  • Moving to a different compiler version might lead to shorter and/or faster code. Most often newer compilers produce better code. You might also want to play with compiler options to see what works best.
  • If possible move from glibc to uClibc. This leads to smaller executables and hence to faster load times.
  • library optimiser tool:
    This will allow you to create an optimised library. As unneeded functions are removed this should lead to a performance gain. Normally there will be library pages which contain unused code (adjacent to code that is used). After optimizing the library this does not occur any more, so less pages are needed and hence less page loads, so some time can be saved.
  • Function reordering:
    This is a technique to rearrange the functions within an executable so they appear in the order they are needed. This improves the load time of the application as all initialization code is grouped into a set of pages, instead of being scattered over a number of pages.

Suspend related improvements

Another approach to improve boot time is to use a suspend related mechanism. Two approaches are known.

  • Using the standard hibernate/resume approach. This is what has been demonstrated by Chan Ju, Park, from Samsung. See sheet 23 and onwards from this PPT and section 2.7 of this paper.
    Issue with this approach is that flash write is much slower than flash read, so the actual creation of the hibernate image might take quite a while.
  • Implementing snapshot boot. This is done by Hiroki Kaminaga from Sony and is described at snapshot boot for ARM and
    This is similar to hibernate and resume, but the hibernate file is retained and used upon every boot. Disadvantage is that no writable partitions should be mounted at the time of making the snapshot. Otherwise inconsistencies will occur if a partition is modified, while applications in the hibernate file might have information in the snapshot related to the unmodified partition.

Miscellaneous topics

About Compression discusses the effects of compression on boot time. This can affect both the kernel boot time as well as user-space startup.

Uninvestigated speedups

This section is a holding pen for ideas for improvement that are not implemented yet but that could result in a boot time gain. Please leave a note here if you are working on one of these items to avoid duplicate work.

  • Prepopulated buffer cache - As initramfs performs an additional copy of the data the idea is to have a prepopulated buffer cache. A simplistic scenario would allow dumping the buffer cache when the booting is completed and the user applications have initialised. This data then could be used in a subsequent boot to initialize the buffer cache (of course without copying). A possible approach would be to have those data to reside into the kernel image and use them directly. Alternately they could be loaded separately.
    Unfortunately my knowledge of the internals in this section is not yet good enough to do a trial implementation.
    • is it possible to have the buffer cache split into two different parts, one which is statically allocated, one which is dynamically allocated?
    • the pages in the prepopulated buffer cache probably cannot be discarded, so they should be pinned
    • apart from the buffer cache data itself also some other variables might need restoring
    • a similar approach could also be used for the cached file data.
  • Dedicated fs - currently a lot of abstraction is done in fs to make a nice abstraction allowing easy addition of new filesystems and creating a unified view of those filesystem. While this is pretty neat, the abstraction layers also introduce some overhead. A solution could be to create a dedicated fs system, which supports only one (or maybe 2) filesystems, and eliminates the abstraction overhead. This will give some benefit, but the chance of getting this into the mainline is zero.

Articles and Presentations

Case Studies

Additional Projects/Mailing Lists/Resources

Replacements for SysV 'init'

The traditional method of starting a Linux system is to use /sbin/init, which processes the file /etc/inittab. This is an init program which processes a series of actions for different run-levels and system events (key-combinations and power events).

See the init(8) man page and the the inittab(5) man page.

busybox init

An 'init' applet is often included in BusyBox

There used to be (as of 2000) some slight differences in the supported features of the 'inittab' file between busybox init and full-blown init. However, I don't know (as of 2010) if that's still the case. (See for some details)

Denys Vlasenko, one of the maintainers of busybox has suggested a replacement for traditional init for that tool called runsv. See


upstart is the name of a newer Linux desktop systems that provides the program /sbin/init, but with different operational semantics.

Android init

Android 'init' is a custom program for booting the Android system.

See Android 'init'


systemd is a new project (as of May 2010) for starting daemons and services on a Linux desktop system



  • Kexec is a system which allows a system to be rebooted without going through BIOS. That is, a Linux kernel can directly boot into another Linux kernel, without going through firmware. See the white paper at: kexec.pdf

Splash Screen projects

  • Splashy - Technology to put up a splash screen early in the boot sequence. This is user-space code.
    • This seems to be the most current splash screen technology, for major distributions. A framebuffer driver for the kernel is required.
  • Gentoo Splashscreen - newer technology to put a splash screen early in the boot sequence
  • PSplash - PSplash is a userspace graphical boot splash screen for mainly embedded Linux devices supporting a 16bpp or 32bpp framebuffer.
  • - put up a splash screen early in boot sequence
    • This project requires kernel patches
    • This project is now abandoned, and work is being done on Splashy.


  • FSMLabs Fastboot - press release by FSMLabs about fast booting of their product. Is any of this published?
  • snapshot boot - a technology uses software resume to boot up the system quickly.

Apparently obsolete or abandoned material

  • Alert.gif in progress - Boot-up Time Reduction Howto - this is a project to catalog existing boot-up time reduction techniques.
    • Was originally intended to be the authoritative source for bootup time reduction information.
    • No one maintains it any more (as of Aug, 2008)
  • Alert.gifno content yet - Boot-up Time Delay Taxonomy - list of delays categorized by boot phase, type and magnitude
    • Was to be a survey of common bootup delays found in embedded devices.
    • Was never really written.


Companies, individuals or projects working on fast booting

Boot time check list

From an August 2009 discussion about boot time on ARM devices, several hints and advice regarding boot time optimization are available. While it may repeat a lot of above, below is a check list extracted from this discussion:

  • Is CPU's clock switched to maximum? If the kernel, bootloader or hardware is in charge of setting CPU power and speed scaling, then you should check that it boots with the CPU set at maximum speed instead of slowest.
  • Is your hardware (register) timing configuration of your SoC's memory interfaces (e.g. RAM and NOR/NAND timing) optimized? A lot of vendors ship their hardware with "well, it works, optimize later" settings. What you want is "as fast as possible, but sill stable and reliable" configuration. This might need some hardware knowledge and has to be customized to the individual memory devices used.
  • Does your boot loader uses I- and D-Cache? E.g. U-Boot doesn't enable D-Cache by default on ARM devices, as it needs customized MMU tables to do so.
  • Does kernel copy from permanent storage (e.g. NOR or NAND) to RAM use optimized functions? E.g. DMA, or on ARM at least load/store multiple commands (ldm/stm)?
  • If you use U-Boot's uImage, set "verify=no" in U-Boot to avoid checksum verification.
  • Optimize size of your kernel.
    • You might even try some of the embedded system kernel config options that, for example, eliminate all the printk strings, reduce data structures, or eliminate unneeded functionality.
  • How often is kernel (image) data copied? First by boot loader from storage to RAM, then by kernel's uncompressor to it's final destination? Once more? If you use compressed kernel and NOR flash, consider running the uncompressor XIP in NOR flash.
  • If you use compressed kernel, check compression algorithm. zlib is slow on decompression, and lzo is much faster. So if you implement lzo compression, you'll probably speed things up a little as well (check LKML for this). Having no compression at all may also be a good thing to try (see next topic).
  • Check to use uncompressed kernel (depends on your system configuration). Using an uncompressed kernel on a flash-based system may improve boot time. The reason is that compressed kernels are faster only when the throughput to the persistent storage is lower than the decompression throughput, and on typical embedded systems with DMA the throughput to memory outperforms the CPU-based decompression. Of course it depends on a lot of stuff like performance of flash controller, kernel storage filesystem performance, DMA controller performance, cache architecture etc. So it's individual per-system. Example: With using an uncompressed kernel (~2.8MB) uncompressing (running the uncompressor XIP in NOR flash) took ~0.5s longer than copying 2.8MB from flash to RAM.
  • Enable precalculated loops-per-jiffy
  • Enable kernel quiet option
  • If you use UBI: UBI is rather slow in attaching MTD devices. Everything is explained at MTD's UBI scalability and UBI fs scalability sections. There is not very much you can do to speed it up but implement UBI2. UBIFS would stay intact. There were discussions about this and it does not seem to be impossibly difficult to do UBI2 (few ideas).
    • In a follow-up e-mail, Sascha Hauer wrote:

"What's interesting about this is that the kernel NAND driver is much slower

than the one in U-Boot. Looking at it it turned out that the kernel driver uses interrupts to wait for the controller to get ready. Switching this to polling nearly doubles the NAND performance. UBI mounts much faster now and this cuts off another few seconds from the boot process  :) "

  • Use static device nodes during boot, and later setup busybox mdev for hotplug.
  • If you have network enabled, there might be some very long timeouts in the network code paths, which appear to be used whether you specify a static address or not. See the definitions of CONF_PRE_OPEN and CON_POST_OPEN in net/ipv4/ipconfig.c. Check ipdelay configuration patch.
  • Parallelize boot process.
  • Disable the option "Set system time from RTC on startup and resume", you can use the command hwclock -s at the of the init instead of slowing down the kernel.