  Battery Powered Linux Mini-HOWTO
  Hanno Mueller, hmueller@kabel.de
  5 May 1997

  This document describes how to reduce a Linux system's power consump­
  tion by tweaking some of its configuration settings. This will be
  helpful for everyone who runs Linux on a portable computer system.
  There is also some general information about how to take care of your
  battery. If you are using Linux on a desktop system, you probably
  don't need to read all this.

  1.  Introduction

  ``Wherever you want to go, there you are.''

  1.1.  Before you ask

  This document does not describe how to install Linux on laptops, but
  how to optimize a ready-configured Linux for use on laptops. Please
  read the Installation-HOWTO or your distributor's handbook for help
  with installing Linux.

  This document does not describe how to use an uninterruptable power
  supply and the powerd daemon, either (even though a UPS is a big
  battery). Read the UPS-HOWTO for details about that subject.

  1.2.  What this document is about

  More and more people own portable computers these days and in turn,
  more and more people install Linux on such machines.

  Installing and using Linux on a laptop is usually no problem at all,
  so go ahead and give it a try. Unlike some other operating systems,
  Linux still supports and runs well on even very old hardware, so you
  might give your outdated portable a new purpose in life by installing
  Linux on it.

  If you need help with installing Linux on a laptop or if you have
  questions about laptop hardware, you can check the excellent Linux
  Laptop webpage at http://www.cs.utexas.edu/users/kharker/linux-laptop/
  where you will find a lot of useful information and detailed help. The
  Linux Laptop page describes hardware configuration for specific laptop
  models and chipsets.

  This HOWTO however will focus on the one problem that is common to all
  portable systems: Power consumption.

  Yet, I have not found a Linux distribution that comes with a
  configuration optimized for laptops. Since I could not find this kind
  of information anywhere else, I have started to collect a few simple
  but effective tricks that will help you save battery power and in turn
  increase your system's uptime while running on battery.

  (Sidenote. I received a complaint by a reader that these tips were not
  very effective with his laptop. So does all this really work? Yes, but
  don't expect miracles. I was able to increase my laptop's battery time
  from 90 minutes to more than 120 minutes.)

  1.3.  Roadmap

  If you are a laptop pro, you can probably skip the ``General
  Information'' section. If you are a Linux pro, what you really want to
  know can be found in the ``changing some general system settings''
  section. If you are a Linux distributor, please read ``a message to
  Linux distributors''.

  1.4.  Feedback

  Your feedback is welcome. Please send comments to hmueller@kabel.de.
  Did it work on your system? Do you have new tips? Are there any
  outdated links or addresses in this text?

  I am sorry, but I will not be able to help you with questions about
  specific laptop models. I don't claim to be a laptop guru, I just
  happen to own one laptop myself and I just want to share the
  information I collected. Please check the Linux Laptop webpage first,
  probably someone else has already written a page dedicated to your
  model. Ask your manufacturer's technical support. Or go the the laptop
  newsgroup comp.sys.laptops and ask there.

  1.5.  Disclaimer

  All methods described here were tested by me and worked fine on my
  laptop, unless noted otherwise. However, I cannot guarantee that any
  of this won't crash or seriously damage your system. Life is
  dangerous, so keep backup copies of your important files before
  playing with your Linux configuration. If things go wrong, I do not
  take any responsibility for your data loss. In other words: Don't sue
  me. Thank you.

  1.6.  Copyright

  This document shall be distributed under the standard HOWTO-copyright
  notice, found in the HOWTO folder at
  http://sunsite.unc.edu/mdw/linux.html.

  2.  General information

  This section describes a few technical things about laptop batteries
  and some general power saving tips. This information is not Linux-
  specific and if you are experienced with laptops, you might already
  know all this.

  2.1.  Be kind to your battery

  (Please note the ``credits'' for this section.)

  There are currently three types of batteries commonly used for
  portable computers.

  ·  NiCd batteries were the standard technology for years, but today
     they are out of date and new laptops don't use them anymore. They
     are heavy and very prone to the ``memory effect''. When recharging
     a NiCd battery that has not been fully discharged, it ``remembers''
     the old charge and continues there the next time you use it.

     The memory effect is caused by crystallization of the battery's
     substances and can permanently reduce your battery's lifetime, even
     make it useless. To avoid it, you should completely discharge the
     battery and then fully recharge it again at least once every few
     weeks.

     (A sidenote about the memory effect. James Youngman knows of a
     rather drastic method to - uhm - ``repair'' batteries: ``If your
     NiCd battery is suffering from the memory effect, remove it from
     your computer, hold it about 30cm above a desk or the floor, and
     drop it (make sure it lands flat).'' He says that this will break
     the whiskers that have formed in the battery and that are the cause
     of the memory effect if your battery is already affected by it. ``I
     don't know if this works for non-NiCd batteries or not.'')

     Cadmium is a very hazardous poison, but if returned to your dealer,
     the material can almost be fully recycled.

     Just in case you might be interested, here are some specs for NiCd:

       Cell voltage: 1,2 V
       Energy / mass: 40 Wh/kg
       Energy / volume: 100 Wh/l
       max. Energy: 20 Wh
       Charge temp.: 10 to 35 C (50 to 95 F)
       Discharge temp.: -20 to 50 C (-5 to 120 F)
       Storage temp.: 0 to 45 C (30 to 115 F)

  ·  NiMh batteries are the current standard used in most low price
     laptops to date. They can be made smaller and are less affected by
     the memory effect than NiCd.

     However, they have problems at very high or low room temperatures.
     And even though they use less hazardous and non-poisonous
     substances, they cannot be fully recycled yet (but this will
     probably change in the future).

     NiMh specs:

       Cell voltage: 1,2 V
       Energy / mass: 55 Wh/kg
       Energy / volume: 160 Wh/l
       max. Energy: 35 Wh
       Charge temp.: 10 to 35 C (50 to 95 F)
       Discharge temp.: 0 to 45 C (30 to 115 F)
       Storage temp.: 0 to 30 C (30 to 85 F)

  ·  The new high performance batteries use LiIon technology. In theory,
     there is no memory effect at all with these batteries, but on
     occasion, they seem to have similar problems. Their substances are
     non-hazardous to the enviroment, but they should be returned for
     recycling as well.

     LiIon specs:

       Cell voltage: 3,6 V
       Energy / mass: 100 Wh/kg
       Energy / volume: 230 Wh/l
       max. Energy: 60 Wh
       Charge temp.: 0 to 45 C (30 to 115 F)
       Discharge temp.: -20 to 60 C (-5 to 140 F)
       Storage temp.: -20 to 60 C (-5 to 140 F)

  Even if the battery case looks the same, you cannot just upgrade to
  another battery technology. The recharging process is different for
  the kind of battery you use, and most manufacturers integrate the
  recharging circuit inside the laptop case. When in doubt, ask your
  manufacturer if your laptop supports a more modern battery.

  A battery that is not used for a long time will slowly discharge
  itself. And even with greatest care, a battery needs to be replaced
  after 500 to 1000 recharges. But still it is not recommended to run a
  laptop without the battery while on ac power - the battery often
  serves as a protection against voltage peaks.

  As the manufacturers change the shapes of their batteries every few
  months, you might have problems to find a new battery for your laptop
  in a few years from now. Buy a spare battery now - before it's out of
  stock.

  2.2.  Power saving - The obvious stuff

  There are some obvious things that you can do to reduce your system's
  power consumption. Well, maybe not so obvious, since not very many
  people follow these rules...

  ·  Decrease or turn off your display's backlight when you don't need
     it. By the way, tft displays use more power than dstn (so now you
     have a fine excuse why you bought the cheaper laptop...).

     (David Bateman tells me that using a crt screen while on battery
     and turning off the laptop display will extend battery time by
     about 30%: ``Not that this is a very useful piece of knowledge
     though, if you've got the crt plugged-in then why not the laptop
     too.'')

  ·  How much processing power do you really need? I doubt that you will
     be doing very much more than text editing when on the road (well,
     at least I don't compile linux kernels then). While on battery,
     reducing the cpu clock speed will decrease power consumption, too.
     Quite a few laptops offer a cpu clock selector that will toggle
     between normal and slow speed.

  ·  Avoid using external devices (printer, crt screen, zip drive,
     portable camera etc.) with your computer while on battery. When
     connected to a standard ink jet printer, my laptop's battery time
     is reduced from up to 120 minutes down to 20 minutes.

  ·  Avoid using any built in device unless necessairy: Diskette drive,
     harddisk, cd-rom. Especially cd-rom access will dramatically
     decrease your battery time.

  ·  Pcmcia cards can also consume a lot of power, so don't leave your
     modem or network adapter plugged in when it is not in use. But this
     is different between the various pcmcia manufacturers, so check the
     product specs before you buy (e. g. some cards never turn
     themselves off even when not in use).

  ·  Use simple software. A full blown multimedia application will
     create a lot more system load and harddisk / cd-rom activity than a
     small simple word processor.

  ·  If you are yet about to buy a laptop - don't buy a laptop with a
     2nd level cache if battery uptime is important. A computer with 2nd
     level cache is about 10% to 20% faster and it will be a lot better
     with multimedia applications, but it consumes a lot of power.
     Bjoern Kriews tells me that he has two almost identical laptops and
     the one without cache ram runs 4h30 compared to 2h30 with cache.

  ·  Another tip for those still buying a laptop - don't buy the latest,
     fastest cpu type. Usually, the older generations are optimized by
     the manufacturer after some time without notice. The ``new''
     versions of old cpu types often create less heat and consume less
     power than the product's premiere version. There are also
     frankenstein laptops available that use cpus not optimized for
     portable systems. As I write this, the newest generation
     pentium-200 laptops run about 20 minutes on battery power and will
     probably burn your lap...

  Well, you get the idea. Most of these are restrictions that will
  probably stop you from doing any serious work with your Linux system.
  (The best way to save power while on battery is... not to do anything
  at all. That increases my laptop's battery uptime by almost 100%.)

  So let's go ahead to some other, more useful measures that will save
  power without disturbing your work.

  3.  Advanced Power Management

  Portable systems in general, but even many desktop computers come
  equipped with support for apm, the ``advanced power management''
  scheme. This section describes how to activate apm support in your
  Linux kernel. People who are experienced with Linux may find this
  section rather boring and want to skip to the next.

  3.1.  What APM can do for you

  I won't describe it in detail here, check the Linux APM drivers page
  at http://www.cs.utexas.edu/users/kharker/linux-laptop/apm.html for
  more information. All that you need to know is that with the help of
  apm, the cpu can tell the bios when there's nothing really exciting
  going on so that the bios can take care of some power saving by itself
  - e. g. reducing the cpu clock, turning off the harddisk, turning off
  the display's backlight etc.

  Apm is also responsible for the ``system suspend'' (or ``sleep'') mode
  and for the ``suspend to disk'' (or ``hiberntation'') mode. And yet
  another cool, though not very important feature is that with the help
  of apm, shutdown -h will not just halt your system, but also turn it
  off.

  Not all manufacturers implement a correct apm bios, so some laptops
  have trouble with the Linux apm drivers (if your machine has trouble
  with apm, it will most likely either lock up at Linux' boot up or
  after returning from suspend). If you are not sure, check the Linux
  laptop page for your specific model.

  3.2.  How to activate APM support in Linux

  It's easy - just recompile the Linux kernel. Check the Kernel-HOWTO if
  you don't know how to do that.

  When the configuration script reaches the ``character devices''
  section, the default setting for full apm bios support in kernel
  version 2.0.30 or higher is:
       Advanced Power Management BIOS support: Yes
       Ignore USER SUSPEND: No
       Enable PM at boot time: Yes
       Make CPU Idle calls when idle: Yes
       Enable console blanking using APM: Yes
       Power off on shutdown: Yes

  Please read the configuration script's help texts. They explain in
  detail what each option does, so I won't repeat them here.

  If your system does not fully support the apm bios standard, some of
  those options might crash your system. Test all apm features with the
  new kernel to make sure that everything works as it should.

  (A sidenote about console blanking: David Bateman tells me that you
  should not enable it because it can cause problems with the current
  version of XFree 3.2: ``The symptoms are that the screen will be blank
  when X starts, and it can be fixed usually by just hitting a key. It's
  a small but annoying problem. The next relase of XFree, will have
  pretty good DPMS support for a lot of laptop chipset, which should
  include code to turn off the LCD. Check out the manpage for xset in
  XFree 3.2A.'' David also notes that the lifetime of your display's
  backlight is determined by the number of times it's switched on and
  off: ``So its a compromise, lifetime of the battery versus lifetime of
  the backlight.'')

  3.3.  APM support and the PCMCIA drivers

  After recompiling the kernel, don't forget to recompile the linux
  pcmcia drivers as well.

  The precompiled pcmcia drivers that come with most linux distribution
  have apm support disabled, so that the bios can't instruct your card
  adapters to turn off.

  Also, you must recompile the drivers if you upgrade to a new kernel
  version and your old kernel was compiled with module version
  information turned on (this option is found in the ``loadable module
  support'' section of the kernel configuration).

  Read the PCMCIA-HOWTO for detailed instructions on how to compile the
  drivers or go to the Linux PCMCIA drivers homepage at
  http://hyper.stanford.edu/HyperNews/get/pcmcia/home.html.

  3.4.  The apmd package

  Now that you have APM support installed, go and get the apmd package
  from the Linux APM drivers page. You don't really need it, but it is a
  very useful collection of programs. The apmd daemon logs your
  battery's behaviour and it will send out a warning if you are on low
  power. The apm command will suspend your system with a shell command
  and xapm shows the current state of your battery.

  (BTW, if you have problems with pcmcia cards after returning from
  suspend, you can check out an alternative apmd package at
  http://www.cut.de/bkr/linux/apmd/apmd.html. It unloads the pcmcia
  driver module before going to suspend and reloads the module on
  resume.)

  3.5.  And if my laptop does not support APM?

  If your computer's bios does not offer any power saving settings (even
  the old ones without apm should at least allow to set harddisk and
  display standby), you can use hdparm -S to define your harddisk's
  standby period. This will already help a lot, since harddisk activity
  consumes a lot of power. Your system should have hdparm installed, so
  read man hdparm for the command syntax.

  4.  Changing some general system settings

  After I got Linux up and running on my laptop, I found it accessing
  the harddisk every few seconds, even when there was no user logged in
  to the system. The harddisk could never enter its power saving mode.
  Reducing harddisk activity can greatly increase the battery runtime,
  so this is why I collected the following recipes.

  I tested all this with RedHat 4.1, the locations of some configuration
  settings may be different for your distribution. (If so, please let me
  know.)

  4.1.  The crond daemon and atrun

  Check your /etc/crontab file if it starts a process every minute. You
  will often find atrun there.

  With the at command, you can spool commands that must be invoked some
  time in the future. Some Linux systems use a dedicated atd daemon to
  take care of this, others (e. g. RedHat) let the crond daemon run
  atrun once every minute.

  This is not really necessairy on most systems, since at commands
  rarely depend upon being invoked on exact time. So if you find a line
  like this in your /etc/crontab:

       # Run any at jobs every minute
       * * * * * root [ -x /usr/sbin/atrun ] && /usr/sbin/atrun

  Then you can safely change this to:

       # Run any at jobs every hour
       00 * * * * root [ -x /usr/sbin/atrun ] && /usr/sbin/atrun

  Read man 5 crontab for details. Some folks can even work fine without
  the crond daemon, so you might want to consider disabling it
  completely.

  4.2.  The update / bdflush daemon

  Linux deals with a lot of open file buffers at any given moment, so
  the system must make sure that file changes are saved to the harddisk
  as soon possible. Otherwise, those changes will be lost after a system
  crash.

  The update / bdflush daemon takes care of this. (These are two names
  for the same program, so you can use either name to start the daemon).
  The default settings will make this daemon call flush every 5 seconds
  and sync every 30 seconds.

  It seems to depend on your harddisk's firmware, but on my machine this
  caused non-stop harddisk access. (Some harddisks seem to flush their
  cache ram even if nothing has changed.)

  Since Linux does not crash very often anymore, I have changed both
  values to 3600 seconds (= one hour). This caused no problems at all
  and the constant disk access has stopped. (But if my system crashes
  now, there will be more broken files, of course.)

  RedHat 4.1: In /etc/inittab, change the update call to:

       ud::once:/sbin/update -s 3600 -f 3600

  Suse 4.4.1: update is called in /sbin/init.d/boot.

  Slackware: update is called in /etc/rc.d/rc.S.

  See man update for details.

  4.3.  The syslogd daemon

  The syslogd daemon is responsible for the various Linux system log
  files that are found in the /var/log/ directory. By default syslogd
  will sync the log file each time after logging a system message.

  You can turn that off by preceding the filename with a dash in
  /etc/syslog.conf. Here's an example as found in my system's
  syslog.conf:

       # Log anything (except mail) of level info or higher.
       # Don't log private authentication messages!
       *.info;mail.none;authpriv.none                  -/var/log/messages

  This again means that if the system crashes, the message that reported
  the problem may not have been stored to disk. Dilemma...

  4.4.  The init command

  During the bootup, the initial processes and daemons will be started
  using the init command. This command (yet again) calls sync before
  each process it creates.

  You can change this by removing the sync() call in the source code and
  recompiling the command.

  To avoid problems with lost file buffers, you should add a call to
  sync in your system's /etc/rc.d/init.d/halt script, right before the
  script unmounts the file systems.

  4.5.  The swap partition

  The Linux swap partition is used to increase the physical ram space
  with virtual memory. This again is a possible reason for harddisk
  access. If your laptop already has a lot of ram or if the applications
  that you use are quite simple (think of vi), you might want to
  consider turning it off.

  This of course depends on what you plan to do. 4 to 8 megs are not
  enough, you must use a swap partition then. With 8 to 16 megs, text
  console applications will work fine and if you can avoid using a lot
  of multitasking features, you can safely disable swap. The X-Windows
  enviroment requires a lot of ram and you should not use it without a
  swap partition unless you really have a lot more than 16 megs.

  (Sidenote: My laptop with 16 megs and disabled swap partition can run
  an emacs session, four bash shells and compile a kernel without
  running out of memory. That's enough for me.)
  If you already have installed a swap partition, you can disable it by
  preceding the swapon command that is called in /etc/rc.d/rc.sysinit
  with a hash mark. If you don't want to make it a permanent move, let
  the system ask during boot if you want to use the swap partition. In
  /etc/rc.d/rc.sysinit (RedHat 4.1) or /sbin/init.d/boot (Suse 4.4.1):

       echo "Should the system use swap?"
       echo "  0: No."
       echo "  1: Yes."
       /bin/echo "Your choice: \c"
       read SWAPCHOICE

       case "$SWAPCHOICE" in
           0)
               # Do nothing.
               echo "(Swap partitions disabled)"
               ;;
           *)
               # Start up swapping.
               echo "Activating swap partitions"
               swapon -a
       esac

  Then you can use the swap partition while on ac power and drop it
  while on battery.

  4.6.  The apache httpd webserver daemon

  I am using my laptop to develop and test cgi scipts for websites, that
  is why I am running a local webserver on it. The standard
  configuration is a bit too much if all you want to do is just test a
  script or check a page from time to time.

  In httpd.conf, just change the values of MinSpareServers and
  StartServers to 1. This will be enough for a local test site.

  If you wish to turn off the webserver's logging, you must recompile
  the httpd daemon. Read the documentation for details.

  4.7.  How to find more ways to optimize

  If your Linux system still seems to access the harddisk too often, you
  can find out what is going on inside by using the ps ax command. This
  will show all running processes and their full name, sometimes it also
  reveals the command line arguments of each process.

  Now read the man page of each process to find out what it does and how
  to change its behaviour. With this method, you will most likely find
  the process that is responsible. You may also find strace helpful.

  Please send me an email if you found something new.

  5.  Appendix

  5.1.  A message to Linux distributors

  If you happen to be a Linux distributor, thank you for reading all
  this. Laptops are becoming more and more popular, but still most Linux
  distributions are not very well prepared for portable computing.
  Please make this document obsolete and change this for your
  distribution.

  ·  The installation routine should include a configuration, optimized
     for laptops. The ``mimimal install'' is often not lean enough.
     There are a lot of things that a laptop user does not need on the
     road. Just a few examples. There is no need for three different
     versions of vi (as found in Suse Linux). Most portable systems do
     not need printing support (they will never be connected to a
     printer, printing is usually done with the desktop system at home).
     Quite a few laptops do not need any network support at all.

  ·  Add better power management and seamless pcmcia support to your
     distribution. Add a precompiled kernel and an alternative set of
     pcmcia drivers with apm support that the user can install on
     demand. Include a precompiled apmd package with your distribution.

  ·  Add support for dynamically switching network configurations. Most
     Linux laptops travel between locations with different network
     settings (e. g. the network at home, the network at the office and
     the network at the university) and have to change the network id
     very often. Changing a Linux system's network id is a pain with
     most distributions.

  Please mail me if your distribution is optimized for portable
  computing and what kind of features you added for that. Future
  versions of this HOWTO will include a section where you can advertise
  your distribution's laptop features.

  5.2.  Credits

  ·  The information about battery technology is mostly based on the
     article ``Stromkonserve'' by Michael Reiter, published in ``c't
     Magazin fuer Computertechnik'' (Heise Verlag Hannover, Germany),
     edition 10/96, page 204. Used by permission. Visit their website at
     http://www.heise.de/.

  ·  The following people contributed to this document:

       Frithjof Anders <anders@goethe.ucdavis.edu>
       David Bateman <dbateman@ee.uts.edu.au>
       Markus Gutschke <gutschk@uni-muenster.de>
       Kenneth E. Harker <kharker@cs.utexas.edu>
       R. Manmatha <manmatha@bendigo.cs.umass.edu>
       Bjoern Kriews <bkr@rrz.uni-hamburg.de>
       James Youngman <Youngman@vggas.com>

  5.3.  About this document

  The current version can be found at the usual HOWTO distribution site
  http://sunsite.unc.edu/mdw/linux.html, at one of its many mirror sites
  or on my homepage at http://www.kabel.de/~hmueller/.

  This text was written during my many trips between Hamburg and
  Hannover on German rail. (The new ice-2 coaches have power outlets for
  laptops, yeah!).

  And now hum with me: ...on the road again...

