2004.09_Wakeup Call-Automatically Power on Your Computers When You Need Them.pdf

SYSADMIN

Wakeup

Wakeup Call

Switching computers that do not need to run 24 by 7 on and off as required can

cut your power consumption. In this article, we will be looking at three differ-

ent methods of programming a wakeup time under Linux without needing to

set the time manually in the computer’s BIOS. BY MIRKO DÖLLE

RAID systems. Most users tend to

leave their desktops running when

they leave the office in the evening,

although computers are rarely needed

for more than 10 hours per day. For spe-

cial applications, like fax servers that

send out fax messages at 2 o’clock in the

morning, why not boot the machine just

before two and switch it off again for the

day until the next nocturnal session is

due?

Implementing these scenarios is quite

simple. All you need is a time switch for

a couple of dollars to switch the

machines on and off as needed. Things

start getting expensive if, as a normal

user, you do not have access to the time

switch in the data center. In this case

expensive power switches and a com-

puter-based controller are typically the

only viable solution.

The BIOS on your computer’s mother-

board will typically allow you to wake

up your computer without additional

hardware. More or less any modern

machine should have a function for

time-controlled booting, and let’s not

forget the additional “Wake on LAN”

and “Wake on Ring” functions that allow

a desktop to wake the server whenever it

needs to do so.

The problem is how to change the

wakeup time for time-controlled power

up on Linux – different manufacturers

and BIOS revisions mean applying differ-

ent approaches as no one method is

ideal for all. For example, ACPI wakeup

does not work on many motherboards

due to different implementations of the

standard. In contrast to this, NVRAM

wakeup can use non-volatile memory,

that is the memory where the BIOS

stores its data, to allow direct editing of

the wakeup time. Of course, this

assumes that you find the right memory

location, and that your BIOS actually

notices that the values have changed.

The third method, settime , uses a simple

trick. The BIOS always wakes up at the

same time and date, and the computer

clock is reset to a specific date just

before you shut down the machine.

The kernel transfers the time directly to

your computer’s RTC (real time clock),

but not the date. This leads to the com-

puter waking up at the same time every

day, and not just on the given date.

The problem is that there is a more-or-

less standardized method of transferring

the time to the RTC, but no such stan-

dard for the wakeup date. Consequently,

the acpi_system_write_alarm functions

in drivers/acpi/system.c for the kernel

2.4, or drivers/acpi/sleep/proc.c for the

kernel 2.6, only support the alarm time

fields.

ACPI – the Secret Weapon?

ACPI is probably the easiest way to

switch on a computer at a preset time,

that is, assuming that the kernel sup-

ports your choice of motherboard. If this

is the case, then you can simply store the

wakeup time for the system in

/proc/acpi/alarm :

Out of Date

Kernel developers Andy Grover and Paul

Diefenbaugh have already prepared the

way for the full functionality, but they

need a usable fixed ACPI table (FACP). If

you check out the comments in the ker-

nel code, you will note that the current

FACP tables for motherboards are unser-

viceable, and this

has led to the code

block being per-

manently

disabled.

The various

motherboards pose

a variety of prob-

lems. Some

motherboards do

not pass the time

correctly to the

computer’s RTC,

storing the last

boot time instead

of the desired

wakeup time. Oth-

ers immediately

crash when an

echo 2004-08-02 20:15:00 >/proc U

/acpi/alarm

Figure 1: Most computer BIOS chips have a wakeup function called RTC Alarm

Resume or something similar.

September 2004

www.linux-magazine.com

Programming a wakeup service for your computer

S ervers need to run 24 by 7, as do

Wakeup

SYSADMIN

attempt to write to /proc/acpi/alarm is

made. ACPI wakeup works with many of

the current crop of motherboards. You

can checkout the distribution section on

the LinVDR (mini video recorder) distri-

bution homepage for a non-exhaustive

list [1].

between midnight and two in the morn-

ing, so booting the machine at two

o’clock would allow you to perform an

EPG scan to support automatic timer

programming.

However, a print server that boots at 9

in the morning and powers down at 7 in

the evening would be up and running for

10 hours on weekends. An additional

routine in the startup scripts that recog-

nizes weekends and public holidays

could shut the machine down, but this is

only a stopgap solution. Although you

might need to use the print server on the

weekend, it would still shut down, even

if you booted it manually.

A script that checks for weekends and

public holidays when shutting down the

computer, and programs the wakeup

time and date, is a far better solution.

NVRAM wakeup and the settime method

provide the answers.

modify the /etc/modules.conf module

configuration file to tell your computer

to load the NVRAM module when

/dev/nvram is accessed or immediately

on booting.

Using BIOS Functions

There are no tricks to watch out for

when building the program from the

source code (available at [2]). The

default installation path is /usr/local , but

admins can edit the Makefile to change

the path to /usr if needed. If the

/dev/nvram , /dev/rtc , and /dev/mem

devices do not exist at this point, make

devices will automatically create them.

In version 0.96, the tr statements in

lines 62 and 75 of the helper script guess-

helper.sh do not work:

Topsy-Turvy

If you want to use ACPI to wake up your

computer, you need to ensure that its

BIOS settings are correct. For years now

most computers have had BIOS func-

tions called Wake on Timer , Resume on

Alarm , RTC Alarm Resume or similar

variations on the same theme. This said,

users were expected to set the time in the

BIOS. ACPI wakeup uses a similar tech-

nique, but it does not use the BIOS

function. This is why you will need to

disable the BIOS wakeup function on

most motherboards if you want to use

ACPI. Also, note that the time for the

next time-controlled power up will not

be displayed in the BIOS.

Yet another hurdle: there are a few

motherboards, such as the Asus A7V133,

that do not take kindly to a shutdown

script using hwclock -w to synchronize

the RTC with the system time after enter-

ing the wakeup time. The computer

simply does not wake up again. You

need to set the --directisa flag when call-

ing hwclock , or as an alternative, you can

set the wakeup time after calling

hwclock .

answers=`$echo $answers | U

tr [:lower:] [:upper:]`

NVRAM Wakeup

NVRAM wakeup uses the BIOS settings

stored in non-volatile RAM (NVRAM).

The nvram kernel module allows Linux

to access a maximum of 128 bytes of

non-volatile memory.

To get this to work, you need to com-

pile the nvram kernel module, and also

create the character devices /dev/nvram

with a major of 10 and a minor of 144,

/dev/rtc with a major of 10 and a minor

of 135, and /dev/mem with a major of

1 and a minor of 1. You also need to

Fortunately, the script worked fine after

adding four backticks

answers=`$echo $answers | U

tr '[:lower:]' '[:upper:]'`

For most manufacturers, the way they

allocate the NVRAM is a big secret – you

might have some difficulty finding out

where the date and time are stored for

lack of documentation. What’s worse,

the memory locations tend to be a mov-

ing target that changes whenever you

update the BIOS.

The NVRAM wakeup sources include

specs for a number of boards. These

details come courtesy of the guess-

helper.sh script. Running nvram-wakeup

with the -D flag launches the program in

debug mode, and tells you if your moth-

erboard is known. If nvram-wakeup tells

you that

Groundhog Day…

As previously men-

tioned, ACPI wakeup

tells the computer to

wake up every day at

the preset time, and this

does tend to restrict the

usefulness of the func-

tion. ACPI wakeup

would be quite useful

for a fax server that

automatically processes

fax messages every

night before switching

off again. ACPI wakeup

is also quite useful for

PC video recorders

(PVRs), such as VDR for

example. Most channels

update their electronic

program guides (EPGs)

Listing 1: nvram-wakeup.conf

01 ##############################

02 ## motherboard autodetection information:

03 ##

04 ## - motherboard vendor: ""

05 ## - motherboard type: "VT8367-8235"

06 ## - motherboard revision: ""

07 ## - BIOS vendor:

"Award Software

International, Inc."

08 ## - BIOS version:

"6.00 PG"

nvram-wakeup: Your motherboard U

is currently not supported.

09 ## - BIOS release:

"04/07/2003"

11 addr_stat

= 0xD2

however, you will need to run guess-

helper.sh .

The guess-helper.sh script tries to

locate the position of the NVRAM mem-

ory fields. You will need to boot your

computer at least four times to do this.

The first time you boot, you need to dis-

able the BIOS wakeup function, set the

time to the 31st of the month at one

12 shift_stat

= 5

13 addr_day

= 0xD8

14 addr_hour

= 0xD9

15 addr_min

= 0xDA

16 addr_sec

= 0xDB

18 upper_method = VT8235_37

www.linux-magazine.com September 2004

SYSADMIN

Wakeup

second to midnight, and then call guess-

helper.sh . Then set the wakeup time to

11th of the month and the time to

12:13:14, then to the 1st of the month,

midnight, and finally disable the wakeup

function again.

Each time the wakeup point is called,

guess-helper.sh compares the content of

the NVRAM with the other calls to estab-

lish the memory locations for wakeup

points. guess-helper.sh uses two methods

to do so: access via /dev/nvram and

direct I/O address access. This results in

two configuration files, nvram-wakeup.

conf in the guess-nvram-module direc-

tory, and guess-directisa below root ‘s

home directory.

Figure 2: For a successful trial run, the values displayed by nvram-

wakeup need to match. The wakeup time set in the BIOS is

shown at the top; the bottom value shows the value as parsed by

the program.

next full quarter hour, and

type reboot to restart your

machine. Then set a wake-

up time for the next full

quarter hour and enter

poweroff to power the com-

puter off.

If the BIOS wakes up at

the next full quarter hour, it

has successfully read its

BIOS to support wakeup.

For some motherboards

the memory size accessible

to /dev/nvram, as defined in

the nvram module, is too

small to access the whole NVRAM area.

Cases where guess-helper.sh is unable to

locate the day, hour, minute, and second

positions, or only finds a few fields, are

indicative of this issue. The kernel

sources define the memory size in

drivers/char/nvram.c :

for the computer to boot. Figure 1 shows

the output for nvram-wakeup . The top

section of Figure 1 shows the wakeup

data that nvram-wakeup located in the

NVRAM; the bottom value shows the

data that would have been program-med

without the write protect parameter, -N .

The values match, so we can assume

that guess-helper.sh really has located the

right memory cells.

The next step is to set a date about 15

minutes in the future, and then shut the

computer down:

Positioning Information

A quick look at the configuration files

shows us where comments about the

motherboard, and address information

are stored. In the case of the Elito Epox

8K5A2+ board, the /root/guess-nvram-

module file was empty apart from a few

comments, whereas the /root/guess-

directisa contained the address entries

shown in Listing 1.

If both configuration files are empty,

you can assume that guess-helper.sh was

unable to locate the addresses.

You need to tell the program the path

to the configuration file, using the -C

parameter to do so, whenever you call

nvram-wakeup . Also, the -A parameter is

required in this case, as we will be using

the I/O addresses for direct access rather

than the /dev/nvram device. Both these

parameters can be left out if you have

a motherboard that nvram-wakeup

supports.

#define NVRAM_BYTES U

(128-NVRAM_FIRST_BYTE)

You need to extend the size to a full 128

bytes:

nvram-wakeup -s `date -d "+15 U

Minutes" +%s` -A -C /etc/nvram U

-wakeup.conf

poweroff

#define NVRAM_BYTES 128

This change may help guess-helper.sh to

locate the necessary fields for your par-

ticular motherboard.

If your computer does not wake up as

expected, this may be due to a basic

APM/ACPI problem, or your computer

may need to run the BIOS routine once

more – that is reboot – to wake up at the

preset time.

Back to the Past

settime is an ingenious method that is

guaranteed to work with any mother-

board. The idea is to program a fixed

wakeup time in the BIOS, for example

day 31 of the month at 23:59:59.

When you shutdown your machine, a

script calculates the time vector between

shutdown and wakeup, subtracts this

vector from say July 31 2004, 23:59:59

and sets the system and

RTC time to an appropri-

ate day and time in July

2004. The next time you

boot your computer, you

only need to correct the

time setting for every-

thing to be as it should

be.

In practical applica-

tions this approach can

be a bit tricky. You have

to ensure that the cor-

Reboot not Shutdown

There is a very easy way to find out

if you are facing the so-called reboot

problem. Set a wakeup point for the

Trial Run

For our trial run with nvram-wakeup , we

need an arbitrary wakeup time in the

BIOS; we will be using

nvram-wakeup to vali-

date the results.

In the following exam-

ple, the BIOS is set up to

wake up the computer at

20:15 on the 15th of the

month. The date for the

nvram-wakeup call also

needs to consider the

time vector between

local time and UTC, and

allow about five minutes

Listing 2: Excerpt from settime

01 #!/bin/bash

02 BiosWakeup="2004-07-31 23:59:59"

03 Wakeup=`date -d "$1" +%s`

04 Now=`date +%s`

05 Bios=`date -u -d "${BiosWakeup}" +%s`

06 Diff=$[${Wakeup}-${Now}]

07 echo "$[${Now}-${Bios}]" > /etc/timediff

08 date -u -s "${BiosWakeup} ${Diff} seconds ago" >/dev/null

09 hwclock -w --utc

September 2004

www.linux-magazine.com

Wakeup

SYSADMIN

rect time is set, whenever you boot the

machine, and before fsck executes – this

could prevent disk checks from taking

place otherwise. If you correct the time

before fsck executes, there is nowhere to

make a note of the fact, as you will only

have read access to the partitions.

Dual boot systems with Windows, or

some other operating system, are tricky

too, as they would wake up with the

wrong time and date. Additionally, this

method only makes sense if you boot the

machine at least once every two months.

To go back to the past, try the following

instead:

Alternatively, if the modification date is

in the past, the system time must be the

current real time.

In lines 6 through 8, correcttime reads

the time vector between the system and

real time from the /etc/timediff file and

adds it to the current system time, thus

taking the machine back to real time.

The real time is then transferred to the

RTC, as most distributions do an explicit

sync of the system time with the time of

the RTC somewhere in the boot scripts.

date -s "3600 seconds ago"

Additionally, date can convert dates in the

international standard notation, which is

seconds since the epoch, as follows:

debian:~# date -d "2004-08-31 U

23:59:59" +%s

1093993199

Save Energy – Save Money!

A glance at your last electricity bill

shows you just how expensive electricity

is. You can save energy. And without suf-

fering as a consequence if you put a bit

of thought into the process. Five normal

desktop PCs (150 watts each) running

day and night, as is the case in many

offices, will consume over 6,500 kilowatt

hours, or £ 800, US$ 1,500, EUR 1,200

per year. Servers are even worse, as val-

ues of 400 watts or more per unit are

quite common. A single server could

consume over 3,500 kilowatt hours, that

is add £ 420, US$ 780, EUR 630 to your

annual electricity bill.

Shutting the computers down auto-

matically at 9.00 pm, and booting them

again at 6.00 o’clock in the morning on

working days would mean a saving of

more than 3,600 kilowatt hours for the

desktops, and about 2,000 kilowatt

hours for the server. That is equivalent to

a saving of almost £ 650, US$ 1,200, EUR

1,000, not to mention the reduced envi-

ronmental pollution.

If you need your computers outside of

normal office hours, just press the power

button. Even in a server-client-environ-

ment you can shut down both the

servers and the clients – just configure

the server to use “Wake on LAN” and

use the etherwake program in the work-

station init scripts.

Delta T

Tw o separate scripts provide the answer:

settime calculates the time vector

between the actual and wakeup times,

stores the difference in a file called

/etc/timediff , sets the system time to a

date in July 2004, and synchronizes the

RTC [3]. When you boot the system, cor-

recttime is called in the first init script,

reads the time vector, corrects the system

time, and synchronizes the RTC. This

ensures that your other start scripts will

not even notice the time shift.

The settime method relies on the fact

that the RTC continues to keep normal

time. This removes the need to synchro-

nize the system time with another

system, or use an accurate clock when-

ever you boot. All you need to know is

the number of seconds that the clock has

lost in comparison to real time.

Also, there is no need to perform com-

plicated date calculations including leap

years; date has all the functions you

require. For example,

The rest is easy. Listing 2 shows an

excerpt from the settime script. Line 3

converts the wakeup time to seconds

past epoch, line 4 establishes the current

date in seconds past epoch, and line 6

uses these values to calculate the time

vector in seconds.

Line 5 of Listing 2 calculates the

wakeup time set in the BIOS, one second

to midnight on July 31 2004 in this exam-

ple, in seconds past epoch, allowing line

7 to calculate the difference to real time

and store the value in /etc/timediff . Now,

the next time the system boots, correct-

time just has to add the difference to the

system time to obtain the real time.

Finally, lines 8 and 9 set the system

and RTC time to a value in the past. As

already seen in line 5, these times are

converted to UTC and stored. To use

local time rather than UTC for the RTC,

you need to change the three lines to

reflect your local timezone.

Correcting the Time

The settime script is only called when

required, that is when you need to

wakeup your machine at a preset time,

and immediately before shutting down.

That means correcttime has to find out if

the RTC time is the real

time, or if the computer

is somewhere in the

past. Listing 3 shows an

excerpt from correcttime .

The call to date in line

3 discovers when

/etc/timediff was last

modified, returning a

value in seconds past the

epoch. If the modifica-

tion date in line 5 is in

the future, the system

time must have been set

to a date in the past.

date -s "+3600 seconds"

sets the system clock forward one hour.

Listing 3: Excerpt from correcttime

01 #!/bin/bash

02 if [-r/etc/timediff ]; then

03 Timediff=`date -r /etc/timediff +%s`

04 Now=`date +%s`

05 if [ "${Timediff}" -gt "${Now}" ]; then

06 Diff=`cat /etc/timediff | head -n 1`

07 date -s "+${Diff} seconds" >/dev/null

08 hwclock -w --noadjfile --utc

09 exit 0

10 fi

11 fi

■

INFO

[1] ACPI compatibility:

http://linvdr.org/wiki/index.php?

pagename=LinVDR-Mainboards

[2] NVRAM wakeup: http://sourceforge.net/

projects/nvram-wakeup/

[3] The settime and correcttime scripts:

http://www.linux-magazine.com/

Downloads/46/wakeup

www.linux-magazine.com September 2004

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