It is said to overclock Raspberry Pi one has to upgrade the system:

sudo apt-get update && sudo apt-get install raspberrypi* raspi-config

Then configure it with raspi-config utility:

sudo raspi-config

It is good anyhow to check the system version first:

uname -a
Linux raspberrystar.pinkaccordions.org 3.2.27+ #174 PREEMPT Wed Sep 26 14:09:47 BST 2012 armv6l GNU/Linux

The kernel is up to date. Inspecting raspi-config I have discovered it is not updated, but I prefer to configure the system via CLI (command line or console) interface rather than GUI one (no need to connect the RPi to a TV set which is in another room:-). So I decided not to upgrade the system but rather manually configure it. To achieve overclocking one has to add the following lines to /boot/config.txt file:

pi@raspberrystar ~ $ sudo vim /boot/config.txt
## add the following:
temp_limit=80
arm_freq=900
sdram_freq=500

Now reboot:

pi@raspberrystar ~ $ sudo reboot

Check the dmesg:

pi@raspberrystar ~ $ dmesg  | grep 7000
[    1.956412] bcm2835-cpufreq: min=700000 max=900000 cur=700000

CPU frequency is still 700Mhz. To increase it one has to edit scaling_governor file:

pi@raspberrystar ~ $ sudo bash
root@raspberrystar:/home/pi# 
echo "performance" > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
## check if the above works:-)
pi@raspberrystar ~ $ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
performance
# display available options:
pi@raspberrystar ~ $ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
conservative ondemand userspace powersave performance

The ondemand option allows for adjusting CPU frequency depending on CPU utilization.

Without any further reboot the new settings work:

# check current CPU frequency
pi@raspberrystar ~ $ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
900000

There is even a temperature sensor available so one can check if the CPU is not overheated:

# check processor temperature:
pi@raspberrystar ~ $ /opt/vc/bin/vcgencmd measure_temp
temp=46.5'C

To boot the system with scaling_governor set to appropriate value one has to edit /etc/rc.local:

pi@raspberrystar ~ $ sudo vim  /etc/rc.local
# Add the following line
# echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

I have performed a simple test:

#!/bin/bash
#
N=5
START=$(date +%s)

for ((i=1;i<=$N;i++ )) ; do

echo "**** Iteration $i ****"

STARTI=$(date +%s)
perl -e 'for ($i=0;$i<=10000000;$i++) { $s .= "xx"; }'
ENDI=$(date +%s) ; TOTALI=$(( $ENDI - $STARTI ))
echo "*** $TOTALI s."

done

END=$(date +%s)
TOTAL=$(( $END - $START ))
MEAN=`awk -v m=$TOTAL -v n=$N 'BEGIN { print m/n }'`

echo "total: " $TOTAL  "mean: " $MEAN
##

The test uses the following perl program:

perl -e 'for ($i=0;$i<=10000000;$i++) { $s .= "xx"; }'

Because computing time can vary, the program has to be run N times and the mean time is reported.

My Rpi runs 28--29s at 700 Mhz, 25,8s at 800 Mhz and 21s at 900 Mhz.

So running at 900 Mhz results in almost 30% reduction of computing time.