Cpu Frequency Daemon is a very configurable program for throttling the cpu's frequency depending on "rules". A rule tests for battery state, cpu temperatur, active processes and so on.

For example, a rule which throttles your cpu when it's to hot might look like this:

[Rule]
name=CPU Too Hot
acpi_temperature=65-100
cpu_interval=50-100
profile=Performance Low
[/Rule]

And if you want full performance when watching movies you might have a rule like this:

[Rule]
name=Movie Watcher
programs=xine,mplayer,gmplayer
battery_interval=0-100
acpi_temperature=0-60
profile=Performance High
[/Rule]

(For more examples, have a look at the manpage)

As a Gentoo user I want full cpu power when emerge is running. But sadly, "programs=emerge" won't work because emerge' process actually is the python interpreter (and surely you don't want to define "programs=python" as it would fire on your processor for all python scripts):

$ ps a | grep emerge
13423 pts/2    SN+    0:02 /usr/bin/python -O /usr/bin/emerge libc

So what we need is a cpufreqd.conf-statement which executes a shell command and scores the rule depending on the process' exit code (0 is good and anything else is bad). There is already a plugin, "exec", which executes a command before and after testing a rule or switching the profile, although this is not exactly what we want. Nevertheless the code of this plugin (cpufreqd_exec.c) only needs a few changes to get a new options called "exec" which does exactly what we need (I've attached patches for cpufreqd-2.2.1 [1] and cpufreqd-2.3.3 [2]).

The rule for switching to "Performance High" when emerge is running now looks like this:

[Rule]
name=emerge is running
exec=ps -u root | grep emerge
battery_interval=0-100
acpi_temperature=0-65
cpu_interval=0-100
profile=Performance High
[/Rule]

(If somebody knows an more efficient method to check wether emerge is running or not, please post it into the comments).