How can I limit the processing power given to a specific program? - limit

Question

I develop on a laptop with a dual-core amd 1.8 GHz processor but people frequently run my programs on much weaker systems (300 MHz ARM for example).
I would like to simulate such weak environments on my laptop so I can observe how my program runs. It is an interactive application.
I looked at qemu and I know how to set up an environment but its a bit painful and I didn't see the exact incantation of switches I would need to make qemu simulate a weaker cpu.
I have virtualbox but it doesn't seem like I can virtualize fewer than 1 full host cpu.
I know about http://cpulimit.sourceforge.net/ which uses sigstop and sigcont to try to limit the cpu given to a process but I am worried this is not really an accurate portrayal of a weaker cpu.
Any ideas?

Answer 1

If your CPU is 1800 MHz and your target is 300 MHz, and your code is like this:
while(1) { /*...*/ }
you can rewrite it like:
long last=gettimestamp();
while(1)
{
long curr=gettimestamp();
if(curr-last>1000) // out of every second...
{
long target=curr+833; // ...waste 5/6 of it
while(gettimestamp()<target);
last=target;
}
// your original code
}
where gettimestamp() is your OS's high frequency timer.
You can choose to work with smaller values for a smoother experience, say 83ms out of every 100ms, or 8ms out of every 10ms, and so on. The lower you go though the more precision loss will ruin your math.
edit: Or how about this? Create a second process that starts the first and attaches itself as a debugger to it, then periodically pauses it and resumes it according to the algorithm above.

Answer 2

You may want to look at an emulator that is built for this. For example, from Microsoft you can find this tech note, http://www.nsbasic.com/ce/info/technotes/TN23.htm.
Without knowing more about the languages you are using, and platforms, it is hard to be more specific, but I would trust the emulator programs to do a good job in providing the test environment.

Answer 3

I've picked a PIIMMX-266 laptop somewhere, and installed a mininal Debian on it. That was a perfect solution until it has died some weeks ago. It is a Panasonic model, which has a non-standard IDE connector (it's not 40-pin, nor 44-pin), so I was unable to replace its HDD with a CF (a CF-to-IDE adapter costs near zero). Also, the price of such a machine is USD 50 / EUR 40.
(I was using it to simulate a slow ARM-based machine for our home aut. system, which is planned to able to run on even smallest-slowest Linux systems. Meanwhile, we've choosen a small and slow computer for home aut. purposes: GuruPlug. It has a cca. 1.2 GByte fast CPU.)
(I'm not familiar with QEMU, but the manual says that you can use KVM (kernel virtualization) in order to run programs at native speed; I assume that if it's an extra feature then it can be turned off, so, strange but true, it can emulate x86 on x86.)