Is there a way to disable CPU cache (L1/L2) on a Linux system?

Question

I am profiling some code on a Linux system (running on Intel Core i7 4500U) to obtain the time of ONLY the execution costs. The application is the demo mpeg2dec from libmpeg2. I am trying to obtain a probability distribution for the mpeg2 execution times. However we want to see the raw execution cost when cache is switched off.

Is there a way I can disable the cpu cache of my system via a Linux command, or via a gcc flag ? or even set the cpu (L1/L2) cache size to 0KB ? or even add some code changed to disable cache ? Of course, without modifying or rebuilding the kernel.

Answer 1

See this 2012 thread, someone posted a tiny kernel module source to disable cache through asm.

http://www.linuxquestions.org/questions/linux-kernel-70/disabling-cpu-caches-936077/

Answer 2

If disabling the cache is really necessary, then so be it.

Otherwise, to know how much time a process takes in terms of user or system "cycles", then I would recommend the getrusage() function.

struct rusage usage;
getrusage(RUSAGE_SELF, &usage);

You can call it before/after your loop/test and subtracted the values to get a good idea of how much time your process took, even if many other processes run in parallel on the same machine. The main problem you'd get is if your process start swapping. In that case your timings will be off.

double user_usage = usage.ru_utime.tv_sec + usage.ru_utime.tv_usec / 1000000.0;
double system_uage = usage.ru_stime.tv_sec + usage.ru_stime.tv_usec / 1000000.0;

This is really precise from my own experience. To increase precision, you could be root when running your test and give it a negative priority (-1 or -2 is enough.) Then it won't be swapped out until you call a function that may require it.

Of course, you still get the effect of the cache... assuming you do not handle very large amount of data with code that goes on and on (opposed to having a loop).