Measuring CPU clocks consumed by a process

Question

Hi, I have written a program in C. Its a program created as result of a research. I want to compute exact CPU cycles which program consumes. Exact number of cycles. Any idea how can I find that?

Answer 1

No you can't. The concept of a 'CPU cycle' is not well defined. Modern chips can run at multiple clock rates, and different parts of them can be doing different things at different times.

The question of 'how many total pipeline steps' might in some cases be meaningful, but there is not likely to be a way to get it.

Answer 2

Sorry, but no, at least not for most practical purposes -- it's simply not possible with most normal OSes. Just for example, quite a few OSes don't do a full context switch to handle an interrupt, so the time spent servicing a interrupt can and often will appear to be time spent in whatever process was executing when the interrupt occurred.

The "not for practical purposes" would indicate the possibility of running your program under a cycle accurate simulator. These are available, but mostly for CPUs used primarily in real-time embedded systems, NOT for anything like a full-blown PC. Worse, they (generally) aren't for running anything like a full-blown OS, but for code that runs on the "bare metal."

In theory, you might be able to do something with a virtual machine running something like Windows or Linux -- but I don't know of any existing virtual machine that attempts to, and it would be decidedly non-trivial and probably have pretty serious consequences in performance as well (to put it mildly).

Answer 3

The valgrind tool cachegrind (valgrind --tool=cachegrind) will give you a detailed output including the number of instructions executed, cache misses and branch prediction misses. These can be accounted down to individual lines of assembler, so in principle (with knowledge of your exact architecture) you could derive precise cycle counts from this output.

Know that it will change from execution to execution, due to cache effects.

The documentation for the cachegrind tool is here.

Answer 4

Try OProfile. It use various hardware counters on the CPU to measure the number of instructions executed and how many cycles have passed. You can see an example of it's use in the article, Memory part 7: Memory performance tools.

Answer 5

I am not entirely sure that I know exactly what you're trying to do, but what can be done on modern x86 processors is to read the time stamp counter (TSC) before and after the block of code you're interested in. On the assembly level, this is done using the RDTSC instruction, which gives you the value of the TSC in the edx:eax register pair.

Note however that there are certain caveats to this approach, e.g. if your process starts out on CPU0 and ends up on CPU1, the result you get from RDTSC will refer to the specific processor core that executed the instruction and hence may not be comparable. (There's also the lack of instruction serialisation with RDTSC, but in this context here, I don't think that's so much of an issue.)