Linux C++: how to profile time wasted due to cache misses?

Question

I know that I can use gprof to benchmark my code.

However, I have this problem -- I have a smart pointer that has an extra level of indirection (think of it as a proxy object).

As a result, I have this extra layer that effects pretty much all functions, and screws with caching.

Is there a way to measure the time my CPU wastes due to cache misses?

Thanks!

Answer 1

You could find a tool that accesses the CPU performance counters. There is probably a register in each core that counts L1, L2, etc misses. Alternately Cachegrind performs a cycle-by-cycle simulation.

However, I don't think that would be insightful. Your proxy objects are presumably modified by their own methods. A conventional profiler will tell you how much time those methods are taking. No profile tool would tell you how performance would improve without that source of cache pollution. That's a matter of reducing the size and structure of the program's working set, which isn't easy to extrapolate.

A quick Google search turned up boost::intrusive_ptr which might interest you. It doesn't appear to support something like weak_ptr, but converting your program might be trivial, and then you would know for sure the cost of the non-intrusive ref counts.

Answer 2

My advice would be to use PTU (Performance Tuning Utility) from Intel.

This utility is the direct descendant of VTune and provide the best available sampling profiler available. You'll be able to track where the CPU is spending or wasting time (with the help of the available hardware events), and this with no slowdown of your application or perturbation of the profile. And of course you'll be able to gather all cache line misses events you are looking for.

Answer 3

You could try cachegrind and it's front-end kcachegrind.

Answer 4

It depends on what OS and CPU you are using. E.g. for Mac OS X and x86 or ppc, Shark will do cache miss profiling. Ditto for Zoom on Linux.

Answer 5

If you're running an AMD processor, you can get CodeAnalyst, apparently free as in beer.

http://developer.amd.com/cpu/codeanalyst/codeanalystlinux/Pages/default.aspx

Answer 6

Another tool for CPU performance counter-based profiling is oprofile. You can view its results using kcachegrind.

Answer 7

Here's kind of a general answer.

For example, if your program is spending, say, 50% of it's time in cache misses, then 50% of the time when you pause it the program counter will be at the exact locations where it is waiting for the memory fetches that are causing the cache misses.