Using qprof

qprof is a tool for quick profiling of executables. It can give you an idea of where the program is spending most of its time without recompiling the program. It works by hooking a few functions using LD_PRELOAD, and installing a SIGVTALRM signal handler to sample the program counter.

Notes for using it with GHC

Installing qprof on most Linux distributions should be easy; e.g. on Debian-based systems use sudo apt-get install qprof.

Profiling a program is straightforward:

qprof ./prog <flags>

The profile will be by basic-block, so you'll need some combination of -ddump-simpl -ddump-stg -ddump-cmm to map back to the Haskell code.

I got some strange results when using the non-threaded RTS, such as the handle_tick function being high up the profile. Presumably because we also use SIGVTALRM. Using the threaded RTS or +RTS -V0 should help here.

I found the resolution of the samples to be way too low. You can increase the frequency of samples with qprof's -i option, but I wasn't able to increase it beyond about 1ms (the default is 10ms). Nevertheless, this is a good way to identify the inner loop quickly. It's not a good way to evaluate small optimisations (for that, PAPI or oprofile would be better).