use valgrind to know time(in seconds) spent in each function
For machine instruction profiling use valgrind's callgrind (also, cachegrind can do cache and branch prediction profiling which is quite nice).
For time measurements use google's cpu profiler, it gives way better results than gprof. You can set sampling frequency and it can show the output as a nice annotated call graph.
Valgrind isn't suited for measuring time, as running an application in valgrind distorts the results (slowdown, CPU vs. I/O). Thus valgrind profiling tool callgrind doesn't measure time but CPU instructions. Callgrind is only useful if your bottleneck is CPU-bound (thus CPU instructions matter), then CPU instructions measured will be in proportion to the time spent. It's not useful if heavy I/O or multiple processes are involved. Then you should use a sampling profiler, like sysprof or gprof (Edit 2020: perf). That checks in intervals which function the process is in, with less distorted results.