Toon Knapen <toon.knapen@fft.be> writes:
The documentation of boost::timer states that 'The maximum measurable elapsed time may be as low as 596.5 hours (or even less) ...'.
Well on many 32bit platforms this would probably even be about 71 hours. Apparantly POSIX requires CLOCKS_PER_SEC to be 10e6, so if clock_t is 32 bit, the maximum of ( clock() / CLOCKS_PER_SEC ) is 4294 seconds.
Because accuracy for clock() is generally about 1 sec, using clock() is not very usefull for doing timings of short (in time) events.
In my experience, clock() is significantly more accurate than 1 second.
Even more, for short events, the boost::timer might also influence the timing too much.
I believe that boost::timer is very light weight. This: { boost::timer t; /* ... */ double elapsed = t.elapsed(); } Should be equivalent (in efficiency) to: { std::clock_t initial = clock(); /* ... */ double elapsed = static_cast<double>(clock() - initial) / CLOCKS_PER_SEC; }
[snip: clock() useful only for longer timings]
But for long events the clock_t wraps around too fast (as mentioned above). And if you're unlucky that you _start_time = clock() is or a bit smaller than std::numeric_limits< clock_t >::max() (thus near the wrap-around point), directly afterwards the expression 'clock() - start' will result in a negative value.
This can be remedied by storing the time internally using a 64-bit integer, and then providing a method which the user must call once per wraparound period in order to achieve accurate results.
So I would like to suggest for boost::timer to use std::time_t and use std::time(NULL) instead of clock() and use std::difftime to calculate the elapsed time.
clock() serves as a process CPU-time timer, while time() depends on the system time, and is also affected by changes to the system clock. (Also, time() provides only second-precision.) There was talk of an additional wallclock_timer being added, however; to be useful, such a facility would ideally be based on a more precise time source than time(), such as gettimeofday. It appears that the date/time library includes the partially documented (only through doxygen) facility microsec_clock, which may or may not be functional on non-POSIX platforms. -- Jeremy Maitin-Shepard