extending the std::chrono functionality to deal with run-time (non compile-time) constant periods

Question

I have been experimenting with all kind of timers on Linux and OSX, and would like to try and wrap some of them with the same interface used by std::chrono.

That's easy to do for timers that have a well-defined "period" at compile time, e.g. the POSIX clock_gettime() familiy, the clock_get_time() family on OSX, or gettimeofday().

However, there are some useful timers for which the "period" - while constant - is only known at runtime. For example: - POSIX states the period of clock(), CLOCKS_PER_SEC, may be a variable on non-XSI systems - on Linux, the period of times() is given at runtime by sysconf(_SC_CLK_TCK) - on OSX, the period of mach_absolute_time() is given at runtime by mach_timebase_info() - on recent Intel processors, the DST register ticks at a constant rate, but of course that can only be determined at runtime

To wrap these timers in the std::chrono interface, one possibility would be to use a period of std::chrono::nanosecond , and convert the value of each timer to nanoseconds. An other approach could be to use a floating point representation. However, both approaches would introduce a (very small) overhead to the now() function, and a (probably small) loss in precision.

The solution I'm trying to pursue is to define a set of classes to represent such "run-time constant" periods, built along the same lines as the std::ratio class. However I expect that will require rewriting all the related template classes and functions (as they assume constexpr values).

How do I wrap these kind of timers a la std:chrono ?

Or use non-constexpr values for the time period of a clock ?

Answer 1

Does anyone have any experience with wrapping these kind of timers a la std:chrono ?

Actually I do. And on OSX, one of your platforms of interest. :-)

You mention:

on OSX, the period of mach_absolute_time() is given at runtime by mach_timebase_info()

Absolutely correct. Also on OSX, the libc++ implementation of high_resolution_clock and steady_clock is actually based on mach_absolute_time. I'm the author of this code, which is open source with a generous license (do anything you want with it as long as you retain the copyright).

Here is the source for libc++'s steady_clock::now(). It is built pretty much the way you surmised. The run time period is converted to nanoseconds prior to returning. On OS X the conversion factor is very often 1, and the code takes advantage of that fact with an optimization. However the code is general enough to handle non-1 conversion factors.

On the first call to now() there's a small cost of querying the run time conversion factor to nanoseconds. In the general case a floating point conversion factor is computed. In the common case (conversion factor == 1) the subsequent cost is calling through a function pointer. I've found that the overhead is really quite reasonable.

On OS X the conversion factor, although not determined until run time, is still a constant (i.e. does not vary as the program executes), so it only needs to be computed once.

If you're in a situation where your period is actually varying dynamically, you'll need more infrastructure to handle this. Essentially you would need to integrate (calculus) the period vs time curve and then compute an average period between two points in time. That would require a constant monitoring of the period as it changes with time, and <chrono> isn't the right tool for that. Such tools are typically handled at the OS level.