From: Beman Dawes (bdawes_at_[hidden])
Date: 2001-10-31 17:07:01
At 10:54 AM 10/31/2001, Toon Knapen wrote:
>I need to be able to toggle the timer on and off. Since I want to
>calculate the total time spend in some function I need to toggle the
>timer off everytime the program leaves the function and back on
>everytime the program enters the function again.
>So I propose to add a member called `toggle()`.
>A drawback though is that you need to `toggle` the timer back on before
>asking the `elapsed()` time (otherwise I would need a bit to indicate if
>the timer is on or off which increases the memory footprint. Maybe I
>could use the MSB of the clock_t as a hack)
I sympathize with the need to measure total time in a function, but I
wonder if toggle() is the way to do it.
Problems I see:
1) User needs to remember to toggle() the time off initially so that the
sequence of toggles is correct, and then toggle() it back on before the
call to elapsed().
2) On many systems, either the timer precision is so low or the time
latency is so high that it is impossible to use to measure the time spent
in a single function invocation, unless the function is a major time
burner. The only reliable way is to invoke the function a very large
number of times, and then divide the elapsed time by the number of
invocations. This problem isn't unique to boost::timer, and is probably
getting much worse a clock speeds increase. How many modern platforms can
actually deliver timings with high enough precision and low enough latency
to accurately measure a single invocation of a smallish function?
I'm inclined to think the solution may not be to try to fix timer, but
to specify a new timing library with much more precise semantics.
If Jeff Garland is reading this, I'd be curious if his date and time
classes would solve the problem. I know they can handle very large
granularity (100 year, for example, used in measuring geological time), but
I don't remember if they can handle very small time periods, and what they
do if the hardware timers can't deliver the precision desired.
>In attachment you can find my proposal implementation (Oh, I also
>aligned with the coding guidelines AFAIK : trailing underscore for
>members, ... )
Always hard to tell exactly what changed when the formatting is also
changed. I'm assuming the toggle() code was the only substantive change.
Thanks,
--Beman