Re: [boost] [timer] Boost Timer Library Version 2

Subject: Re: [boost] [timer] Boost Timer Library Version 2
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-09-28 08:38:33

Vicente J. Botet Escriba wrote:
>
> * Why does stop returns cpu-times? Why by reference? Why
> elapsed is not returned by reference?
>
> cpu_times         elapsed() const noexcept;
>
> const cpu_times&   stop() noexcept;

The interface is, of course, directed by the current
implementation.  Still, it would be appropriate for stop() to
return by value to avoid forcing the implementation to store a
cpu_times instance in the timer.

> * I would associate the operations pairwise start/stop and
> suspend/resume. I guess it would be easier to explain the
> semantics.

Yes.  Of course.  I knew I didn't quite like the current
semantics, but it just didn't click.  Another advantage of
start/stop and suspend/resume is that RAII classes can be used to
control the timer.

Here's the new interface:

   void      start();
   void      stop();
   bool      is_stopped();
   void      suspend();
   void      resume();
   bool      is_suspended();
   cpu_times elapsed();

start() notes the current time as the start time and marks the
timer as not stopped, if is_stopped(), and marks the timer as not
suspended.

stop() notes the current time as the end time and marks the timer
as stopped, if !is_stopped(), and marks the timer as not
suspended.

suspend() notes the current time as the suspension time and marks
the object as being suspended, if !is_stopped() &&
!is_suspended().

resume() calls start() and arranges for a future call to
elapsed() to include the time elapsed until suspend() was called
plus the time that elapses subsequently, if is_suspended().

Reasonable?  I know that means that both start and end times must
be stored with this interface (two cpu_times instances, for
example), but that seems a worthwhile price.

Here's a use case addressed by these ideas:

   nanosecond_type status_time(0);
   cpu_timer timer;
   for (int i(0); i < 100; ++i)
   {
      task_to_time();
      scoped_suspender _(timer);
      nanosecond_type const current_time(timer.elapsed().user);
      if (1000000000 < current_time - status_time)
      {
         report(timer);
         status_time = current_time;
      }
   }

scoped_suspender is an RAII class that calls suspend() and
resume().  Thus, the overhead of checking the elapsed time and
writing output is not included in the time tracked by the timer.
This approach permits doing non-timed work of various sorts in
the midst of timed work.

scoped_suspender might be a class template with a generator:

   auto _(make_scoped_suspender(timer));

That would allow supporting any timer type that provides
suspend() and resume().

I suspect that this use case would be pretty common, so it could
be captured:

   timer_reporter report(1000000000); // once per second
   cpu_timer timer;
   for (int i(0); i < 100; ++i)
   {
      task_to_time();
      report(timer);
   }

   void
   timer_reporter::operator ()(cpu_timer & _timer)
   {
      if (_timer.is_stopped())
      {
         return;
      }
      scoped_suspender _(_timer);
      nanosecond_type const current(_timer.elapsed().user);
      if (delay_ < current - last_)
      {
         std::string const formatted(
            _timer.format(precision_, format_));
         os_ << formatted;
         last_ = current;
      }
   }

Obviously, I'm suggesting that timer_reporter is somewhat similar
to auto_cpu_timer in that it would save a std::ostream & for
output, as well as the precision and a format string for calling
cpu_timer::format().

I purposely did not supply the timer as a constructor argument
because that would require construction after a timer already
started or else stopping the timer, constructing the
timer_reporter, and then starting the timer.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer                     using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP  http://www.sig.com

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