[Boost-users] date_time microsec_clock::univeral_time() implementation

From: Brian Neal (bgneal_at_[hidden])
Date: 2006-06-20 10:47:42

We have an application that was using our own custom functions for
getting the OS time. We were basically representing time as the number
of milliseconds since the UNIX epoch in an unsigned long, or as the
number of nanoseconds since the epoch in a 64-bit type.

We later discovered boost, and cut everything over to
boost::date_time. This has worked out very, very well and has
uncovered several time related bugs in our old code. However, we do
have a small percentage of code that is time critical, and we have
noticed that calling microsec_clock::univeral_time() is 2.6 times
slower than our old function, and for this one bit of code that is
significant.

Looking at the implementation for UNIX like systems, I see that
create_time() in microsec_time_clock.hpp is calling gettimeofday()
followed by a call to gmtime() or localtime() as appropriate. I think
it's this second call that is the difference between our old code and
boost.

In our time critical chunk of code we are just time tagging events,
and we aren't concerned about timezone. I was thinking about adding a
function to our code that called clock_gettime() (which is what our
old code did), but then constructing a time_type out of that using the
UNIX epoch for the date part and the results of clock_gettime() for
the time_duration part, i.e, in pseudo code:

time_type our_get_time() {
clock_gettime(CLOCK_REALTIME, &timespec);
time_duration td = massage(timespec);
return time_type(unix_epoch, td);
}

This still won't be as fast as our old code, because we essentially
ignored the date part of the times and simply worked with durations
since the epoch. But I think the killer for us is the call to gmtime,
which the above avoids.

Comments? I'm curious why this approach wasn't taken in boost. Was it
the timezone factor?

I'm also curious why gettimeofday() was chosen over clock_gettime()?
Is one more available than the other?

Thanks.