How to use boost to track real time instead of user+sys?

Question

I'm using boost::timer to time a section of my code. If I run the code with one thread:

$ time ./runfoo 1
Took 2.08s

real    0m2.086s
user    0m1.611s
sys     0m0.475s

2.08 is the output of t.elapsed().

Yet if I run the code with 4 threads:

$ time ./runfoo 4
Took 2.47s

real    0m1.022s
user    0m1.834s
sys     0m0.628s

It seems to be tracking user+sys time, not real time.

How do I make it track real time? I'm using Boost 1.46. To be more specific, the timer is in the main thread, which just calls a function that ends up using the multiple threads.

EDIT: The relevant source looks something like this:

boost::asio::io_service ios;
boost::thread_group pool; 
boost::asio::io_service::work work(ios);
pool.create_thread(boost::bind(&boost::asio::io_service::run, &ioService));
pool.create_thread(boost::bind(&boost::asio::io_service::run, &ioService));
pool.create_thread(boost::bind(&boost::asio::io_service::run, &ioService));
pool.create_thread(boost::bind(&boost::asio::io_service::run, &ioService));
{
    boost::timer t;

    function_which_posts_to_ios(ios);

    printf("Took %.2fs\n", t.elapsed();
}

As to what output I expect, I'd like the program in the 2nd run to print "Took 1.02s" instead of "Took 2.47s", as 1.02s was the actual amount of seconds that elapsed.

"Real" time, like the one returned by [`gettimeofday`](http://pubs.opengroup.org/onlinepubs/009695399/functions/gettimeofday.html)? — Some programmer dude, Apr 25 '14 at 22:43
Could you show the source to `runfoo`? Without it, we can't begin to assess what output you might expect — sehe, Apr 25 '14 at 22:48

score 2 · Accepted Answer · edited May 23 '17 at 10:26

2

It appears that you are using the deprecated Version 1 timers where the semantics of elapsed() was not consistent across platforms, wall-clock time on some and CPU time on others. In the Version 2 cpu_timer, the elapsed() method returns a struct which has distinct members for real, user, and system time.

If you cannot use the Version 2 API, you can use boost::posix_time instead to measure wall clock time. See c++ boost get current time in milliseconds.

edited May 23 '17 at 10:26

Community

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answered Apr 26 '14 at 01:01

rhashimoto

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Yea those don't exist for the version of boost I'm using. Looks like the question you linked has something I could use - thanks! – Claudiu Apr 26 '14 at 01:45

How to use boost to track real time instead of user+sys?

1 Answers1