Re: [Boost-bugs] [Boost C++ Libraries] #11806: microsec_clock::universal_time() is needlessly slow

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #11806: microsec_clock::universal_time() is needlessly slow
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2018-01-06 15:31:19

#11806: microsec_clock::universal_time() is needlessly slow
-------------------------------+--------------------------------
  Reporter:  jzwinck@…         |      Owner:  James E. King, III
      Type:  Bugs              |     Status:  closed
 Milestone:  To Be Determined  |  Component:  date_time
   Version:  Boost 1.55.0      |   Severity:  Optimization
Resolution:  wontfix           |   Keywords:
-------------------------------+--------------------------------
Changes (by James E. King, III):

 * status:  assigned => closed
 * resolution:   => wontfix

Comment:

 I benchmarked calling the current implementation (boost 1.66.0) in release
 mode on linux (ubuntu artful) 1M times and each call takes less than
 100ns:

 {{{
 test:
   // benchmark for trac 11806
   {
     check("calling microsec_clock::universal_time 1000000 times", true);
     boost::timer::auto_cpu_timer t;
     for (size_t i = 0; i < 1000000; ++i) {
       ptime x = microsec_clock::universal_time();
     }
     return 0;
   }

 results:
 jking_at_ubuntu:~/boost/libs/date_time/test$
 ../../../bin.v2/libs/date_time/test/testmicrosec_time_clock.test/gcc-
 gnu-7/release/threadapi-pthread/testmicrosec_time_clock
 Pass :: calling microsec_clock::universal_time 1000000 times
  0.088597s wall, 0.090000s user + 0.000000s system = 0.090000s CPU
 (101.6%)
 jking_at_ubuntu:~/boost/libs/date_time/test$
 ../../../bin.v2/libs/date_time/test/testmicrosec_time_clock.test/gcc-
 gnu-7/release/threadapi-pthread/testmicrosec_time_clock
 Pass :: calling microsec_clock::universal_time 1000000 times
  0.103956s wall, 0.100000s user + 0.000000s system = 0.100000s CPU (96.2%)
 jking_at_ubuntu:~/boost/libs/date_time/test$
 ../../../bin.v2/libs/date_time/test/testmicrosec_time_clock.test/gcc-
 gnu-7/release/threadapi-pthread/testmicrosec_time_clock
 Pass :: calling microsec_clock::universal_time 1000000 times
  0.096352s wall, 0.090000s user + 0.000000s system = 0.090000s CPU (93.4%)
 }}}

 I then changed the code such that if the clock source is universal time
 (i.e. gmtime_r) passed into create_time, we take (what I thought would be
 a) shortcut:

 {{{
 #if defined(BOOST_HAS_GETTIMEOFDAY)
       timeval tv;
       if (-1 == gettimeofday(&tv, 0)) { //gettimeofday does not support TZ
 adjust on Linux.
         boost::throw_exception(std::runtime_error("gettimeofday failed"));
       }
       if (universal) {
         // if the source is UTC we can take a fast shortcut here instead
         // of calling gmtime_r, which is slow (see Boost Trac 11806)
         static date_type posix_epoch(1970, 1, 1);
         static uint64_t res = resolution_traits_type::res_adjust();
         return time_type(date_type(1970, 1, 1),
                          time_duration_type(0, 0,
                            tv.tv_sec, (static_cast<uint64_t>(tv.tv_usec) *
 res) / 1000000ull));
       }
       std::time_t t = tv.tv_sec;
       boost::uint32_t sub_sec = tv.tv_usec;
 #elif defined(BOOST_HAS_FTIME)
 }}}

 This slowed down the program so much I didn't even let it finish.  Asking
 for a ptime based on a large number of seconds is incredibly inefficient.
 So I am resolving this as wontfix, unless you have a better suggestion for
 implementing it.

-- 
Ticket URL: <https://svn.boost.org/trac10/boost/ticket/11806#comment:2>
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