[Boost-bugs] [Boost C++ Libraries] #7719: C++11 steady_clock is not monotonic on Win32 & OSX

Subject: [Boost-bugs] [Boost C++ Libraries] #7719: C++11 steady_clock is not monotonic on Win32 & OSX
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-11-21 16:26:07

#7719: C++11 steady_clock is not monotonic on Win32 & OSX
-------------------------------+--------------------------------------------
 Reporter:  fnjordy@…          |       Owner:  viboes 
     Type:  Bugs               |      Status:  new    
Milestone:  To Be Determined   |   Component:  chrono 
  Version:  Boost 1.52.0       |    Severity:  Problem
 Keywords:                     |  
-------------------------------+--------------------------------------------
 Boost 1.52 defers to different OS specific calls for implementation of the
 std::chrono::steady_clock monotonic interface.  There are supposed to be
 two characteristics: 1)  monotonic, 2) independent of wall time.  The
 current Boost implementation only achieves the latter.

 Issue raised from Stackoverflow:  stackoverflow.com/q/13478093/175849

 Per platform discussion:

 '''POSIX''':  defers to `clock_gettime (CLOCK_MONOTONIC)`.  Generally
 sufficient excepting peculiarities of hyper-threading.


 '''OSX''':  defers to `mach_absolute_time()` which is implemented by
 `RDTSC` which is only monotonic on the same core.

 Apple recommend per their documentation to use
 `clock_get_time(SYSTEM_CLOCK)` as a monotonic source.  References:

 www.opensource.apple.com/source/xnu/xnu-1486.2.11/osfmk/man/clock_get_time.html

 stackoverflow.com/q/11680461/175849

 An important caveat is that the clock is not monotonic if
 `clock_set_time()` is called.


 '''Win32''':  defers to `QueryPerformanceCounter()` which on many
 platforms is not monotonic due to dependency on `RDTSC` as a direct source
 or interpolation of times from more reliable sources such as HPET, RTC, or
 PC AT.

 Microsoft in MSVC2012 implement `std::chrono` using `_Xtime_get_ticks()`
 which defers to `GetSystemTimeAsFileTime()` as discussed here (in
 Japanese):

 gist.github.com/3763854

 This however is wall time and subject to clock changes, a more suitable
 source would be `GetTick64Count()` which requires Vista+, the alternative
 `GetTickCount()` wraps after 49.7 days.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7719>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.