id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
7719	C++11 steady_clock is not monotonic on Win32 & OSX	fnjordy@…	viboes	"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."	Bugs	closed		chrono	Boost 1.52.0	Problem	invalid