> Monotonic > --------- > > This is a particularly tricky term, as there are several subtly > incompatible definitions in use.
Is it a definition for the glossary? > C++ followed the mathematical > definition, so that a monotonic clock only promises not to go > backwards. The "C++ Timeout Specification" doesn't have any monotonic anymore. It has a steady_clock, but it's something different. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3128.html#time.clock.monotonic > In practice, that is not sufficient to be useful, and no > Operating System provides such a weak guarantee. Most discussions > of a "Monotonic *Clock*" will also assume several additional > guarantees, some of which are explicitly required by the POSIX > specification. What do you mean for POSIX? The definition of CLOCK_MONOTONIC by the POSIX specification is: "The identifier for the system-wide monotonic clock, which is defined as a clock whose value cannot be set via clock_settime() and which cannot have backward clock jumps. The maximum possible clock jump shall be implementation-defined." http://pubs.opengroup.org/onlinepubs/000095399/basedefs/time.h.html time.monotonic() of the PEP 418 gives the same guarantee (cannot go backward, cannot be set), except for "system-wide" (Python cannot give this guarantee because of Windows older than Vista). > The tradeoffs > often include lack of a defined Epoch_ or mapping to `Civil Time`_, I don't know any monotonic with a defined epoch or mappable to the civil time. > and being more expensive (in `Latency`_, power usage, or duration spent > within calls to the clock itself) to use. CLOCK_MONOTONIC and CLOCK_REALTIME have the same performances on Linux and FreeBSD. Why would a monotonic clock be more expensive? > For example, the clock may > represent (a constant multiplied by) ticks of a specific quartz timer > on a specific CPU core, and calls would therefore require > synchronization between cores. I don't think that synchronizing a counter between CPU cores is something expensive. See the following tables for details: http://www.python.org/dev/peps/pep-0418/#performance CLOCK_MONOTONIC and CLOCK_REALTIME use the same hardware clocksource and so have the same latency depending on the hardware. Victor _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
