[Python-checkins] peps: PEP 418: Integrate Jim Jewett's Glossary
victor.stinner
python-checkins at python.org
Thu Apr 12 01:07:04 CEST 2012
http://hg.python.org/peps/rev/23127be44be3
changeset: 4226:23127be44be3
user: Victor Stinner <victor.stinner at gmail.com>
date: Thu Apr 12 01:06:31 2012 +0200
summary:
PEP 418: Integrate Jim Jewett's Glossary
files:
pep-0418.txt | 121 ++++++++++++++++++++++++++++++++++++++-
1 files changed, 118 insertions(+), 3 deletions(-)
diff --git a/pep-0418.txt b/pep-0418.txt
--- a/pep-0418.txt
+++ b/pep-0418.txt
@@ -2,7 +2,7 @@
Title: Add a monotonic time functions
Version: $Revision$
Last-Modified: $Date$
-Author: Victor Stinner <victor.stinner at gmail.com>
+Author: Jim Jewett <jimjjewett at gmail.com>, Victor Stinner <victor.stinner at gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
@@ -247,8 +247,123 @@
daemon)
-Definitions
-===========
+Glossary
+========
+
+:Accuracy:
+ Is the answer correct? Any clock will eventually <drift>; if a clock is
+ intended to match <Civil Time>, it will need to be <adjusted> back to the
+ "true" time.
+
+:Adjusted:
+ Resetting a clock to the correct time. This may be done either with a
+ <Step> or by <Slewing>.
+
+:Civil Time:
+ Time of day; external to the system. 10:45:13am is a Civil time; 45 seconds
+ is not. Provided by existing function time.localtime() and time.gmtime().
+ Not changed by this PEP.
+
+:Clock:
+ An instrument for measuring time. Different clocks have different
+ characteristics; for example, a clock with <nanonsecond> <precision> may
+ start to <drift> after a few minutes, while a less precise clock remained
+ accurate for days. This PEP is primarily concerned with clocks which use a
+ unit of seconds.
+
+:Counter:
+ A clock which increments each time a certain event occurs. A counter is
+ <strictly monotonic>, but not <clock_monotonic>. It can be used to generate
+ a unique (and ordered) timestamp, but these timestamps cannot be mapped to
+ <civil time>; tick creation may well be bursty, with several advances in the
+ same millisecond followed by several days without any advance.
+
+:CPU Time:
+ A measure of how much CPU effort has been spent on a certain task. CPU
+ seconds are often normalized (so that a variable number can occur in the
+ same actual second). CPU seconds can be important when profiling, but they
+ do not map directly to user response time, nor are they directly comparable
+ to (real time) seconds.
+
+:Duration:
+ Elapsed time. The difference between the starting and ending times. A
+ defined <epoch> creates an implicit (and usually large) duration. More
+ precision can generally be provided for a relatively small <duration>.
+
+:Drift:
+ The accumulated error against "true" time, as defined externally to the
+ system.
+
+:Epoch:
+ The reference point of a clock. For clocks providing <civil time>, this is
+ often midnight as the day (and year) rolled over to January 1, 1970. For a
+ <clock_monotonic> clock, the epoch may be undefined (represented as None).
+
+:Latency:
+ Delay. By the time a clock call returns, the <real time> has advanced,
+ possibly by more than the precision of the clock.
+
+:Monotonic:
+ The characteristics expected of a monotonic clock in practice. Moving in at
+ most one direction; for clocks, that direction is forward. The <clock>
+ should also be <steady>, and should be convertible to a unit of seconds.
+ The tradeoffs often include lack of a defined <epoch> or mapping to <Civil
+ Time>, and being more expensive (in <latency>, power usage, or <duration>
+ spent within calls to the clock itself) to use. 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.
+
+:Precision:
+ Significant Digits. What is the smallest duration that the clock can
+ distinguish? This differs from <resolution> in that a difference greater
+ than the minimum precision is actually meaningful.
+
+:Process Time:
+ Time elapsed since the process began. It is typically measured in <CPU
+ time> rather than <real time>, and typically does not advance while the
+ process is suspended.
+
+:Real Time:
+ Time in the real world. This differs from <Civil time> in that it is not
+ <adjusted>, but they should otherwise advance in lockstep. It is not
+ related to the "real time" of "Real Time [Operating] Systems". It is
+ sometimes called "wall clock time" to avoid that ambiguity; unfortunately,
+ that introduces different ambiguities.
+
+:Resolution:
+ Represented Digits. Note that many clocks will have a resolution greater
+ than their actual <precision>.
+
+:Slew:
+ A slight change to a clock's speed, usually intended to correct <drift> with
+ respect to an external authority.
+
+:Stability:
+ Persistence of accuracy. A measure of expected <drift>.
+
+:Steady:
+ A clock with high <stability> and relatively high <accuracy> and
+ <precision>. In practice, it is often used to indicate a <clock_monotonic>
+ clock, but places greater emphasis on the consistency of the duration
+ between subsequent ticks.
+
+:Step:
+ An instantaneous change in the represented time. Instead of speeding or
+ slowing the clock (<slew>), a single offset is permanently added.
+
+:System Time:
+ Time as represented by the Operating System.
+
+:Thread Time:
+ Time elapsed since the thread began. It is typically measured in <CPU time>
+ rather than <real time>, and typically does not advance while the thread is
+ idle.
+
+:Wallclock:
+ What the clock on the wall says. This is typically used as a synonym for
+ <real time>; unfortunately, wall time is itself ambiguous.
+
Resolution
----------
--
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