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| <h3 class="section">21.1 Time Basics</h3> |
| |
| <p><a name="index-time-2588"></a> |
| Discussing time in a technical manual can be difficult because the word |
| “time” in English refers to lots of different things. In this manual, |
| we use a rigorous terminology to avoid confusion, and the only thing we |
| use the simple word “time” for is to talk about the abstract concept. |
| |
| <p>A <dfn>calendar time</dfn> is a point in the time continuum, for example |
| November 4, 1990 at 18:02.5 UTC. Sometimes this is called “absolute |
| time”. |
| <a name="index-calendar-time-2589"></a> |
| We don't speak of a “date”, because that is inherent in a calendar |
| time. |
| <a name="index-date-2590"></a> |
| An <dfn>interval</dfn> is a contiguous part of the time continuum between two |
| calendar times, for example the hour between 9:00 and 10:00 on July 4, |
| 1980. |
| <a name="index-interval-2591"></a> |
| An <dfn>elapsed time</dfn> is the length of an interval, for example, 35 |
| minutes. People sometimes sloppily use the word “interval” to refer |
| to the elapsed time of some interval. |
| <a name="index-elapsed-time-2592"></a><a name="index-time_002c-elapsed-2593"></a> |
| An <dfn>amount of time</dfn> is a sum of elapsed times, which need not be of |
| any specific intervals. For example, the amount of time it takes to |
| read a book might be 9 hours, independently of when and in how many |
| sittings it is read. |
| |
| <p>A <dfn>period</dfn> is the elapsed time of an interval between two events, |
| especially when they are part of a sequence of regularly repeating |
| events. |
| <a name="index-period-of-time-2594"></a> |
| <dfn>CPU time</dfn> is like calendar time, except that it is based on the |
| subset of the time continuum when a particular process is actively |
| using a CPU. CPU time is, therefore, relative to a process. |
| <a name="index-CPU-time-2595"></a> |
| <dfn>Processor time</dfn> is an amount of time that a CPU is in use. In |
| fact, it's a basic system resource, since there's a limit to how much |
| can exist in any given interval (that limit is the elapsed time of the |
| interval times the number of CPUs in the processor). People often call |
| this CPU time, but we reserve the latter term in this manual for the |
| definition above. |
| <a name="index-processor-time-2596"></a> |
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