Last updated 2004-06-28 by Roedy
Green ©1996-2004 Canadian Mind Products
Last updated 2004-06-28 by Roedy
Green ©1996-2004 Canadian Mind Products
Java definitions: 0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
You are here : home : Java Glossary : T words : time.
You may be interested in performance monitoring. You want to know how much CPU time your program used in various sections as opposed to elapsed time. For this you properly need help from the operating system or a profiler. The CPU is always being used by other tasks, the operating system, or even the system idle loop when you are not using it. You need OS help to avoid counting that time too. In a pinch, you have to add code that marks any time you start/return a call to the OS, so that you can discount that time. That still is not accurate because other tasks will interrupt you at any time, and because that scheme would not count the cpu time spent by the OS on your behalf.
The accuracy of System.currentTimeMillis() varies with platform. It is only nominally accurate to the millisecond.
| 55 ms | Windows 9X |
| 10 ms | Windows NT, Windows 2000, Windows XP single processor |
| 15.625 ms | Windows NT, Windows 2000, Windows XP dual processor |
In addition the Thread.sleep method will achieve delays of 1 ms on a single processor and 2 ms minimum on a dual processor.
In the Pentium there are RDTSC instructions to read special timestamp performance monitoring register which are more accurate than time of day clocks. RDTSC reads the cycle counter into EDX EAX. Some operating systems may make this a privileged instruction. See Intel's documentation for more details. You could access them via a bit of JNI. If you do that, make sure you discount the overhead of the JNI calls themselves.
If you want a task to be run at some time in the future or repeatedly at regular intervals, you can schedule it with the java.util.Timer class. See also javax.swing.Timer.
You can also use the JVMPI (Java Virtual Machine Profiler Interface) for accurate timing.
Imagine trying to explain this to a space faring species that used but a single unit of measure for time.
| Unit Of Measure | Definition |
|---|---|
| millenium | 1000 years, 10 centuries. |
| century | 100 years. |
| leap year | the approximate time for the earth to revolve around the sun. 366 days. |
| year | the approximate time for the earth to revolve around the sun. Sometimes 365 and sometimes 366 days. |
| season | approximately 3 months, 1/4 year. Interval between solstice and equinox. |
| quarter | approximately 3 months, 1/4 year. Business term for an arbitrary accounting period. |
| month | 28 to 31 days. |
| fortnight | 2 weeks, 7 days. |
| week | 7 days. |
| day | the approximate time for the earth to rotate on its axis. 24 hours. |
| hour | 60 minutes, 1/24 of a day. |
| minute | 60 seconds, 1/60 hour, 1/1440 day. |
| second | 1000 milliseconds, 1/60 minute, 1/3600 hour, 1/86,400 day. |
| millisecond | 1000 microseconds, 10-3 seconds, 1/1000 second, 1/3,600,000 hour, 1/86,400,000 day. |
| microsecond | 1000 nanoseconds, 10-6 seconds, 1/1,000,000 second, 1/3,600,000,000 hour, 1/86,400,000,000 day. |
| nanosecond | 1000 picoseconds, 10-9 seconds, 1/1,000,000,000 second, 1/3,600,000,000,000 hour, 1/86,400,000,000,000 day. |
| picosecond | 1000 femtoseconds, 10-12 seconds, 1/1,000,000,000,000 second, 1/3,600,000,000,000,000 hour, 1/86,400,000,000,000 day. |
In JDK 1.5+ you have System.nanoTime which has much greater resolution, but is not not synchronized to UTC time in any way. Presumably it is implemented with the Pentium RDTSC instruction in Windows.
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