[Commons Wiki] Update of "Monitoring/Precision" by nicolasDeLoof

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The following page has been changed by nicolasDeLoof:
http://wiki.apache.org/commons/Monitoring/Precision

New page:
= Performance monitoring precision =

Monitoring application performance is a primary use case of commons-monitoring. Java provides 2 timers implementation, as 
[http://java.sun.com/j2se/1.5.0/docs/api/java/lang/System.html#currentTimeMillis() System.currentTimeMillis] and 
[http://java.sun.com/j2se/1.5.0/docs/api/java/lang/System.html#nanoTime() System.nanoTime()].

Those timers methods performances and real precision depends on the hardware, OS and JRE implementation. 

== MS Windows ==

Based on [http://blogs.sun.com/dholmes/entry/inside_the_hotspot_vm_clocks SUN] technical documentation, both implementation precision depend on underlying hardware. 

  * {{{currentTimeMillis}}} uses a native windows low resolution timer, and for this reason is VERY quick (6 cycles), with a precision of only 10 to 15ms. Using this method as lower impacts on the application, but monitoring a process taking less than 15ms may produce elapsedTime=0 !

  * {{{nanoTime}}} uses Windows Performance API (when available) that is both hardware and version (and service pack) dependent (user timers, ACPI timers or CPU-level timers...). In most cases, this requires slow I/O instruction and result in microsecond accuracy. In better case requesting an on-chip timer requires ~100 cycles.


Both SUN JRE and Bea Jrockit implementation shows bad performances of {{{nanoTime}}} on Windows, but the {{{currentTimeMillis}}} precision is too limited for use as fine-grained performance counters.

== Sun Solaris ==

On this platform, a quick & dirty bench [1] demonstrates both methods have comparable performances. No technical info on real precision, but {{{nanoTime}}} would give the better one available.

== Other systems ==

?? any info available ??


== Java 1.3 / Java 1.4 ==

backport-util-concurrent (used to backport commons-monitoring on pre-java5) provides an implementation of {{{System.nanoTime}}} that delegates to {{{currentTimeMillis}}} with unit conversion. On Sun JRE 1.4 it also uses an internal SUN performance package to get a better precision.

== conclusion ==

Based on those informations, {{{currentTimeMillis}}} - even with nice performances on Windows - cannot be used for fine-grained monitoring. In all cases {{{nanoTime}}} provides the better time-counter available on the system, in terms of both precision and fiability


[2]
{{{#!java
public class CurrentTimeMillisVsNanoTime
{
    static int loops = 1000000;
    public static void main( String[] args )
    {
        long time = System.nanoTime();
        for ( int i = 0; i < loops; i++ )
        {
            System.nanoTime();
        }
        System.out.println( "System.nanoTime took     : " + (System.nanoTime() - time) + "ns" );

        time = System.nanoTime();
        for ( int i = 0; i < loops; i++ )
        {
            System.currentTimeMillis();
        }
        System.out.println( "System.currentTimeMillis : " + (System.nanoTime() - time) + "ns" );
    }
}
}}}

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