JVM time drifting

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04.09.2008 at 08:29AM PDT, ID: 23308523 | Points: 500

	[x] Attachment Details

JVM time drifting

Zones: Java Programming Language, Unix Operating Systems, Application Servers

HP_UX OS time varies from Websphere systemout log.

Why OS time(date) different from systemout log.

When i restart servers, its getting OS time. However, its drifting 2 minutes after one day... I greatly appreciate anyone inputs.

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Question Stats

Zone: Software

Question Asked By: wasuser

Question Asked On: 04.09.2008

Participating Experts: 2

Points: 500

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04.09.2008 at 08:31AM PDT, ID: 21315984

Rank: Genius

CEHJ:

Are you sure you're looking at system time on the OS and not hardware time?

04.09.2008 at 08:33AM PDT, ID: 21315998

wasuser:

I hope, i am looking at OS time by using "date" command .... How can i check Hardware time????

Thanks for any inputs...

04.09.2008 at 08:40AM PDT, ID: 21316074

Rank: Genius

CEHJ:

Yes, you would be. I've never come across this before. I would check to see if you can reproduce it in *any* java program

04.09.2008 at 09:05AM PDT, ID: 21316311

wasuser:

Please pardon my dumb question

From where Systemout log timestamp getting ?????

Why its drifting from Unix date command?

Thanks

04.09.2008 at 09:31AM PDT, ID: 21316546

Rank: Genius

CEHJ:

Java's meant to get it from the system. How far does it differ right now?

04.09.2008 at 09:34AM PDT, ID: 21316568

wasuser:

3 minutes after 40 hours ....

04.09.2008 at 09:40AM PDT, ID: 21316626

Rank: Genius

CEHJ:

That could be due to threading issues. The time may be got but unable to print immediately. Run the following and tell me what the difference is:

                       
                                public class DatePrinter {
    public static void main(String[] args) {
        System.out.println(new java.util.Date());
    }
}

Open in New Window

04.09.2008 at 09:48AM PDT, ID: 21316700

wasuser:

When i use follwoing TimeTest class:

//TimeTest.java
import java.util.Date;
public class TimeTest
{
public static void main(String[] args)
{
System.out.println(new Date());
}
}

RESULTS:
-----------
Wed Apr 09 12:45:58 EDT 2008

Date command results as follows:
Wed Apr 9 12:46:01 2008

When i start Application servers .... i am getting same time as OS time (date) .... However, if it is runs for 2 days then we can see 3 minutes drifting .... I am guessing May be JVM calculte time with CPU clicks ....

04.09.2008 at 10:03AM PDT, ID: 21316844

Rank: Genius

CEHJ:

You could try the following and see if that delta grows

                       
                                while [ true ];do date >>/tmp/clock.log;java TimeTest >>/tmp/clock.log;sleep $((60 * 30));done

Open in New Window

04.09.2008 at 10:32AM PDT, ID: 21317093

wasuser:

I am running that script ..... i let you know how it goes!!!

04.09.2008 at 12:02PM PDT, ID: 21317989

wasuser:

Out put of that script:

Wed Apr 9 13:27:02 2008
Wed Apr 09 13:27:02 EDT 2008
Wed Apr 9 13:57:05 2008
Wed Apr 09 13:57:05 EDT 2008
Wed Apr 9 14:27:08 2008
Wed Apr 09 14:27:08 EDT 2008
Wed Apr 9 14:57:10 2008
Wed Apr 09 14:57:11 EDT 2008

see 1 second drifted for 1:30 hour

Thanks

04.09.2008 at 12:10PM PDT, ID: 21318060

Rank: Genius

CEHJ:

>>see 1 second drifted for 1:30 hour

I think you can call that 'no drift' ;-)

That was what i expected

04.09.2008 at 12:26PM PDT, ID: 21318206

wasuser:

I didn't get you ... However, why we are getting 1 second diffence ?

04.09.2008 at 12:36PM PDT, ID: 21318284

Rank: Genius

CEHJ:

1 second is by no means surprising, especially since the two different 'programs' run serially

04.09.2008 at 12:42PM PDT, ID: 21318327

wasuser:

OK ... i will let it run and see how it goes!!!

While googling, i found follwoing link ... in Reply 17 someone mentioned some patches .... Do you have any idea what are those ....????

http://forum.java.sun.com/thread.jspa?threadID=765165&start=15&tstart=0

Thank you very much!!!!

04.09.2008 at 12:51PM PDT, ID: 21318404

blu:

Okay, you found the thread that states that the HP JVM maintains its own internal clock, which is pretty dumb if you ask me. However, it says that if you start the JVM with the "-XX:+UseGetTimeOfDay" flag, it will revert to using the system clock. Have you tried that?

04.09.2008 at 01:01PM PDT, ID: 21318494

wasuser:

OK ...

1. When i restart my Application servers .. OS time match with Systemout logs ....
2. After one day ... difference between my OS time (running on HP-UX) and JVM time (represented by my app servers) is around 2 minutes...

Thanks in advance

04.10.2008 at 05:04AM PDT, ID: 21323734

blu:

Was that last comment with out without the suggested flag?

04.10.2008 at 08:41AM PDT, ID: 21326030

wasuser:

Without flag!!!!

Do you want me start JVM with "-XX:+UseGetTimeOfDay" as command line argument?

04.10.2008 at 10:00AM PDT, ID: 21326939

blu:

Yes!!!!

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