Re: Time travel? Or vaticinating web server? - SOLUTION

[Jordi Salvat i Alabart <js...@atg.com>]

I've finally solved this issue. Posting here for the benefit of list 
lurkers and archive readers who may hit the same problem in the future:

This was actually a Linux Kernel "bug". I've fixed it by upgrading to a 
2.4.20 kernel built with CONFIG_X86_TSC_DISABLE set. I don't know 
whether there's a solution for earlier kernels (this config var doesn't 
exist in 2.4.18).

 From the 2.4.20 kernel Configure.help:

Unsynced TSC support
CONFIG_X86_TSC_DISABLE
   This option is used for getting Linux to run on a NUMA multi-node
   boxes, laptops and other systems suffering from unsynced TSCs or
   TSC drift, which can cause gettimeofday to return non-monotonic values.
   Choosing this option will disable the CONFIG_X86_TSC optimization,
   and allows you to then specify "notsc" as a boot option regardless of
   which processor you have compiled for.
 

   NOTE: If your system hangs when init should run, you are probably
   using a i686 compiled glibc which reads the TSC without checking for
   availability. Boot without "notsc" and install a i386 compiled glibc
   to solve the problem.
 

   If unsure, say N.

Salut,

Jordi.

Jordi Salvat i Alabart wrote:
> Hi.
> 
> While trying to test JMeter for measurement accuracy, I've discovered 
> the principle for time-travel. Or maybe it's my Apache web server, which 
> is capable of responding to requests before it receives them... I think 
> I'll request a patent for that and make my first million...
> 
> Seriously: I'm seeing negative times in my samples.
> 
> So, I've created the attached bit of code, which shows that 
> System.currentTimeMillis() is not reliable. I'm using Linux with a 
> 2.4.18 kernel and libc6 2.3.1-5. The problem occurs with the following 
> JDKs:
> 
> java version "1.3.1"
> Java(TM) 2 Runtime Environment, Standard Edition (build 
> Blackdown-1.3.1-02b-FCS)Java HotSpot(TM) Client VM (build 
> Blackdown-1.3.1_02b-FCS, mixed mode)
> 
> java version "1.4.1_01"
> Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.1_01-b01)
> Java HotSpot(TM) Client VM (build 1.4.1_01-b01, mixed mode)
> 
> Can someone please try this on a different O.S., to see if it's my 
> system or the JDK?
> 
> Thanks,
> 
> Jordi.
> 
> 
> 
> 
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