negative timings in GC log

[Mikael Gerdin]

On Friday 21 February 2014 10.09.21 Bernd Eckenfels wrote:
> Just a BTW: NTPd is known to not cause those problems as it does not "step"
> the clock, this does more happen if you use ntpdate in cron or the VMTools.
> I would advice against both in Java systems.
> 
> However I wonder if - similar to other timed primitives - it is time to
> reconsider switching of the clock source for those timings?

This has in fact been done in 8.
The following change makes the VM use CLOCK_MONOTONIC for the 
os::elapsed_counter which is used for the GC duration times.

https://bugs.openjdk.java.net/browse/JDK-8027294

However, there are quirks with CLOCK_MONOTONIC. See the man page for 
clock_gettime(3).

/Mikael

> > Am 21.02.2014 um 09:27 schrieb Kirk Pepperdine <kirk at kodewerk.com>:
> > 
> > Hi all,
> > 
> > More of an FYI but I sometimes run across things that look like this.
> > 
> > [PSYoungGen: 2096928K->64K(2097024K)] 3981201K->1884337K(4194176K),
> > -0.2652920 secs] [Times: user=0.04 sys=0.00, real=0.02 secs]
> > 
> > This is coming from a 1.7.0 JVM but I fear I won’t be able to determine
> > the exact build. The JVM was running in a virtualized environment. My
> > guess is something like this could happen if something (NTP?) has diddled
> > with the clock. My question is, would it be better to leave it as is or
> > report it as 0 or unknown?
> > 
> > Regards,
> > Kirk