[infinispan-dev] Time measurement and expiry

[Dan Berindei]

On Tue, Oct 18, 2011 at 1:32 AM, Mircea Markus <mircea.markus at jboss.com> wrote:
>
> On 17 Oct 2011, at 14:13, Sanne Grinovero wrote:
>
>>> Very interesting, I knew that in Windows currentTimeMillis() basically
>>> just reads a volatile because I got bit by the 15 millisecond accuracy
>>> issue before, so I thought it would always be very fast. I had no idea
>>> on Linux it would have the same performance as nanoTime().
> Indeed very nice!
> I miss the part where the article says nanoTime has the same performance on modern Linux as currentTimeMillis, are you sure?

Yeah, the article didn't talk about Linux but I found this article:
http://blogs.oracle.com/ksrini/entry/we_take_java_performance_very
There's also this JDK bug complaining about both being slow:
http://bugs.sun.com/view_bug.do?bug_id=6876279  (the test output is in
a weird format, ignore the 1st number, the 2nd one is nanos/call).
Except when I actually ran the code in the bug report I my timings
were much better than those reported in the bug description:

java version "1.6.0_22"
OpenJDK Runtime Environment (IcedTea6 1.10.3) (fedora-59.1.10.3.fc15-x86_64)
OpenJDK 64-Bit Server VM (build 20.0-b11, mixed mode)
Intel(R) Core(TM) i5 CPU       M 540  @ 2.53GHz

currentTimeMillis: 36ns
nanoTime: 28ns

-server or -XX:AggressiveOpts don't seem to make a difference.


I also ran the test on cluster10 and the results are slightly worse,
but not significantly:

java version "1.6.0_17"
OpenJDK Runtime Environment (IcedTea6 1.7.10) (rhel-1.39.b17.el6_0-x86_64)
OpenJDK 64-Bit Server VM (build 14.0-b16, mixed mode)
Intel(R) Xeon(R) CPU           E5640  @ 2.67GHz

currentTimeMillis: 40ns
nanoTime: 35ns


It would be interesting if we could run the test on all our machines
and see how the timings vary by machine and OS.


It seems we're not the only ones with this problem either, Oracle (the
database) apparently calls gettimeofday() a lot so RHEL has some
optimizations to remove the system call overhead and make it even
faster (more like Windows I presume, but I don't have a Windows
machine on hand to confirm):
http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_MRG/1.3/html/Realtime_Tuning_Guide/sect-Realtime_Tuning_Guide-General_System_Tuning-gettimeofday_speedup.html
http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_MRG/1.3/html/Realtime_Tuning_Guide/sect-Realtime_Tuning_Guide-Realtime_Specific_Tuning-RT_Specific_gettimeofday_speedup.html