Obtaining the time in Java with nano-second precision
On Linux the code that the OpenJDK JVM executes is:
The full source can be found here. The code makes the function call gettimeofday(). This returns the time since the Linux epoch in micro seconds however it is rounded to the nearest millisecond. On newer Linux kernels this is an alias for clock_gettime(). The POSIX spec has deprecated gettimeofday() for clock_gettime() so this should be used instead. On modern Linux systems the function call is implemented as a Virtual Dynamic Shared Object or (VDSO) as opposed to a system call for improved performance.
The accuracy and of the underlying clock is dependent on the clock source being used by the kernel. To view all clock sources availble execute
cat /sys/devices/system/clocksource/*/available_clocksourceTo view the current clock source in use execute
Calling clock_gettime() to obtain a nanosecond precision time from the JVM can be achieved using Java using JNA or JNI. We implement both clock_gettime() using JNA and JNI and performance compared with System.currentTimeMillis() as a baseline. The tests were conducted on a Intel Xeon CPU E5-1650 v3 @ 3.50GHz using the Java Benchmark Harness (JMH).
All the code and benchmarks are available here to be executed independently.
|Source JMH output is here|
For the most part this is a comparison between JNA and JNI performance. JNA has the advantage of not requiring the user to compile any native code but this comes at the cost of performance. For each call to obtain the time JNA constructs Structure objects that represent native structures. This results in a worse median latency as well as generating significant amounts of garbage degrading long tail latency. The JNA direct mapping showed improved performance past the 99.99 percentiles but was still significantly slower than JNI.
In the future there will be some support for higher precision time in Java 9.