time : Java Glossary

Benchmarking, Measuring Elapsed Time with System.nanoTime

For benchmarking, use System.nanoTime which has much greater resolution than System.currentTimeMillis, but is not synchronized to UTC (Coordinated Universal Time/Temps Universel Coordonné) time in any way. There is no guarantee of resolution. It is not even synchonised between different JVMs (Java Virtual Machines) running on the same machine! I have already heard that you can’t even trust it to be synchronised between threads. All you can count on is that is will not run backwards even when a thread runs on different cores. You may well find every value it returns ends in 000000 because the clock cannot provide better than millisecond resolution/granularity. On a 2 GHz machine, 1 nanosecond represents about two machine cycles or the time at takes to do two additions.

// elapsed time with System.nanoTime
// Works only in JDK 1.5+

long start = System.nanoTime();

// .. do something

// elapsed time in nanoseconds, billionths of a second
long elapsed = System.nanoTime() - start;

On Windows boxes, a 1.5 JVM (Java Virtual Machine) uses the Win32 function QueryPerformanceCounter() which has no defined resolution. It reports clock cycles since bootup. The JVM has to use QueryPerformanceFrequency() to interpret the results. The JVM does this for you and reports the results of System. nanoTime in true nanoseconds. If you want the finer resolution of clock cycles, use the assembler instruction RDTSC (Read Time Stamp Counter). This means that System. nanoTime results are useful both for comparing algorithms on the same hardware and for benchmarking different hardware on the same algorithm.

Unfortunately, if you have multiple CPUs (Central Processing Units) each has its own nanotimer and Windows does not properly keep them synchronised. So you hop back and forth between different timers as your task runs on different CPUs. Linux has ways of dealing with this. Power Management also causes drift between multiple CPUs.

If you have to code for 1.4-, then you can only get millisecond accuracy. You need to use:

// elapsed time with System.currentTimeMillis
// Works in all JDK versions

long start = System.currentTimeMillis();

// .. do something

// elapsed time in microseconds, millionths of a second
long elapsed = System.currentTimeMillis() - start;

Pentium RDTSC

If you are stuck using a JDK (Java Development Kit) 1.4-, then you could roll your own nanoTime method. RDTSC reads the cycle counter into EDX:EAX. Some operating systems may make this a privileged instruction. See Intel’s documentation for more details. You could access them via a bit of JNI (Java Native Interface) with the Pentium class that I wrote and posted the code for. If you use that method, make sure you calibrate so you can discount the considerable overhead of the JNI calls themselves.

Non-Java Timestamp Schemes

Microsoft timestamps files with 64-bit hundreds of nanoseconds since 1601-01-01 UTC Gregorian. I presume they use Pope Gregory’s flip date to avoid the complication of the missing days. The FileTimes package converts from Java time to Windows native time to access the create date and last access date of a Windows file. In DOS, W3.1, OS2, W95, W98 and Me the dates were kept internally in local time.

Performance Monitoring

System.currentTimeMillis Timer Accuracy

Measuring Resolution/Granularity of System.currentMillis

Here is the source code for measuring the resolution of System.currentMillis on your own computer.

In addition, the Thread.sleep method will achieve delays of 1 ms on a single processor and 2 ms minimum on a dual processor.

You can also use the JVMPI (Java Virtual Machine Profiler Interface) for accurate timing.

System.nanoTime() returns the current value of the most precise available system timer, in nanoseconds. It is usually the CPU instruction clock, calibrated and converted into nanoseconds.

Units of Time

Imagine trying to explain this to a space faring species that used but a single unit of measure for time. Java 1.5 added the TimeUnit class to help convert between all these different ways of measuring it.

Mixed Base Conversion

Repeating Events

Learning More