Is System.nanoTime() in Java Still a Reliable Time Measurement Tool?

Is System.nanoTime() in Java Truly Useless?

Question:

A blog post titled "Beware of System.nanoTime() in Java" raises concerns that on x86 systems, System.nanoTime() can potentially return incorrect time values due to the use of CPU-specific counters. In a multi-core system, thread scheduling could result in a processor with a lower counter being assigned, leading to negative values for time differences.

Answer:

The blog post's claims were written in 2011 and are no longer accurate. Subsequent operating system updates have resolved these issues on Windows and Linux.

Windows:

On Windows XP Service Pack 2 and later, System.nanoTime() uses the QueryPerformanceCounter/QueryPerformanceFrequency API, which is implemented by the Hardware Abstraction Layer (HAL) to provide a consistent time source across processors.

Linux:

The Linux clock_gettime(CLOCK_REALTIME) function ensures that time is consistent across all processors. As of kernel version 2.6.18, a safe clocksource is used if unsynced or variable frequency time-stamp counters are detected.

Other Architectures:

There is limited information available about other architectures, such as SPARC and Solaris. However, it is generally assumed that modern virtual machines (VMs) provide a consistent time source for guest operating systems, regardless of the underlying hardware.

Conclusion:

System.nanoTime() remains a valid method for measuring time differences in Java. The concerns raised in the 2011 blog post have been addressed and do not apply to modern system implementations.

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