System.currentTimeMillis() is slow on Linux, especially with the HPET time source

A DESCRIPTION OF THE REQUEST :
See http://pzemtsov.github.io/2017/07/23/the-slow-currenttimemillis.html

System.currentTimeMillis() takes about 650 ns on Linux with the HPET time source, compared to 4 ns on Windows.

The Linux implementation of currentTimeMillis calls gettimeofday. gettimeofday has higher precision than what's needed for currentTimeMillis. clock_gettime with a COARSE timer would be much faster, which would help a lot with the HPET time, but also with the TSC timer.

JUSTIFICATION :
If System.currentTimeMillis is called in a time-sensitive manner, the slower behaviour on Linux can be damaging, especially if the HPET timer happens to be used. Using a faster implementation would be beneficial without a drawback, since the granularity of the result is 1ms.


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See http://pzemtsov.github.io/2017/07/23/the-slow-currenttimemillis.html for extensive tests and analysis.
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