os::javaTimeSystemUTC to call nanosecond precision OS API, so Clock.systemUTC() can give nanosecond precision UTC

[Florian Weimer]

* Mark Kralj-Taylor:

> The wording of Linux docs suggests that clock_gettime(CLOCK_REALTIME)
> should be supported if the clock_gettime() API exists. But other clock
> sources are not mandatory.

Really old glibc emulates clock_gettime (CLOCK_REALTIME) using
gettimeofday, yes.

clock_gettime was already in Linux 2.12 (and possibly quite a bit
earlier, I did not check), so that is not likely to be a limitation.

A tricky question is whether it's possible to avoid loading librt.
The present code already uses dlopen, but I think it could try a
little bit harder (try resolving clock_gettime first, and then load
librt, and try again).  For distribution builds that do not need to be
compatible with glibc versions before 2.17, directly calling
clock_gettime would also be an option. (clock_gettime moved to
libc.so.6 in glibc 2.17, but a lot of software keeps linking against
librt for the definition of clock_gettime, something that we are
finally tackling on the glibc side.)

Making a direct system call instead is a bit tricky because it's
absolutely required to use the vDSO if possible, for performance
reasons.  But it's possible to obtain the address of the vDSO function
using dlvsym, so that might be an option as well.