RFR(S) 8242504: Enhance the system clock to nanosecond precision

[Daniel Fuchs]

Hi David,

This is not a review for the posix code.

Your webrev looks good to me and corresponds to what I expected
to see. I understand that not all operating systems / platforms
are expected to have the nano second precision, so your test
probably can't go much beyond what is currently being tested.

Last time I upgraded the system clock to micro second precision [1]
I had to write a CSR and release notes. That was the first time
the clock went beyond millisecond precision however - and my
expectation is that your proposed change should no longer
require a CSR as potential nanosecond precision should
now be covered by the spec.

I had to modify a few other places as well (see [1]) - where precision
greater than 1ms was not expected, but these modifications
should cover your current changes too, so I don't think anything
more is required. Some --test-repeat might be in order
to verify things are stable but I don't expect any issue there :-)

LGTM.

best regards,

-- daniel

[1] https://bugs.openjdk.java.net/browse/JDK-8068730

On 26/05/2020 05:59, David Holmes wrote:
> bug: https://bugs.openjdk.java.net/browse/JDK-8242504
> webrev: http://cr.openjdk.java.net/~dholmes/8242504/webrev/
> 
> This work was contributed by Mark Kralj-Taylor:
> 
> https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-April/038975.html 
> 
> 
> On the hotspot side we change the Linux implementations of 
> javaTimeMillis() and javaTimeSystemUTC() so that they use 
> clock_gettime(CLOCK_REALTIME) instead of gettimeofday(). In keeping with 
> our conditional use of this API I added a new guard
> 
> os::Posix::supports_clock_gettime()
> 
> and refined the existing supports_monotonic_clock() so that we can still 
> use CLOCK_REALTIME if CLOCK_MONOTONIC does not exist. In the future 
> (hopefully very near future) we will simply assume these APIs always exist.
> 
> On the core-libs side the existing test:
> 
> test/jdk/java/time/test/java/time/TestClock_System.java
> 
> is adjusted to track the precision in more detail.
> 
> Finally Mark has added instantNowAsEpochNanos() to the benchmark 
> previously known as
> 
> test/micro/org/openjdk/bench/java/lang/Systems.java
> 
> which I've renamed to SystemTime.java as Mark suggested. I agree having 
> these three functions measured together makes sense.
> 
> Testing:
>    - test/jdk/java/time/test/java/time/TestClock_System.java
>    - test/micro/org/openjdk/bench/java/lang/SystemTime.java
>    - Tiers 1-3
> 
> Thanks,
> David