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

[David Holmes]

Hi Vyom,

Thanks for looking at this.

On 26/05/2020 6:44 pm, Vyom Tiwari wrote:
> Hi David,
> 
> we can remove the redundant local variable(jlong result) from if block 
> as follows.
> 
> return jlong(ts.tv_sec) * MILLIUNITS +  jlong(ts.tv_nsec) / NANOUNITS_PER_MILLIUNIT;

Sure. I copied the code from os::javaTimeNanos. IIRC I introduced the 
local to allow adding a printf before the return when debugging the 
conversion :) I should have removed it again.

Thanks,
David

> Vyom
> 
> 
> On Tue, May 26, 2020 at 10:29 AM David Holmes <david.holmes at oracle.com 
> <mailto:david.holmes at oracle.com>> 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
> 
> 
> 
> -- 
> Thanks,
> Vyom