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

[David Holmes]

Thanks Roger!

David

On 27/05/2020 12:28 am, Roger Riggs wrote:
> Looks good.
> 
> Thanks to Mark and you for the improvement and testing.
> 
> 
> 
> On 5/26/20 12:59 AM, 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
>