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

David Holmes david.holmes at oracle.com
Tue Apr 14 22:32:36 UTC 2020

Hi Mark,

On 15/04/2020 3:09 am, Mark Kralj-Taylor wrote:
> David, Daniel,
> What is the oldest (lowest) version of Linux and glibc for openjdk 15?

I'm not clear on that.

> The availability of clock_gettime(CLOCK_REALTIME) on the oldest
> Linux/glibc supported by openjdk 15 is likely to be the deciding
> factor on if Hotspot Linux code can call
> clock_gettime(CLOCK_REALTIME).
> doc/building.md suggests minimum Linux is Oracle Enterprise Linux 6.4
> (i.e. RHEL 6.4).
> Which I think uses glibc 2.12-1.107 (based on
> https://yum.oracle.com/repo/OracleLinux/OL6/4/base/x86_64/index_src.html).
> Searching for glibc sources it looks like this supports
> clock_gettime(CLOCK_REALTIME).
> 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. So CLOCK_REALTIME should be available even
> if CLOCK_MONOTONIC is not.
> - See: https://linux.die.net/man/2/clock_gettime.
> - Also "POSIX.1-2008 marks gettimeofday() as obsolete, recommending
> the use of clock_gettime(2) instead." from:
> https://linux.die.net/man/2/gettimeofday
> Note that Hotspot os_posix.cpp checks for non-error return from
> clock_gettime(CLOCK_MONOTONIC) to guard setting the _clock_gettime
> function pointer. Which was why in the patch I called clock_gettime
> directly for Linux specific os_linux.cpp (a subset of Posix OS-s).
> Also os_linux.cpp has:
> #error "Build platform doesn't support clock_gettime and related functionality"
> #endif
> Which made me wonder if openjdk might require CLOCK_MONOTONIC - which
> would mean clock_gettime(CLOCK_REALTIME) is supported.

So we require that the build platform supports CLOCK_MONOTONIC and 
clock_gettime, but not that the runtime platform supports 
CLOCK_MONOTONIC (without which we don't/didn't need clock_gettime().

So we can switch to using clock_gettime(CLOCK_REALTIME) at build time 
with no problem.

We can probably also require clock_gettime(CLOCK_REALTIME) at runtime, 
but we need to double-check that. I recall encountering a platform where 
clock_gettime was not available, but I can't recall if it was mainstream 
or on one of the other OpenJDK projects - nor do I recall exactly how 
long ago this was. Keeping the dynamic lookup of clock_gettime would be 
a conservative approach here - but we would need to make the distinction 
between the ability to use CLOCK_REALTIME and CLOCK_MONOTONIC.

Or someone puts in the time and effort to establish exactly what the 
kernel and glibc dependencies are and whether we can just rely on 
everything existing on all platforms we care about. I don't have time to 
do that nor validate the results if someone else does it.


> Mark
> On Tue, 14 Apr 2020 at 18:04, Mark Kralj-Taylor <kralj.mark at gmail.com> wrote:
>> Daniel,
>> Yes System.currentTimeMillis() and Clock.systemUTC() must be
>> consistent, so should use the same OS time source (as shown by ).
>> The patch to os_linux.cpp ensures this by calling the same Linux API:
>> clock_gettime(CLOCK_REALTIME) for both, from:
>> - os::javaTimeMillis() that backs System.currentTimeMillis()
>> - os::javaTimeSystemUTC() that backs Clock.systemUTC()
>> Looking at Linux / glibc source I think that gettimeofday() and
>> clock_gettime(CLOCK_REALTIME) are supposed to be the same clocksource.
>> i.e. that given by: cat
>> /sys/devices/system/clocksource/clocksource0/current_clocksource
>> Mark
>> On Tue, 14 Apr 2020 at 13:29, Daniel Fuchs <daniel.fuchs at oracle.com> wrote:
>>> Hi,
>>> On 11/04/2020 00:53, David Holmes wrote:
>>>> Update:
>>>>> It's a holiday weekend so I can't dig into this right now but we tried
>>>>> using a high-precision clock source for systemUTC() in the past but it
>>>>> didn't work because systemUTC() and currentTimeMillis() have to use
>>>>> the same time base, and currentTimeMillis() has to use gettimeofday().
>>>>> I thought this cross-dependency was documented somewhere but can't
>>>>> find it right now. If gettimeofday and clock_gettime(CLOCK_REALTIME)
>>>>> actually have the same time characteristics wrt. wall-clock time then
>>>>> changing both as suggested may indeed work.
>>> Just to emphasize David's comment: System::currentTimeMillis() and
>>> Clock::systemUTC() should use the same time source: if they don't - then
>>> some tests will fail, and because it can be tricky to assert things
>>> in tests, they might (and probably will) fail only intermittently.
>>> I'm probably the culprit here, I added those tests when I upgraded
>>> Clock::systemUTC() to report sub millisecond granularity [1] - as was
>>> available in the underlying clock that System::currentTimeMillis()
>>> already used.
>>> However, I think I would be disturbed if System::currentTimeMillis()
>>> and Clock::systemUTC() were using different clocks and could
>>> report a different notion of time (by drifting away from each other).
>>> best regards,
>>> -- daniel
>>> [1] https://bugs.openjdk.java.net/browse/JDK-8068730

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