Issue #8658 has been updated by headius (Charles Nutter).


I missed the discussion on this, but here's the summary of JRuby/JVM case:

If monotonic clock is available at OS level, System.nanoTime is equivalent to clock_gettime(CLOCK_MONOTONIC). I suppose there may be some embedded systems or obscure platforms that don't have a monotonic clock, but otherwise I'm guessing this is going to be pretty universal across *nixes. The code in JDK is in http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/9b0ca45cd756/src/os/linux/vm/os_linux.cpp at line 1453.

If monotonic clock is not available on *nix, JVM will fall back on gettimeofday transparently. I'm investigating whether it is possible for us to query this behavior.

System.currentTimeMillis is just a plain gettimeofday call, which in the Process::clock_gettime API is called GETTIMEOFDAY_BASED_CLOCK_REALTIME.

So JRuby will always be able to support GETTIMEOFDAY_BASED_CLOCK_REALTIME via System.currentTimeMillis, CLOCK_MONOTONIC when nanoTime is monotonic, and the other forms when we're able to make a native downcall. Initially, we will probably just support these two.

I HAVE A QUESTION, however... what about Windows? There's no mention at all in the rdoc about Windows support. I need to investigate what currentTimeMillis and nanoTime do on JVM on Windows.
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Feature #8658: Process.clock_gettime
https://bugs.ruby-lang.org/issues/8658#change-42175

Author: akr (Akira Tanaka)
Status: Closed
Priority: Normal
Assignee: 
Category: 
Target version: 


How about adding a new method, Process.clock_gettime(clk_id) ?

Recently there were two feature request for measuring time.
Feature #8640 https://bugs.ruby-lang.org/issues/8640
Feature #8096 https://bugs.ruby-lang.org/issues/8096

It seems they are somewhat different.

clock_gettime() function defined by POSIX is a good
candidate for providing as a method.
I think it can supports the both request.

Also, it has less possible design choices than the requests
because clock_gettime() is defined by POSIX.
People familiar to POSIX can learn the method more easily.

I wrote a patch to implement Process.clock_gettime.
This method can be used as follows.

  % ./ruby -e 'p Process.clock_gettime(Process::CLOCK_MONOTONIC)'
  2701692957811563

Several considerations:

I implemented the method as a module function of Process.
It is same as Process.times.
I expect clock_gettime is used mainly for measuring
time interval and wall clock time is not important.
So I didn't use Time.

The method returns a number of nanoseconds as an integer.
It is not so unexpected if user knows clock_gettime() in POSIX.

clock_gettime() returns it as struct timespec
which contains two fields: tv_sec and tv_nsec.

Although tv_sec is time_t, Time is not appropriate because
the origin (zero) can be other than the Epoch.
Actually CLOCK_MONOTONIC means elapsed time since
the system start-up time on Linux.

Also, I expect the result is subtracted in most case:
  t1 = Process.clock_gettime(...)
  ...
  t2 = Process.clock_gettime(...)
  t = t2 - t1
So the result should be easy to subtract.
An array such as [sec, nsec] is difficult to subtract.

The result is an integer, not a float.
IEEE 754 double is not enough to represent the result
of clock_gettime(CLOCK_REALTIME).
It contains 19 digits in decimal now but IEEE 754 double
can represent only 15 digits.

On LP64 systems, Fixnum can represent 2**62-1.
So (2**62-1)/(365.25*24*60*60*1e9)=146.1 years are representable
without object allocation.

On ILP32 and LLP64 systems, Fixnum can represent 2**30-1.
So (2**30-1)/1e9=1.07 seconds are representable
without object allocation.
This means Bignum allocations are mostly required except
the origin is very recent.

clock_gettime() is defined by POSIX.
Linux, NetBSD, FreeBSD, OpenBSD has it, at least.

If clock_gettime() is not available,
an emulation layer for CLOCK_REALTIME is implementable
using gettimeofday().
(not implemented yet, though.)

Any comments?



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