8234397: add OS uptime information to os::print_os_info output

[Schmidt, Lutz]

Matthias, 

your change looks good to me overall. Please note: I'm not a Reviewer!

I feel the urge to complain about one thing, though:
When calculating the uptime in days, you divide the time retrieved from the system (usually seconds or milliseconds) by a large number. Why do you force that number to be a float? I would prefer the denominator to be an "int" value. 

Rationale: floats (32bits) are very limited in precision, only y6 to 7 decimal digits. At least in the windows case, where you obtain milliseconds from the system, your denominator is 86,400,000. At first glance, that does not fit into a float mantissa. What saves you here are the prime factors "2" (10 in total). As a result, you only need 17 mantissa bits to represent the denominator. 

Thanks,
Lutz

On 25.11.19, 09:06, "hotspot-dev on behalf of Baesken, Matthias" <hotspot-dev-bounces at openjdk.java.net on behalf of matthias.baesken at sap.com> wrote:

    > 
    > The comment in the posix code mentions that it doesn't work on macOS but
    > doesn't say anything about Linux. Has it been tested on Solaris?
    >
    
    Hi David,  it works on Solaris .
    I think I should adjust the comment (saying macOS AND Linux) .
    
    Best regards, Matthias
    
    
    > >
    > > One example that occurred last week  -  my colleague Christoph and me
    > were browsing through an hs_err  file of a crash on AIX .
    > > When looking into the hs_err  we wanted to know the  uptime because
    > our latest  fontconfig - patches   (for getting rid of the crash)    needed a
    > reboot too to really work .
    > > Unfortunately we  could not find the info , and we were  disappointed  (
    > then we noticed the crash is from  OpenJDK and not our internal JVM ).
    > >
    > >
    > >>> Bug/webrev :
    > >>> https://bugs.openjdk.java.net/browse/JDK-8234397
    > >>> http://cr.openjdk.java.net/~mbaesken/webrevs/8234397.1/
    > >>
    > >> Can Linux not use the POSIX version?
    > >>
    > >
    > > Unfortunately the posix code does not give the desired result  on Linux (at
    > least on my test machines).
    > 
    > The comment in the posix code mentions that it doesn't work on macOS but
    > doesn't say anything about Linux. Has it been tested on Solaris?
    > 
    > I'm really unsure about this code and am hoping someone more
    > knowledgeable in this areas can chime in. I'd be less concerned if there
    > was a single POSIX implementation that worked everywhere. :( Though I
    > have my general concern about adding yet another potential point of
    > failure in the error reporting logic.
    > 
    > Thanks,
    > David
    > 
    > > Best regards, Matthias
    > >