RFR: JDK-8160350 cannot truss jdk9 [solaris]

[Kim Barrett]

> On Jul 1, 2016, at 9:29 AM, Alan Burlison <Alan.Burlison at oracle.com> wrote:
> 
> Summary:
> 
> Microstate accounting has been always-on on Solaris since Solaris 9, the use of /proc/self/ctl to enable it is redundant and has the side-effect of breaking the use of truss on JVM processes. Microstate accounting code removed and other now-redundant *vtime* methods also removed.
> 
> Tested with JPRT hostpot testset, all tests passed
> 
> Webrev:   http://cr.openjdk.java.net/~coleenp/JDK-8160350/
> Bug link: https://bugs.openjdk.java.net/browse/JDK-8160350
> 
> -- 
> Alan Burlison
> --

This doesn't look right to me.

The BSD implementation of os::elapsedVTime presently just calls
elapsedTime, with a comment "better than nothing, but not much".  I
think it's worse than nothing, and should actually fail, with
os::supports_vtime returning false for this platform.  Similarly,
elapsedVtime on some other platforms falls back to elapsedTime, which
I suspect is a mistake.

The problem is that the way one deals with vtime is different from the
way one deals with elapsedTime values.  The predicate is needed so
that code that wants to use vtime if available can be properly written
to fall back to something else when it's not.  (Note that I think
there might be bugs in how G1 is attempting to do that in some
places.)

I have a note to self to look into this and probably report some bugs,
but haven't gotten around to doing so.

So while we might no longer need os::enable_vtime and
os::vtime_enabled, I think getting rid of os::supports_vtime isn't
right.

OTOH, the present usage of vtime is quite limited.  As noted, it's
only used by G1, and there I think only for some logging / stats
reporting.  Maybe we don't really need it at all?  I don't have an
opinion on that, but other folks in the GC team (at least) might.