RFR(s): 8170520: Make Metaspace ChunkManager counters non-atomic

[Thomas Stüfe]

Hi all,

please take a look at this small fix. (This is one of the smaller fixes in
preparation for a prototype for JDK-8166690, I try to move the smaller
stuff out of the way first).

Issue: https://bugs.openjdk.java.net/browse/JDK-8170520

webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8170520-Make-
Metaspace-ChunkManager-Counters-NonAtomic/webrev.00/webrev/

Small discussion here:
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-October/021709.html

Background: ChunkManager keeps counters (_free_chunks_count and
free_chunks_total). These counters are updated using Atomics. Because this
is expensive, code goes thru some lengths to accumulate updates, which in
turn makes the code more complicated and error prone as necessary. But
updating the counters atomically is not needed, because updates happen
under lock protection anyway.

The proposed fix makes updates non-atomic and moved the call to
inc_free_chunks_total() into ChunkManager::return_chunks() - close to where
the chunk is actually returned to the freelist, so the time window where
the counters are invalid is very small now.

It also fixes an assert and makes inc_free_chunks_total() private because
now it can be private.

Note that I do not intend to push this into 9, so this is for the upcoming
10 repository. I would like to get some reviews nevertheless, so this small
change can be pushed quickly once the 10 repo exists.

Thanks!

Best Regards, Thomas