RFR(xs): 8245926: JFR: Remove chunkSz and rename blockSz from classloader statistics

[Thomas Stüfe]

Hi,

may I have opinions/reviews for this change please:

JBS: https://bugs.openjdk.java.net/browse/JDK-8245926
Webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8245926-jfr-remove-chunksz-rename-blocksz/webrev.00/webrev/

This is part of an effort to reduce exposure of Metaspace implementation
details somewhat in preparation for JDK-8221173.

In JFR, the classloader statistics event contains the following fields:

chunkSize
blockSize
hiddenChunkSize
hiddenBlockSize

blockSize is the interesting one, the number of bytes of Metaspace used by
this class loader for storing metadata. I propose to rename this to
"totalMetaspaceUsed" resp. "hiddenClassesMetaspaceUsed".

chunkSize is the total size of chunks given to a class loader. Delta
between blockSize and chunkSize constitutes waste if the loader stops
loading; however, as far as waste goes this is not very informative since
it only is a usually small part of waste in the current metaspace
allocator.

Moreover, with JDK-8221173 chunks may be partially uncommitted, so that the
delta between chunkSize and blockSize may or may not contribute to RSS. So
the chunkSize stat becomes more meaningless.

I'd argue that this stat is quite useless to the average JFR user; and to
the VM developer interested in fixing up Metaspace there are better tools
(e.g. jcmd VM.metaspace). Therefore I would like to simply remove this stat.

--

Please note that this is my first change to JFR events. Some things are
unclear to me, e.g. how backward compatibility in JMC is handled. So if
something is missing from the patch, please advice.


Thanks!

.. Thomas