RFR(xs): 8185033: On Metaspace OOM, ChunkManager composition should be logged.

[Thomas Stüfe]

Dear all,

May I please get reviews and a sponsor for the following small enhancement.

Issue: https://bugs.openjdk.java.net/browse/JDK-8185033
Webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8185033-On-Metaspace-OOM-chunkManager-composition-should-be-logged/webrev.00/webrev/

This patch adds a bit of logging if a Metaspace OOM happens. We will now
get a printout of the composition of the global chunk managers (non-class
and, if necessary, class).

Before it looked like this:

[272.727s][info][gc,metaspace,freelist] All Metaspace:
[272.728s][info][gc,metaspace,freelist] data space:   Chunk accounting:
used in chunks 100752K + unused in chunks 650K  +  capacity in free chunks
8293K = 109696K  capacity in allocated chunks 101403K
[272.728s][info][gc,metaspace,freelist] class space:   Chunk accounting:
used in chunks 4047K + unused in chunks 97K  +  capacity in free chunks
6085K = 10230K  capacity in allocated chunks 4145K
[272.729s][info][gc,metaspace,freelist] Total fragmentation waste (words)
doesn't count free space
[272.729s][info][gc,metaspace,freelist]   data: 183 specialized(s) 960, 169
small(s) 41, 1507 medium(s) 4480, large count 1
[272.729s][info][gc,metaspace,freelist]  class: 183 specialized(s) 0, 13
small(s) 12, 111 medium(s) 8, large count 1

Now:
[250.548s][info][gc,metaspace,freelist] All Metaspace:
[250.549s][info][gc,metaspace,freelist] data space:   Chunk accounting:
used in chunks 100748K + unused in chunks 638K  +  capacity in free chunks
10357K = 111744K  capacity in allocated chunks 101387K
[250.550s][info][gc,metaspace,freelist] class space:   Chunk accounting:
used in chunks 4047K + unused in chunks 97K  +  capacity in free chunks
6085K = 10230K  capacity in allocated chunks 4145K
[250.550s][info][gc,metaspace,freelist] Total fragmentation waste (words)
doesn't count free space
[250.550s][info][gc,metaspace,freelist]   data: 183 specialized(s) 960, 165
small(s) 27, 1507 medium(s) 4504, large count 1
[250.550s][info][gc,metaspace,freelist]  class: 183 specialized(s) 0, 13
small(s) 12, 111 medium(s) 14, large count 1
[250.550s][info][gc,metaspace,freelist] Chunkmanager (non-class):
[250.550s][info][gc,metaspace,freelist]   89 specialized (128 bytes)
chunks, total 91136 bytes
[250.550s][info][gc,metaspace,freelist]   2567 small (512 bytes) chunks,
total 10514432 bytes
[250.550s][info][gc,metaspace,freelist]   0 medium (8192 bytes) chunks,
total 0 bytes
[250.550s][info][gc,metaspace,freelist]   0 humongous chunks, total 0 bytes
[250.550s][info][gc,metaspace,freelist]   total size: 10605568.
[250.550s][info][gc,metaspace,freelist] Chunkmanager (class):
[250.550s][info][gc,metaspace,freelist]   87 specialized (128 bytes)
chunks, total 89088 bytes
[250.550s][info][gc,metaspace,freelist]   2999 small (256 bytes) chunks,
total 6141952 bytes
[250.550s][info][gc,metaspace,freelist]   0 medium (4096 bytes) chunks,
total 0 bytes
[250.550s][info][gc,metaspace,freelist]   0 humongous chunks, total 0 bytes
[250.550s][info][gc,metaspace,freelist]   total size: 6231040.

This helps in understanding the underlying cause of the OOM, if that cause
is related to chunks lingering around in the freelist. See also [1], a Jep
proposing to improve the metaspace chunk allocation to increase the chance
of chunk reuse. This patch here is also a preparation for the prototype
work of [1].

Note that admittedly, metaspace.cpp will again gain a bit of complexity by
this patch. When examining the coding, I was tempted to refactor but
refrained from doing so to keep the patch focused. But I think
metaspace.cpp (especially the coding associated with statistics) could be
cleaned up a bit. I opened a follow up item [2] to collect cleanup-issues.
I am willing to do that myself, but would prefer it to be done after
functional patches - which still have a slight chance of being backported
to jdk9 - are all being added.

Patch was built and tested on Windows x64, Linux x64 and AIX. I also ran
jtreg hotspot/runtime/Metaspace tests, no issues.

Kind Regards, Thomas

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[1] https://bugs.openjdk.java.net/browse/JDK-8166690
[2] https://bugs.openjdk.java.net/browse/JDK-8185034