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

[Mikael Gerdin]

Hi Thomas,

On 2017-07-21 08:54, Thomas Stüfe wrote:
> 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.

I know this is a bit counter to what you said about keeping the patch 
small but what do you think about doing this on top of your change?
http://cr.openjdk.java.net/~mgerdin/8185033/webrev.00/src/share/vm/memory/metaspace.cpp.udiff.html

I'm not sure why you went with uint32_t for the lengths since it's just 
a couple of bytes and you can get rid of the casts.
I also took the liberty to squash out two const_casts which were making 
my eyes twitch... :)

/Mikael

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